Re: LLVM and dynamic linking
Replying to include the email list. You’re right, the llvm backend and the gmp licensing issues are orthogonal - or should be. The problem is I get build errors when trying to build GHC with LLVM and dynamic libraries. The result is that I get a few different choices when producing a platform image for development, with some uncomfortable tradeoffs: 1. LLVM-built GHC, dynamic libs - doesn’t build. 2. LLVM-built GHC, static libs - potential licensing oddities with me shipping a statically linked ghc binary that is now gpled. I am not a lawyer, but the situation makes me uncomfortable. 3. GCC/ASM-built GHC, dynamic libs - this is the *standard* for most platforms shipping ghc binaries, but it means that one of the biggest and most critical users of the LLVM backend is neglecting it. It also bifurcates development resources for GHC. Optimization work is duplicated and already devs are getting into the uncomfortable position of suggesting to users that they should trust GHC to build your programs in a particular way, but not itself. 4. GCC/ASM-built GHC, static libs - worst of all possible worlds. Because of this, the libgmp and llvm-backend issues aren’t entirely orthogonal. Trac ticket #7885 is exactly the issue I get when trying to compile #1. From: Carter Schonwald<mailto:carter.schonwald@gmail.com> Sent: Monday, December 30, 2013 1:05 PM To: Aaron Friel<mailto:aaron@frieltek.com> Good question but you forgot to email the mailing list too :-) Using llvm has nothing to do with Gmp. Use the native code gen (it's simper) and integer-simple. That said, standard ghc dylinks to a system copy of Gmp anyways (I think ). Building ghc as a Dylib is orthogonal. -Carter On Dec 30, 2013, at 1:58 PM, Aaron Friel <aaron@frieltek.com<mailto:aaron@frieltek.com>> wrote: Excellent research - I’m curious if this is the right thread to inquire about the status of trying to link GHC itself dynamically. I’ve been attempting to do so with various LLVM versions (3.2, 3.3, 3.4) using snapshot builds of GHC (within the past week) from git, and I hit ticket #7885 [https://ghc.haskell.org/trac/ghc/ticket/7885] every time (even the exact same error message). I’m interested in dynamically linking GHC with LLVM to avoid the entanglement with libgmp’s license. If this is the wrong thread or if I should reply instead to the trac item, please let me know. From: Carter Schonwald<mailto:carter.schonwald@gmail.com> Sent: Friday, December 27, 2013 2:41 PM To: Ben Gamari<mailto:bgamari.foss@gmail.com> Cc: ghc-devs@haskell.org<mailto:ghc-devs@haskell.org> great work! :) On Fri, Dec 27, 2013 at 3:21 PM, Ben Gamari <bgamari.foss@gmail.com<mailto:bgamari.foss@gmail.com>> wrote: Simon Marlow <marlowsd@gmail.com<mailto:marlowsd@gmail.com>> writes:
This sounds right to me. Did you submit a patch?
Note that dynamic linking with LLVM is likely to produce significantly worse code that with the NCG right now, because the LLVM back end uses dynamic references even for symbols in the same package, whereas the NCG back-end uses direct static references for these.
Today with the help of Edward Yang I examined the code produced by the LLVM backend in light of this statement. I was surprised to find that LLVM's code appears to be no worse than the NCG with respect to intra-package references. My test case can be found here[2] and can be built with the included `build.sh` script. The test consists of two modules build into a shared library. One module, `LibTest`, exports a few simple members while the other module (`LibTest2`) defines members that consume them. Care is taken to ensure the members are not inlined. The tests were done on x86_64 running LLVM 3.4 and GHC HEAD with the patches[1] I referred to in my last message. Please let me know if I've missed something. # Evaluation ## First example ## The first member is a simple `String` (defined in `LibTest`), helloWorld :: String helloWorld = "Hello World!" The use-site is quite straightforward, testHelloWorld :: IO String testHelloWorld = return helloWorld With `-O1` the code looks reasonable in both cases. Most importantly, both backends use IP relative addressing to find the symbol. ### LLVM ### 0000000000000ef8 <rKw_info>: ef8: 48 8b 45 00 mov 0x0(%rbp),%rax efc: 48 8d 1d cd 11 20 00 lea 0x2011cd(%rip),%rbx # 2020d0 <libtestzm0zi1zi0zi0_LibTest_helloWorld_closure> f03: ff e0 jmpq *%rax 0000000000000f28 <libtestzm0zi1zi0zi0_LibTest2_testHelloWorld_info>: f28: eb ce jmp ef8 <rKw_info> f2a: 66 0f 1f 44 00 00 nopw 0x0(%rax,%rax,1) ### NCG ### 0000000000000d58 <rH1_info>: d58: 48 8d 1d 71 13 20 00 lea 0x201371(%rip),%rbx # 2020d0 <libtestzm0zi1zi0zi0_LibTest_helloWorld_closure> d5f: ff 65 00 jmpq *0x0(%rbp) 0000000000000d88 <libtestzm0zi1zi0zi0_LibTest2_testHelloWorld_info>: d88: eb ce jmp d58 <rH1_info> With `-O0` the code is substantially longer but the relocation behavior is still correct, as one would expect. Looking at the definition of `helloWorld`[3] itself it becomes clear that the LLVM backend is more likely to use PLT relocations over GOT. In general, `stg_*` primitives are called through the PLT. As far as I can tell, both of these call mechanisms will incur two memory accesses. However, in the case of the PLT the call will consist of two JMPs whereas the GOT will consist of only one. Is this a cause for concern? Could these two jumps interfere with prediction? In general the LLVM backend produces a few more instructions than the NCG although this doesn't appear to be related to handling of relocations. For instance, the inexplicable (to me) `mov` at the beginning of LLVM's `rKw_info`. ## Second example ## The second example demonstrates an actual call, -- Definition (in LibTest) infoRef :: Int -> Int infoRef n = n + 1 -- Call site testInfoRef :: IO Int testInfoRef = return (infoRef 2) With `-O1` this produces the following code, ### LLVM ### 0000000000000fb0 <rLy_info>: fb0: 48 8b 45 00 mov 0x0(%rbp),%rax fb4: 48 8d 1d a5 10 20 00 lea 0x2010a5(%rip),%rbx # 202060 <rLx_closure> fbb: ff e0 jmpq *%rax 0000000000000fe0 <libtestzm0zi1zi0zi0_LibTest2_testInfoRef_info>: fe0: eb ce jmp fb0 <rLy_info> ### NCG ### 0000000000000e10 <rI3_info>: e10: 48 8d 1d 51 12 20 00 lea 0x201251(%rip),%rbx # 202068 <rI2_closure> e17: ff 65 00 jmpq *0x0(%rbp) 0000000000000e40 <libtestzm0zi1zi0zi0_LibTest2_testInfoRef_info>: e40: eb ce jmp e10 <rI3_info> Again, it seems that LLVM is a bit more verbose but seems to handle intra-package calls efficiently. [1] https://github.com/bgamari/ghc/commits/llvm-dynamic [2] https://github.com/bgamari/ghc-linking-tests/tree/master/ghc-test [3] `helloWorld` definitions: LLVM: 00000000000010a8 <libtestzm0zi1zi0zi0_LibTest_helloWorld_info>: 10a8: 50 push %rax 10a9: 4c 8d 75 f0 lea -0x10(%rbp),%r14 10ad: 4d 39 fe cmp %r15,%r14 10b0: 73 07 jae 10b9 <libtestzm0zi1zi0zi0_LibTest_helloWorld_info+0x11> 10b2: 49 8b 45 f0 mov -0x10(%r13),%rax 10b6: 5a pop %rdx 10b7: ff e0 jmpq *%rax 10b9: 4c 89 ef mov %r13,%rdi 10bc: 48 89 de mov %rbx,%rsi 10bf: e8 0c fd ff ff callq dd0 <newCAF@plt> 10c4: 48 85 c0 test %rax,%rax 10c7: 74 22 je 10eb <libtestzm0zi1zi0zi0_LibTest_helloWorld_info+0x43> 10c9: 48 8b 0d 18 0f 20 00 mov 0x200f18(%rip),%rcx # 201fe8 <_DYNAMIC+0x228> 10d0: 48 89 4d f0 mov %rcx,-0x10(%rbp) 10d4: 48 89 45 f8 mov %rax,-0x8(%rbp) 10d8: 48 8d 05 21 00 00 00 lea 0x21(%rip),%rax # 1100 <cJC_str> 10df: 4c 89 f5 mov %r14,%rbp 10e2: 49 89 c6 mov %rax,%r14 10e5: 58 pop %rax 10e6: e9 b5 fc ff ff jmpq da0 <ghczmprim_GHCziCString_unpackCStringzh_info@plt> 10eb: 48 8b 03 mov (%rbx),%rax 10ee: 5a pop %rdx 10ef: ff e0 jmpq *%rax NCG: 0000000000000ef8 <libtestzm0zi1zi0zi0_LibTest_helloWorld_info>: ef8: 48 8d 45 f0 lea -0x10(%rbp),%rax efc: 4c 39 f8 cmp %r15,%rax eff: 72 3f jb f40 <libtestzm0zi1zi0zi0_LibTest_helloWorld_info+0x48> f01: 4c 89 ef mov %r13,%rdi f04: 48 89 de mov %rbx,%rsi f07: 48 83 ec 08 sub $0x8,%rsp f0b: b8 00 00 00 00 mov $0x0,%eax f10: e8 1b fd ff ff callq c30 <newCAF@plt> f15: 48 83 c4 08 add $0x8,%rsp f19: 48 85 c0 test %rax,%rax f1c: 74 20 je f3e <libtestzm0zi1zi0zi0_LibTest_helloWorld_info+0x46> f1e: 48 8b 1d cb 10 20 00 mov 0x2010cb(%rip),%rbx # 201ff0 <_DYNAMIC+0x238> f25: 48 89 5d f0 mov %rbx,-0x10(%rbp) f29: 48 89 45 f8 mov %rax,-0x8(%rbp) f2d: 4c 8d 35 1c 00 00 00 lea 0x1c(%rip),%r14 # f50 <cGG_str> f34: 48 83 c5 f0 add $0xfffffffffffffff0,%rbp f38: ff 25 7a 10 20 00 jmpq *0x20107a(%rip) # 201fb8 <_DYNAMIC+0x200> f3e: ff 23 jmpq *(%rbx) f40: 41 ff 65 f0 jmpq *-0x10(%r13) _______________________________________________ ghc-devs mailing list ghc-devs@haskell.org<mailto:ghc-devs@haskell.org> http://www.haskell.org/mailman/listinfo/ghc-devs
7.8 should have working dylib support on the llvm backend. (i believe some of the relevant patches are in head already, though Ben Gamari can opine on that) why do you want ghc to be built with llvm? (i know i've tried myself in the past, and it should be doable with 7.8 using 7.8 soon too) On Wed, Jan 1, 2014 at 5:38 PM, Aaron Friel <aaron@frieltek.com> wrote:
Replying to include the email list. You’re right, the llvm backend and the gmp licensing issues are orthogonal - or should be. The problem is I get build errors when trying to build GHC with LLVM and dynamic libraries.
The result is that I get a few different choices when producing a platform image for development, with some uncomfortable tradeoffs:
1. LLVM-built GHC, dynamic libs - doesn’t build. 2. LLVM-built GHC, static libs - potential licensing oddities with me shipping a statically linked ghc binary that is now gpled. I am not a lawyer, but the situation makes me uncomfortable. 3. GCC/ASM-built GHC, dynamic libs - this is the *standard* for most platforms shipping ghc binaries, but it means that one of the biggest and most critical users of the LLVM backend is neglecting it. It also bifurcates development resources for GHC. Optimization work is duplicated and already devs are getting into the uncomfortable position of suggesting to users that they should trust GHC to build your programs in a particular way, but not itself. 4. GCC/ASM-built GHC, static libs - worst of all possible worlds.
Because of this, the libgmp and llvm-backend issues aren’t entirely orthogonal. Trac ticket #7885 is exactly the issue I get when trying to compile #1.
*From:* Carter Schonwald <carter.schonwald@gmail.com> *Sent:* Monday, December 30, 2013 1:05 PM *To:* Aaron Friel <aaron@frieltek.com>
Good question but you forgot to email the mailing list too :-)
Using llvm has nothing to do with Gmp. Use the native code gen (it's simper) and integer-simple.
That said, standard ghc dylinks to a system copy of Gmp anyways (I think ). Building ghc as a Dylib is orthogonal.
-Carter
On Dec 30, 2013, at 1:58 PM, Aaron Friel <aaron@frieltek.com> wrote:
Excellent research - I’m curious if this is the right thread to inquire about the status of trying to link GHC itself dynamically.
I’ve been attempting to do so with various LLVM versions (3.2, 3.3, 3.4) using snapshot builds of GHC (within the past week) from git, and I hit ticket #7885 [https://ghc.haskell.org/trac/ghc/ticket/7885] every time (even the exact same error message).
I’m interested in dynamically linking GHC with LLVM to avoid the entanglement with libgmp’s license.
If this is the wrong thread or if I should reply instead to the trac item, please let me know.
*From:* Carter Schonwald <carter.schonwald@gmail.com> *Sent:* Friday, December 27, 2013 2:41 PM *To:* Ben Gamari <bgamari.foss@gmail.com> *Cc:* ghc-devs@haskell.org
great work! :)
On Fri, Dec 27, 2013 at 3:21 PM, Ben Gamari <bgamari.foss@gmail.com>wrote:
Simon Marlow <marlowsd@gmail.com> writes:
This sounds right to me. Did you submit a patch?
Note that dynamic linking with LLVM is likely to produce significantly worse code that with the NCG right now, because the LLVM back end uses dynamic references even for symbols in the same package, whereas the NCG back-end uses direct static references for these.
Today with the help of Edward Yang I examined the code produced by the LLVM backend in light of this statement. I was surprised to find that LLVM's code appears to be no worse than the NCG with respect to intra-package references.
My test case can be found here[2] and can be built with the included `build.sh` script. The test consists of two modules build into a shared library. One module, `LibTest`, exports a few simple members while the other module (`LibTest2`) defines members that consume them. Care is taken to ensure the members are not inlined.
The tests were done on x86_64 running LLVM 3.4 and GHC HEAD with the patches[1] I referred to in my last message. Please let me know if I've missed something.
# Evaluation
## First example ##
The first member is a simple `String` (defined in `LibTest`),
helloWorld :: String helloWorld = "Hello World!"
The use-site is quite straightforward,
testHelloWorld :: IO String testHelloWorld = return helloWorld
With `-O1` the code looks reasonable in both cases. Most importantly, both backends use IP relative addressing to find the symbol.
### LLVM ###
0000000000000ef8 <rKw_info>: ef8: 48 8b 45 00 mov 0x0(%rbp),%rax efc: 48 8d 1d cd 11 20 00 lea 0x2011cd(%rip),%rbx # 2020d0 <libtestzm0zi1zi0zi0_LibTest_helloWorld_closure> f03: ff e0 jmpq *%rax
0000000000000f28 <libtestzm0zi1zi0zi0_LibTest2_testHelloWorld_info>: f28: eb ce jmp ef8 <rKw_info> f2a: 66 0f 1f 44 00 00 nopw 0x0(%rax,%rax,1)
### NCG ###
0000000000000d58 <rH1_info>: d58: 48 8d 1d 71 13 20 00 lea 0x201371(%rip),%rbx # 2020d0 <libtestzm0zi1zi0zi0_LibTest_helloWorld_closure> d5f: ff 65 00 jmpq *0x0(%rbp)
0000000000000d88 <libtestzm0zi1zi0zi0_LibTest2_testHelloWorld_info>: d88: eb ce jmp d58 <rH1_info>
With `-O0` the code is substantially longer but the relocation behavior is still correct, as one would expect.
Looking at the definition of `helloWorld`[3] itself it becomes clear that the LLVM backend is more likely to use PLT relocations over GOT. In general, `stg_*` primitives are called through the PLT. As far as I can tell, both of these call mechanisms will incur two memory accesses. However, in the case of the PLT the call will consist of two JMPs whereas the GOT will consist of only one. Is this a cause for concern? Could these two jumps interfere with prediction?
In general the LLVM backend produces a few more instructions than the NCG although this doesn't appear to be related to handling of relocations. For instance, the inexplicable (to me) `mov` at the beginning of LLVM's `rKw_info`.
## Second example ##
The second example demonstrates an actual call,
-- Definition (in LibTest) infoRef :: Int -> Int infoRef n = n + 1
-- Call site testInfoRef :: IO Int testInfoRef = return (infoRef 2)
With `-O1` this produces the following code,
### LLVM ###
0000000000000fb0 <rLy_info>: fb0: 48 8b 45 00 mov 0x0(%rbp),%rax fb4: 48 8d 1d a5 10 20 00 lea 0x2010a5(%rip),%rbx # 202060 <rLx_closure> fbb: ff e0 jmpq *%rax
0000000000000fe0 <libtestzm0zi1zi0zi0_LibTest2_testInfoRef_info>: fe0: eb ce jmp fb0 <rLy_info>
### NCG ###
0000000000000e10 <rI3_info>: e10: 48 8d 1d 51 12 20 00 lea 0x201251(%rip),%rbx # 202068 <rI2_closure> e17: ff 65 00 jmpq *0x0(%rbp)
0000000000000e40 <libtestzm0zi1zi0zi0_LibTest2_testInfoRef_info>: e40: eb ce jmp e10 <rI3_info>
Again, it seems that LLVM is a bit more verbose but seems to handle intra-package calls efficiently.
[1] https://github.com/bgamari/ghc/commits/llvm-dynamic [2] https://github.com/bgamari/ghc-linking-tests/tree/master/ghc-test [3] `helloWorld` definitions:
LLVM: 00000000000010a8 <libtestzm0zi1zi0zi0_LibTest_helloWorld_info>: 10a8: 50 push %rax 10a9: 4c 8d 75 f0 lea -0x10(%rbp),%r14 10ad: 4d 39 fe cmp %r15,%r14 10b0: 73 07 jae 10b9 <libtestzm0zi1zi0zi0_LibTest_helloWorld_info+0x11> 10b2: 49 8b 45 f0 mov -0x10(%r13),%rax 10b6: 5a pop %rdx 10b7: ff e0 jmpq *%rax 10b9: 4c 89 ef mov %r13,%rdi 10bc: 48 89 de mov %rbx,%rsi 10bf: e8 0c fd ff ff callq dd0 <newCAF@plt> 10c4: 48 85 c0 test %rax,%rax 10c7: 74 22 je 10eb <libtestzm0zi1zi0zi0_LibTest_helloWorld_info+0x43> 10c9: 48 8b 0d 18 0f 20 00 mov 0x200f18(%rip),%rcx # 201fe8 <_DYNAMIC+0x228> 10d0: 48 89 4d f0 mov %rcx,-0x10(%rbp) 10d4: 48 89 45 f8 mov %rax,-0x8(%rbp) 10d8: 48 8d 05 21 00 00 00 lea 0x21(%rip),%rax # 1100 <cJC_str> 10df: 4c 89 f5 mov %r14,%rbp 10e2: 49 89 c6 mov %rax,%r14 10e5: 58 pop %rax 10e6: e9 b5 fc ff ff jmpq da0 <ghczmprim_GHCziCString_unpackCStringzh_info@plt> 10eb: 48 8b 03 mov (%rbx),%rax 10ee: 5a pop %rdx 10ef: ff e0 jmpq *%rax
NCG:
0000000000000ef8 <libtestzm0zi1zi0zi0_LibTest_helloWorld_info>: ef8: 48 8d 45 f0 lea -0x10(%rbp),%rax efc: 4c 39 f8 cmp %r15,%rax eff: 72 3f jb f40 <libtestzm0zi1zi0zi0_LibTest_helloWorld_info+0x48> f01: 4c 89 ef mov %r13,%rdi f04: 48 89 de mov %rbx,%rsi f07: 48 83 ec 08 sub $0x8,%rsp f0b: b8 00 00 00 00 mov $0x0,%eax f10: e8 1b fd ff ff callq c30 <newCAF@plt> f15: 48 83 c4 08 add $0x8,%rsp f19: 48 85 c0 test %rax,%rax f1c: 74 20 je f3e <libtestzm0zi1zi0zi0_LibTest_helloWorld_info+0x46> f1e: 48 8b 1d cb 10 20 00 mov 0x2010cb(%rip),%rbx # 201ff0 <_DYNAMIC+0x238> f25: 48 89 5d f0 mov %rbx,-0x10(%rbp) f29: 48 89 45 f8 mov %rax,-0x8(%rbp) f2d: 4c 8d 35 1c 00 00 00 lea 0x1c(%rip),%r14 # f50 <cGG_str> f34: 48 83 c5 f0 add $0xfffffffffffffff0,%rbp f38: ff 25 7a 10 20 00 jmpq *0x20107a(%rip) # 201fb8 <_DYNAMIC+0x200> f3e: ff 23 jmpq *(%rbx) f40: 41 ff 65 f0 jmpq *-0x10(%r13)
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Because I think it’s going to be an organizational issue and a duplication of effort if GHC is built one way but the future direction of LLVM is another. Imagine if GCC started developing a new engine and it didn’t work with one of the biggest, most regular consumers of GCC. Say, the Linux kernel, or itself. At first, the situation is optimistic - if this engine doesn’t work for the project that has the smartest, brightest GCC hackers potentially looking at it, then it should fix itself soon enough. Suppose the situation lingers though, and continues for months without fix. The new GCC backend starts to become the default, and the community around GCC advocates for end-users to use it to optimize code for their projects and it even becomes the default for some platforms, such as ARM. What I’ve described is analogous to the GHC situation - and the result is that GHC isn’t self-hosting on some platforms and the inertia that used to be behind the LLVM backend seems to have stagnated. Whereas LLVM used to be the “new hotness”, I’ve noticed that issues like Trac #7787 no longer have a lot of eyes on them and externally it seems like GHC has accepted a bifurcated approach for development. I dramatize the situation above, but there’s some truth to it. The LLVM backend needs some care and attention and if the majority of GHC devs can’t build GHC with LLVM, then that means the smartest, brightest GHC hackers won’t have their attention turned toward fixing those problems. If a patch to GHC-HEAD broke compilation for every backend, it would be fixed in short order. If a new version of GCC did not work with GHC, I can imagine it would be only hours before the first patches came in resolving the issue. On OS X Mavericks, an incompatibility with GHC has led to a swift reaction and strong support for resolving platform issues. The attention to the LLVM backend is visibly smaller, but I don’t know enough about the people working on GHC to know if it is actually smaller. The way I am trying to change this is by making it easier for people to start using GHC (by putting images on Docker.io) and, in the process, learning about GHC’s build process and trying to make things work for my own projects. The Docker image allows anyone with a Linux kernel to build and play with GHC HEAD. The information about building GHC yourself is difficult to approach and I found it hard to get started, and I want to improve that too, so I’m learning and asking questions. From: Carter Schonwald<mailto:carter.schonwald@gmail.com> Sent: Wednesday, January 1, 2014 5:54 PM To: Aaron Friel<mailto:aaron@frieltek.com> Cc: ghc-devs@haskell.org<mailto:ghc-devs@haskell.org> 7.8 should have working dylib support on the llvm backend. (i believe some of the relevant patches are in head already, though Ben Gamari can opine on that) why do you want ghc to be built with llvm? (i know i've tried myself in the past, and it should be doable with 7.8 using 7.8 soon too) On Wed, Jan 1, 2014 at 5:38 PM, Aaron Friel <aaron@frieltek.com<mailto:aaron@frieltek.com>> wrote: Replying to include the email list. You’re right, the llvm backend and the gmp licensing issues are orthogonal - or should be. The problem is I get build errors when trying to build GHC with LLVM and dynamic libraries. The result is that I get a few different choices when producing a platform image for development, with some uncomfortable tradeoffs: 1. LLVM-built GHC, dynamic libs - doesn’t build. 2. LLVM-built GHC, static libs - potential licensing oddities with me shipping a statically linked ghc binary that is now gpled. I am not a lawyer, but the situation makes me uncomfortable. 3. GCC/ASM-built GHC, dynamic libs - this is the *standard* for most platforms shipping ghc binaries, but it means that one of the biggest and most critical users of the LLVM backend is neglecting it. It also bifurcates development resources for GHC. Optimization work is duplicated and already devs are getting into the uncomfortable position of suggesting to users that they should trust GHC to build your programs in a particular way, but not itself. 4. GCC/ASM-built GHC, static libs - worst of all possible worlds. Because of this, the libgmp and llvm-backend issues aren’t entirely orthogonal. Trac ticket #7885 is exactly the issue I get when trying to compile #1. From: Carter Schonwald<mailto:carter.schonwald@gmail.com> Sent: Monday, December 30, 2013 1:05 PM To: Aaron Friel<mailto:aaron@frieltek.com> Good question but you forgot to email the mailing list too :-) Using llvm has nothing to do with Gmp. Use the native code gen (it's simper) and integer-simple. That said, standard ghc dylinks to a system copy of Gmp anyways (I think ). Building ghc as a Dylib is orthogonal. -Carter On Dec 30, 2013, at 1:58 PM, Aaron Friel <aaron@frieltek.com<mailto:aaron@frieltek.com>> wrote: Excellent research - I’m curious if this is the right thread to inquire about the status of trying to link GHC itself dynamically. I’ve been attempting to do so with various LLVM versions (3.2, 3.3, 3.4) using snapshot builds of GHC (within the past week) from git, and I hit ticket #7885 [https://ghc.haskell.org/trac/ghc/ticket/7885] every time (even the exact same error message). I’m interested in dynamically linking GHC with LLVM to avoid the entanglement with libgmp’s license. If this is the wrong thread or if I should reply instead to the trac item, please let me know.
well, please feel welcome to ask for help as much as you need! To repeat: if you use ghc HEAD, it should be doable to build GHC head (using head as the bootstrap compiler) using LLVM. Once Ben's llvm dy linking patches land, you should be able to do both dynamic and static linking with llvm. As for your Mavericks example, if you review ghc trac and the mailing lists plus irc logs, it took the effort of several folks spread over several months to make sure that once Mavericks / Xcode 5 landed, that it would be "easy" to fix. that said, theres no need to take such a polarizing tone, with speculations about the priorities of the various GHC devs. We're all volunteers (ok, theres a some who are paid volunteers) who care about making sure ghc works as well as possible for everyone, but have finite time in the day, and so many different ways to ghc can be made better. (and in many cases, have a day job that also needs attention too). please test things and holler when they don't work, and if you can debug problems and cook up good patches, great! in the case of llvm and dynamic linking, the root cause was actually pretty darn subtle, and I'm immensely grateful that Ben Gamari got to the root of it. (I'd definitely hit the problem myself, and I was absolutely stumped when I tried to investigate it.) On Wed, Jan 1, 2014 at 10:03 PM, Aaron Friel <aaron@frieltek.com> wrote:
Because I think it’s going to be an organizational issue and a duplication of effort if GHC is built one way but the future direction of LLVM is another.
Imagine if GCC started developing a new engine and it didn’t work with one of the biggest, most regular consumers of GCC. Say, the Linux kernel, or itself. At first, the situation is optimistic - if this engine doesn’t work for the project that has the smartest, brightest GCC hackers potentially looking at it, then it should fix itself soon enough. Suppose the situation lingers though, and continues for months without fix. The new GCC backend starts to become the default, and the community around GCC advocates for end-users to use it to optimize code for their projects and it even becomes the default for some platforms, such as ARM.
What I’ve described is analogous to the GHC situation - and the result is that GHC isn’t self-hosting on some platforms and the inertia that used to be behind the LLVM backend seems to have stagnated. Whereas LLVM used to be the “new hotness”, I’ve noticed that issues like Trac #7787 no longer have a lot of eyes on them and externally it seems like GHC has accepted a bifurcated approach for development.
I dramatize the situation above, but there’s some truth to it. The LLVM backend needs some care and attention and if the majority of GHC devs can’t build GHC with LLVM, then that means the smartest, brightest GHC hackers won’t have their attention turned toward fixing those problems. If a patch to GHC-HEAD broke compilation for every backend, it would be fixed in short order. If a new version of GCC did not work with GHC, I can imagine it would be only hours before the first patches came in resolving the issue. On OS X Mavericks, an incompatibility with GHC has led to a swift reaction and strong support for resolving platform issues. The attention to the LLVM backend is visibly smaller, but I don’t know enough about the people working on GHC to know if it is actually smaller.
The way I am trying to change this is by making it easier for people to start using GHC (by putting images on Docker.io) and, in the process, learning about GHC’s build process and trying to make things work for my own projects. The Docker image allows anyone with a Linux kernel to build and play with GHC HEAD. The information about building GHC yourself is difficult to approach and I found it hard to get started, and I want to improve that too, so I’m learning and asking questions.
*From:* Carter Schonwald <carter.schonwald@gmail.com> *Sent:* Wednesday, January 1, 2014 5:54 PM *To:* Aaron Friel <aaron@frieltek.com> *Cc:* ghc-devs@haskell.org
7.8 should have working dylib support on the llvm backend. (i believe some of the relevant patches are in head already, though Ben Gamari can opine on that)
why do you want ghc to be built with llvm? (i know i've tried myself in the past, and it should be doable with 7.8 using 7.8 soon too)
On Wed, Jan 1, 2014 at 5:38 PM, Aaron Friel <aaron@frieltek.com> wrote:
Replying to include the email list. You’re right, the llvm backend and the gmp licensing issues are orthogonal - or should be. The problem is I get build errors when trying to build GHC with LLVM and dynamic libraries.
The result is that I get a few different choices when producing a platform image for development, with some uncomfortable tradeoffs:
1. LLVM-built GHC, dynamic libs - doesn’t build. 2. LLVM-built GHC, static libs - potential licensing oddities with me shipping a statically linked ghc binary that is now gpled. I am not a lawyer, but the situation makes me uncomfortable. 3. GCC/ASM-built GHC, dynamic libs - this is the *standard* for most platforms shipping ghc binaries, but it means that one of the biggest and most critical users of the LLVM backend is neglecting it. It also bifurcates development resources for GHC. Optimization work is duplicated and already devs are getting into the uncomfortable position of suggesting to users that they should trust GHC to build your programs in a particular way, but not itself. 4. GCC/ASM-built GHC, static libs - worst of all possible worlds.
Because of this, the libgmp and llvm-backend issues aren’t entirely orthogonal. Trac ticket #7885 is exactly the issue I get when trying to compile #1.
*From:* Carter Schonwald <carter.schonwald@gmail.com> *Sent:* Monday, December 30, 2013 1:05 PM *To:* Aaron Friel <aaron@frieltek.com>
Good question but you forgot to email the mailing list too :-)
Using llvm has nothing to do with Gmp. Use the native code gen (it's simper) and integer-simple.
That said, standard ghc dylinks to a system copy of Gmp anyways (I think ). Building ghc as a Dylib is orthogonal.
-Carter
On Dec 30, 2013, at 1:58 PM, Aaron Friel <aaron@frieltek.com> wrote:
Excellent research - I’m curious if this is the right thread to inquire about the status of trying to link GHC itself dynamically.
I’ve been attempting to do so with various LLVM versions (3.2, 3.3, 3.4) using snapshot builds of GHC (within the past week) from git, and I hit ticket #7885 [https://ghc.haskell.org/trac/ghc/ticket/7885] every time (even the exact same error message).
I’m interested in dynamically linking GHC with LLVM to avoid the entanglement with libgmp’s license.
If this is the wrong thread or if I should reply instead to the trac item, please let me know.
I eagerly look forward to these patches, I hope they are able to land in time for the 7.8 release as well. Do you have any additional information on them - or is it part of a branch I could look at? And I apologize for the polarizing tone - I’m overdramatizing the situation and I’m new to following GHC at the root (or head, whichever). Regardless, the LLVM dynamic linking issue has popped up now and again (there are a good number of trac issues) and I’m eager to see that GHC is able to be built properly with it and have it stay working. I have no doubt the issues Ben and others have been working with are subtle and complex. There are absolutely brilliant people here working on GHC, so any problem left unsolved is bound to be uniquely difficult. From: Carter Schonwald<mailto:carter.schonwald@gmail.com> Sent: Wednesday, January 1, 2014 9:53 PM To: Aaron Friel<mailto:aaron@frieltek.com> Cc: ghc-devs@haskell.org<mailto:ghc-devs@haskell.org> well, please feel welcome to ask for help as much as you need! To repeat: if you use ghc HEAD, it should be doable to build GHC head (using head as the bootstrap compiler) using LLVM. Once Ben's llvm dy linking patches land, you should be able to do both dynamic and static linking with llvm. As for your Mavericks example, if you review ghc trac and the mailing lists plus irc logs, it took the effort of several folks spread over several months to make sure that once Mavericks / Xcode 5 landed, that it would be "easy" to fix. that said, theres no need to take such a polarizing tone, with speculations about the priorities of the various GHC devs. We're all volunteers (ok, theres a some who are paid volunteers) who care about making sure ghc works as well as possible for everyone, but have finite time in the day, and so many different ways to ghc can be made better. (and in many cases, have a day job that also needs attention too). please test things and holler when they don't work, and if you can debug problems and cook up good patches, great! in the case of llvm and dynamic linking, the root cause was actually pretty darn subtle, and I'm immensely grateful that Ben Gamari got to the root of it. (I'd definitely hit the problem myself, and I was absolutely stumped when I tried to investigate it.) On Wed, Jan 1, 2014 at 10:03 PM, Aaron Friel <aaron@frieltek.com<mailto:aaron@frieltek.com>> wrote: Because I think it’s going to be an organizational issue and a duplication of effort if GHC is built one way but the future direction of LLVM is another. Imagine if GCC started developing a new engine and it didn’t work with one of the biggest, most regular consumers of GCC. Say, the Linux kernel, or itself. At first, the situation is optimistic - if this engine doesn’t work for the project that has the smartest, brightest GCC hackers potentially looking at it, then it should fix itself soon enough. Suppose the situation lingers though, and continues for months without fix. The new GCC backend starts to become the default, and the community around GCC advocates for end-users to use it to optimize code for their projects and it even becomes the default for some platforms, such as ARM. What I’ve described is analogous to the GHC situation - and the result is that GHC isn’t self-hosting on some platforms and the inertia that used to be behind the LLVM backend seems to have stagnated. Whereas LLVM used to be the “new hotness”, I’ve noticed that issues like Trac #7787 no longer have a lot of eyes on them and externally it seems like GHC has accepted a bifurcated approach for development. I dramatize the situation above, but there’s some truth to it. The LLVM backend needs some care and attention and if the majority of GHC devs can’t build GHC with LLVM, then that means the smartest, brightest GHC hackers won’t have their attention turned toward fixing those problems. If a patch to GHC-HEAD broke compilation for every backend, it would be fixed in short order. If a new version of GCC did not work with GHC, I can imagine it would be only hours before the first patches came in resolving the issue. On OS X Mavericks, an incompatibility with GHC has led to a swift reaction and strong support for resolving platform issues. The attention to the LLVM backend is visibly smaller, but I don’t know enough about the people working on GHC to know if it is actually smaller. The way I am trying to change this is by making it easier for people to start using GHC (by putting images on Docker.io) and, in the process, learning about GHC’s build process and trying to make things work for my own projects. The Docker image allows anyone with a Linux kernel to build and play with GHC HEAD. The information about building GHC yourself is difficult to approach and I found it hard to get started, and I want to improve that too, so I’m learning and asking questions. From: Carter Schonwald<mailto:carter.schonwald@gmail.com> Sent: Wednesday, January 1, 2014 5:54 PM To: Aaron Friel<mailto:aaron@frieltek.com> Cc: ghc-devs@haskell.org<mailto:ghc-devs@haskell.org> 7.8 should have working dylib support on the llvm backend. (i believe some of the relevant patches are in head already, though Ben Gamari can opine on that) why do you want ghc to be built with llvm? (i know i've tried myself in the past, and it should be doable with 7.8 using 7.8 soon too) On Wed, Jan 1, 2014 at 5:38 PM, Aaron Friel <aaron@frieltek.com<mailto:aaron@frieltek.com>> wrote: Replying to include the email list. You’re right, the llvm backend and the gmp licensing issues are orthogonal - or should be. The problem is I get build errors when trying to build GHC with LLVM and dynamic libraries. The result is that I get a few different choices when producing a platform image for development, with some uncomfortable tradeoffs: 1. LLVM-built GHC, dynamic libs - doesn’t build. 2. LLVM-built GHC, static libs - potential licensing oddities with me shipping a statically linked ghc binary that is now gpled. I am not a lawyer, but the situation makes me uncomfortable. 3. GCC/ASM-built GHC, dynamic libs - this is the *standard* for most platforms shipping ghc binaries, but it means that one of the biggest and most critical users of the LLVM backend is neglecting it. It also bifurcates development resources for GHC. Optimization work is duplicated and already devs are getting into the uncomfortable position of suggesting to users that they should trust GHC to build your programs in a particular way, but not itself. 4. GCC/ASM-built GHC, static libs - worst of all possible worlds. Because of this, the libgmp and llvm-backend issues aren’t entirely orthogonal. Trac ticket #7885 is exactly the issue I get when trying to compile #1. From: Carter Schonwald<mailto:carter.schonwald@gmail.com> Sent: Monday, December 30, 2013 1:05 PM To: Aaron Friel<mailto:aaron@frieltek.com> Good question but you forgot to email the mailing list too :-) Using llvm has nothing to do with Gmp. Use the native code gen (it's simper) and integer-simple. That said, standard ghc dylinks to a system copy of Gmp anyways (I think ). Building ghc as a Dylib is orthogonal. -Carter On Dec 30, 2013, at 1:58 PM, Aaron Friel <aaron@frieltek.com<mailto:aaron@frieltek.com>> wrote: Excellent research - I’m curious if this is the right thread to inquire about the status of trying to link GHC itself dynamically. I’ve been attempting to do so with various LLVM versions (3.2, 3.3, 3.4) using snapshot builds of GHC (within the past week) from git, and I hit ticket #7885 [https://ghc.haskell.org/trac/ghc/ticket/7885] every time (even the exact same error message). I’m interested in dynamically linking GHC with LLVM to avoid the entanglement with libgmp’s license. If this is the wrong thread or if I should reply instead to the trac item, please let me know.
you can try it out yourself pretty easily, linked from the master ticket on this https://ghc.haskell.org/trac/ghc/ticket/4210#comment:27 bens ghc repo is at https://github.com/bgamari/ghc/compare/llvm-intra-package (nb: its a work in progress of his) On Thu, Jan 2, 2014 at 2:31 AM, Aaron Friel <aaron@frieltek.com> wrote:
I eagerly look forward to these patches, I hope they are able to land in time for the 7.8 release as well. Do you have any additional information on them - or is it part of a branch I could look at?
And I apologize for the polarizing tone - I’m overdramatizing the situation and I’m new to following GHC at the root (or head, whichever). Regardless, the LLVM dynamic linking issue has popped up now and again (there are a good number of trac issues) and I’m eager to see that GHC is able to be built properly with it and have it stay working.
I have no doubt the issues Ben and others have been working with are subtle and complex. There are absolutely brilliant people here working on GHC, so any problem left unsolved is bound to be uniquely difficult.
*From:* Carter Schonwald <carter.schonwald@gmail.com> *Sent:* Wednesday, January 1, 2014 9:53 PM
*To:* Aaron Friel <aaron@frieltek.com> *Cc:* ghc-devs@haskell.org
well, please feel welcome to ask for help as much as you need! To repeat: if you use ghc HEAD, it should be doable to build GHC head (using head as the bootstrap compiler) using LLVM. Once Ben's llvm dy linking patches land, you should be able to do both dynamic and static linking with llvm.
As for your Mavericks example, if you review ghc trac and the mailing lists plus irc logs, it took the effort of several folks spread over several months to make sure that once Mavericks / Xcode 5 landed, that it would be "easy" to fix.
that said, theres no need to take such a polarizing tone, with speculations about the priorities of the various GHC devs. We're all volunteers (ok, theres a some who are paid volunteers) who care about making sure ghc works as well as possible for everyone, but have finite time in the day, and so many different ways to ghc can be made better. (and in many cases, have a day job that also needs attention too).
please test things and holler when they don't work, and if you can debug problems and cook up good patches, great!
in the case of llvm and dynamic linking, the root cause was actually pretty darn subtle, and I'm immensely grateful that Ben Gamari got to the root of it. (I'd definitely hit the problem myself, and I was absolutely stumped when I tried to investigate it.)
On Wed, Jan 1, 2014 at 10:03 PM, Aaron Friel <aaron@frieltek.com> wrote:
Because I think it’s going to be an organizational issue and a duplication of effort if GHC is built one way but the future direction of LLVM is another.
Imagine if GCC started developing a new engine and it didn’t work with one of the biggest, most regular consumers of GCC. Say, the Linux kernel, or itself. At first, the situation is optimistic - if this engine doesn’t work for the project that has the smartest, brightest GCC hackers potentially looking at it, then it should fix itself soon enough. Suppose the situation lingers though, and continues for months without fix. The new GCC backend starts to become the default, and the community around GCC advocates for end-users to use it to optimize code for their projects and it even becomes the default for some platforms, such as ARM.
What I’ve described is analogous to the GHC situation - and the result is that GHC isn’t self-hosting on some platforms and the inertia that used to be behind the LLVM backend seems to have stagnated. Whereas LLVM used to be the “new hotness”, I’ve noticed that issues like Trac #7787 no longer have a lot of eyes on them and externally it seems like GHC has accepted a bifurcated approach for development.
I dramatize the situation above, but there’s some truth to it. The LLVM backend needs some care and attention and if the majority of GHC devs can’t build GHC with LLVM, then that means the smartest, brightest GHC hackers won’t have their attention turned toward fixing those problems. If a patch to GHC-HEAD broke compilation for every backend, it would be fixed in short order. If a new version of GCC did not work with GHC, I can imagine it would be only hours before the first patches came in resolving the issue. On OS X Mavericks, an incompatibility with GHC has led to a swift reaction and strong support for resolving platform issues. The attention to the LLVM backend is visibly smaller, but I don’t know enough about the people working on GHC to know if it is actually smaller.
The way I am trying to change this is by making it easier for people to start using GHC (by putting images on Docker.io) and, in the process, learning about GHC’s build process and trying to make things work for my own projects. The Docker image allows anyone with a Linux kernel to build and play with GHC HEAD. The information about building GHC yourself is difficult to approach and I found it hard to get started, and I want to improve that too, so I’m learning and asking questions.
*From:* Carter Schonwald <carter.schonwald@gmail.com> *Sent:* Wednesday, January 1, 2014 5:54 PM *To:* Aaron Friel <aaron@frieltek.com> *Cc:* ghc-devs@haskell.org
7.8 should have working dylib support on the llvm backend. (i believe some of the relevant patches are in head already, though Ben Gamari can opine on that)
why do you want ghc to be built with llvm? (i know i've tried myself in the past, and it should be doable with 7.8 using 7.8 soon too)
On Wed, Jan 1, 2014 at 5:38 PM, Aaron Friel <aaron@frieltek.com> wrote:
Replying to include the email list. You’re right, the llvm backend and the gmp licensing issues are orthogonal - or should be. The problem is I get build errors when trying to build GHC with LLVM and dynamic libraries.
The result is that I get a few different choices when producing a platform image for development, with some uncomfortable tradeoffs:
1. LLVM-built GHC, dynamic libs - doesn’t build. 2. LLVM-built GHC, static libs - potential licensing oddities with me shipping a statically linked ghc binary that is now gpled. I am not a lawyer, but the situation makes me uncomfortable. 3. GCC/ASM-built GHC, dynamic libs - this is the *standard* for most platforms shipping ghc binaries, but it means that one of the biggest and most critical users of the LLVM backend is neglecting it. It also bifurcates development resources for GHC. Optimization work is duplicated and already devs are getting into the uncomfortable position of suggesting to users that they should trust GHC to build your programs in a particular way, but not itself. 4. GCC/ASM-built GHC, static libs - worst of all possible worlds.
Because of this, the libgmp and llvm-backend issues aren’t entirely orthogonal. Trac ticket #7885 is exactly the issue I get when trying to compile #1.
*From:* Carter Schonwald <carter.schonwald@gmail.com> *Sent:* Monday, December 30, 2013 1:05 PM *To:* Aaron Friel <aaron@frieltek.com>
Good question but you forgot to email the mailing list too :-)
Using llvm has nothing to do with Gmp. Use the native code gen (it's simper) and integer-simple.
That said, standard ghc dylinks to a system copy of Gmp anyways (I think ). Building ghc as a Dylib is orthogonal.
-Carter
On Dec 30, 2013, at 1:58 PM, Aaron Friel <aaron@frieltek.com> wrote:
Excellent research - I’m curious if this is the right thread to inquire about the status of trying to link GHC itself dynamically.
I’ve been attempting to do so with various LLVM versions (3.2, 3.3, 3.4) using snapshot builds of GHC (within the past week) from git, and I hit ticket #7885 [https://ghc.haskell.org/trac/ghc/ticket/7885] every time (even the exact same error message).
I’m interested in dynamically linking GHC with LLVM to avoid the entanglement with libgmp’s license.
If this is the wrong thread or if I should reply instead to the trac item, please let me know.
Aaron, The LLVM backend needs some care and attention I’m sure you are right about this. Could you become one of the people offering that care and attention. Who are the GHC developers? They are simply volunteers who make time to give something back to their community, and GHC relies absolutely on their commitment and expertise. So do please join in if you can; it’s clearly something you care about, and have some knowledge of. With thanks and best wishes, Simon From: ghc-devs [mailto:ghc-devs-bounces@haskell.org] On Behalf Of Aaron Friel Sent: 02 January 2014 03:03 To: Carter Schonwald Cc: ghc-devs@haskell.org Subject: Re: LLVM and dynamic linking Because I think it’s going to be an organizational issue and a duplication of effort if GHC is built one way but the future direction of LLVM is another. Imagine if GCC started developing a new engine and it didn’t work with one of the biggest, most regular consumers of GCC. Say, the Linux kernel, or itself. At first, the situation is optimistic - if this engine doesn’t work for the project that has the smartest, brightest GCC hackers potentially looking at it, then it should fix itself soon enough. Suppose the situation lingers though, and continues for months without fix. The new GCC backend starts to become the default, and the community around GCC advocates for end-users to use it to optimize code for their projects and it even becomes the default for some platforms, such as ARM. What I’ve described is analogous to the GHC situation - and the result is that GHC isn’t self-hosting on some platforms and the inertia that used to be behind the LLVM backend seems to have stagnated. Whereas LLVM used to be the “new hotness”, I’ve noticed that issues like Trac #7787 no longer have a lot of eyes on them and externally it seems like GHC has accepted a bifurcated approach for development. I dramatize the situation above, but there’s some truth to it. The LLVM backend needs some care and attention and if the majority of GHC devs can’t build GHC with LLVM, then that means the smartest, brightest GHC hackers won’t have their attention turned toward fixing those problems. If a patch to GHC-HEAD broke compilation for every backend, it would be fixed in short order. If a new version of GCC did not work with GHC, I can imagine it would be only hours before the first patches came in resolving the issue. On OS X Mavericks, an incompatibility with GHC has led to a swift reaction and strong support for resolving platform issues. The attention to the LLVM backend is visibly smaller, but I don’t know enough about the people working on GHC to know if it is actually smaller. The way I am trying to change this is by making it easier for people to start using GHC (by putting images on Docker.io) and, in the process, learning about GHC’s build process and trying to make things work for my own projects. The Docker image allows anyone with a Linux kernel to build and play with GHC HEAD. The information about building GHC yourself is difficult to approach and I found it hard to get started, and I want to improve that too, so I’m learning and asking questions. From: Carter Schonwald<mailto:carter.schonwald@gmail.com> Sent: Wednesday, January 1, 2014 5:54 PM To: Aaron Friel<mailto:aaron@frieltek.com> Cc: ghc-devs@haskell.org<mailto:ghc-devs@haskell.org> 7.8 should have working dylib support on the llvm backend. (i believe some of the relevant patches are in head already, though Ben Gamari can opine on that) why do you want ghc to be built with llvm? (i know i've tried myself in the past, and it should be doable with 7.8 using 7.8 soon too) On Wed, Jan 1, 2014 at 5:38 PM, Aaron Friel <aaron@frieltek.com<mailto:aaron@frieltek.com>> wrote: Replying to include the email list. You’re right, the llvm backend and the gmp licensing issues are orthogonal - or should be. The problem is I get build errors when trying to build GHC with LLVM and dynamic libraries. The result is that I get a few different choices when producing a platform image for development, with some uncomfortable tradeoffs: 1. LLVM-built GHC, dynamic libs - doesn’t build. 1. LLVM-built GHC, static libs - potential licensing oddities with me shipping a statically linked ghc binary that is now gpled. I am not a lawyer, but the situation makes me uncomfortable. 1. GCC/ASM-built GHC, dynamic libs - this is the *standard* for most platforms shipping ghc binaries, but it means that one of the biggest and most critical users of the LLVM backend is neglecting it. It also bifurcates development resources for GHC. Optimization work is duplicated and already devs are getting into the uncomfortable position of suggesting to users that they should trust GHC to build your programs in a particular way, but not itself. 1. GCC/ASM-built GHC, static libs - worst of all possible worlds. Because of this, the libgmp and llvm-backend issues aren’t entirely orthogonal. Trac ticket #7885 is exactly the issue I get when trying to compile #1. From: Carter Schonwald<mailto:carter.schonwald@gmail.com> Sent: Monday, December 30, 2013 1:05 PM To: Aaron Friel<mailto:aaron@frieltek.com> Good question but you forgot to email the mailing list too :-) Using llvm has nothing to do with Gmp. Use the native code gen (it's simper) and integer-simple. That said, standard ghc dylinks to a system copy of Gmp anyways (I think ). Building ghc as a Dylib is orthogonal. -Carter On Dec 30, 2013, at 1:58 PM, Aaron Friel <aaron@frieltek.com<mailto:aaron@frieltek.com>> wrote: Excellent research - I’m curious if this is the right thread to inquire about the status of trying to link GHC itself dynamically. I’ve been attempting to do so with various LLVM versions (3.2, 3.3, 3.4) using snapshot builds of GHC (within the past week) from git, and I hit ticket #7885 [https://ghc.haskell.org/trac/ghc/ticket/7885] every time (even the exact same error message). I’m interested in dynamically linking GHC with LLVM to avoid the entanglement with libgmp’s license. If this is the wrong thread or if I should reply instead to the trac item, please let me know.
I am eager to learn and try to work on this :) From: Simon Peyton-Jones<mailto:simonpj@microsoft.com> Sent: Thursday, January 2, 2014 8:17 AM To: Aaron Friel<mailto:aaron@frieltek.com>, Carter Schonwald<mailto:carter.schonwald@gmail.com> Cc: ghc-devs@haskell.org<mailto:ghc-devs@haskell.org> Aaron, The LLVM backend needs some care and attention I’m sure you are right about this. Could you become one of the people offering that care and attention. Who are the GHC developers? They are simply volunteers who make time to give something back to their community, and GHC relies absolutely on their commitment and expertise. So do please join in if you can; it’s clearly something you care about, and have some knowledge of. With thanks and best wishes, Simon From: ghc-devs [mailto:ghc-devs-bounces@haskell.org] On Behalf Of Aaron Friel Sent: 02 January 2014 03:03 To: Carter Schonwald Cc: ghc-devs@haskell.org Subject: Re: LLVM and dynamic linking Because I think it’s going to be an organizational issue and a duplication of effort if GHC is built one way but the future direction of LLVM is another. Imagine if GCC started developing a new engine and it didn’t work with one of the biggest, most regular consumers of GCC. Say, the Linux kernel, or itself. At first, the situation is optimistic - if this engine doesn’t work for the project that has the smartest, brightest GCC hackers potentially looking at it, then it should fix itself soon enough. Suppose the situation lingers though, and continues for months without fix. The new GCC backend starts to become the default, and the community around GCC advocates for end-users to use it to optimize code for their projects and it even becomes the default for some platforms, such as ARM. What I’ve described is analogous to the GHC situation - and the result is that GHC isn’t self-hosting on some platforms and the inertia that used to be behind the LLVM backend seems to have stagnated. Whereas LLVM used to be the “new hotness”, I’ve noticed that issues like Trac #7787 no longer have a lot of eyes on them and externally it seems like GHC has accepted a bifurcated approach for development. I dramatize the situation above, but there’s some truth to it. The LLVM backend needs some care and attention and if the majority of GHC devs can’t build GHC with LLVM, then that means the smartest, brightest GHC hackers won’t have their attention turned toward fixing those problems. If a patch to GHC-HEAD broke compilation for every backend, it would be fixed in short order. If a new version of GCC did not work with GHC, I can imagine it would be only hours before the first patches came in resolving the issue. On OS X Mavericks, an incompatibility with GHC has led to a swift reaction and strong support for resolving platform issues. The attention to the LLVM backend is visibly smaller, but I don’t know enough about the people working on GHC to know if it is actually smaller. The way I am trying to change this is by making it easier for people to start using GHC (by putting images on Docker.io) and, in the process, learning about GHC’s build process and trying to make things work for my own projects. The Docker image allows anyone with a Linux kernel to build and play with GHC HEAD. The information about building GHC yourself is difficult to approach and I found it hard to get started, and I want to improve that too, so I’m learning and asking questions. From: Carter Schonwald<mailto:carter.schonwald@gmail.com> Sent: Wednesday, January 1, 2014 5:54 PM To: Aaron Friel<mailto:aaron@frieltek.com> Cc: ghc-devs@haskell.org<mailto:ghc-devs@haskell.org> 7.8 should have working dylib support on the llvm backend. (i believe some of the relevant patches are in head already, though Ben Gamari can opine on that) why do you want ghc to be built with llvm? (i know i've tried myself in the past, and it should be doable with 7.8 using 7.8 soon too) On Wed, Jan 1, 2014 at 5:38 PM, Aaron Friel <aaron@frieltek.com<mailto:aaron@frieltek.com>> wrote: Replying to include the email list. You’re right, the llvm backend and the gmp licensing issues are orthogonal - or should be. The problem is I get build errors when trying to build GHC with LLVM and dynamic libraries. The result is that I get a few different choices when producing a platform image for development, with some uncomfortable tradeoffs: 1. LLVM-built GHC, dynamic libs - doesn’t build. 1. LLVM-built GHC, static libs - potential licensing oddities with me shipping a statically linked ghc binary that is now gpled. I am not a lawyer, but the situation makes me uncomfortable. 1. GCC/ASM-built GHC, dynamic libs - this is the *standard* for most platforms shipping ghc binaries, but it means that one of the biggest and most critical users of the LLVM backend is neglecting it. It also bifurcates development resources for GHC. Optimization work is duplicated and already devs are getting into the uncomfortable position of suggesting to users that they should trust GHC to build your programs in a particular way, but not itself. 1. GCC/ASM-built GHC, static libs - worst of all possible worlds. Because of this, the libgmp and llvm-backend issues aren’t entirely orthogonal. Trac ticket #7885 is exactly the issue I get when trying to compile #1. From: Carter Schonwald<mailto:carter.schonwald@gmail.com> Sent: Monday, December 30, 2013 1:05 PM To: Aaron Friel<mailto:aaron@frieltek.com> Good question but you forgot to email the mailing list too :-) Using llvm has nothing to do with Gmp. Use the native code gen (it's simper) and integer-simple. That said, standard ghc dylinks to a system copy of Gmp anyways (I think ). Building ghc as a Dylib is orthogonal. -Carter On Dec 30, 2013, at 1:58 PM, Aaron Friel <aaron@frieltek.com<mailto:aaron@frieltek.com>> wrote: Excellent research - I’m curious if this is the right thread to inquire about the status of trying to link GHC itself dynamically. I’ve been attempting to do so with various LLVM versions (3.2, 3.3, 3.4) using snapshot builds of GHC (within the past week) from git, and I hit ticket #7885 [https://ghc.haskell.org/trac/ghc/ticket/7885] every time (even the exact same error message). I’m interested in dynamically linking GHC with LLVM to avoid the entanglement with libgmp’s license. If this is the wrong thread or if I should reply instead to the trac item, please let me know.
Hi all, Apologies for the late reply. First off, one thing to note wrt GMP: GMP is an LGPL library which we link against. Technically, we need to allow relinking to be compliant and free of of the LGPL for our own executables, but this should be reasonably possible - on systems where there is a system-wide GMP installed, we use that copy (this occurs mostly on OSX and Linux.) And so do executables compiled by GHC. Even when GHC uses static linking or dynamic linking for haskell code in this case, it will still always dynamically link to libgmp - meaning replacing the shared object should be possible. This is just the way modern Linux/OSX systems distribute system-wide C libraries, as you expect. In the case where we don't have this, we build our own copy of libgmp inside the source tree and use that instead. That said there are other reasons why we might want to be free of GMP entirely, but that's neither here nor there. In any case, the issue is pretty orthogonal to LLVM, dynamic haskell linking, etc - on a Linux system, you should reasonably be able to swap out a `libgmp.so` for another modified copy[1], and your Haskell programs should be compliant in this regard.[2] Now, as for LLVM. For one, LLVM actually is a 'relatively' cheap backend to have around. I say LLVM is 'relatively' cheap because All External Dependencies Have A Cost. The code is reasonably small, and in any case GHC still does most of the heavy lifting - the LLVM backend and native code generator share a very large amount of code. We don't really duplicate optimizations ourselves, for example, and some optimizations we do perform on our IR can't be done by LLVM anyway (it doesn't have enough information.) But LLVM has some very notable costs for GHC developers: * It's slower to compile with, because it tries to re-optimize the code we give it, but it mostly accomplishes nothing beyond advanced optimizations like vectorization/scalar evolution. * We support a wide range of LLVM versions (a nightmare IMO) which means pinning down specific versions and supporting them all is rather difficult. Combined with e.g. distro maintainers who may patch bugs themselves, and the things you're depending on in the wild (or what users might report bugs with) aren't as solid or well understood. * LLVM is extremely large, extremely complex, and the amount of people who can sensibly work on both GHC and LLVM are few and far inbetween. So fixing these issues is time consuming, difficult, and mostly tedious grunt work. All this basically sums up to the fact that dealing with LLVM comes with complications all on its own that makes it a different kind of beast to handle. So, the LLVM backend definitely needs some love. All of these things are solveable (and I have some ideas for solving most of them,) but none of them will quite come for free. But there are some real improvements that can be made here I think, and make LLVM much more smoothly supported for GHC itself. If you'd like to help it'd be really appreciated - I'd like to see LLVM have more love put forth, but it's a lot of work of course!. (Finally, in reference to the last point: I am in the obvious minority, but I am favorable to having the native code generator around, even if it's a bit old and crufty these days - at least it's small, fast and simple enough to be grokked and hacked on, and I don't think it fragments development all that much. By comparison, LLVM is a mammoth beast of incredible size with a sizeable entry barrier IMO. I think there's merit to having both a simple, 'obviously working' option in addition to the heavy duty one.) [1] Relevant tool: http://nixos.org/patchelf.html [2] Of course, IANAL, but there you go. On Wed, Jan 1, 2014 at 9:03 PM, Aaron Friel <aaron@frieltek.com> wrote:
Because I think it’s going to be an organizational issue and a duplication of effort if GHC is built one way but the future direction of LLVM is another.
Imagine if GCC started developing a new engine and it didn’t work with one of the biggest, most regular consumers of GCC. Say, the Linux kernel, or itself. At first, the situation is optimistic - if this engine doesn’t work for the project that has the smartest, brightest GCC hackers potentially looking at it, then it should fix itself soon enough. Suppose the situation lingers though, and continues for months without fix. The new GCC backend starts to become the default, and the community around GCC advocates for end-users to use it to optimize code for their projects and it even becomes the default for some platforms, such as ARM.
What I’ve described is analogous to the GHC situation - and the result is that GHC isn’t self-hosting on some platforms and the inertia that used to be behind the LLVM backend seems to have stagnated. Whereas LLVM used to be the “new hotness”, I’ve noticed that issues like Trac #7787 no longer have a lot of eyes on them and externally it seems like GHC has accepted a bifurcated approach for development.
I dramatize the situation above, but there’s some truth to it. The LLVM backend needs some care and attention and if the majority of GHC devs can’t build GHC with LLVM, then that means the smartest, brightest GHC hackers won’t have their attention turned toward fixing those problems. If a patch to GHC-HEAD broke compilation for every backend, it would be fixed in short order. If a new version of GCC did not work with GHC, I can imagine it would be only hours before the first patches came in resolving the issue. On OS X Mavericks, an incompatibility with GHC has led to a swift reaction and strong support for resolving platform issues. The attention to the LLVM backend is visibly smaller, but I don’t know enough about the people working on GHC to know if it is actually smaller.
The way I am trying to change this is by making it easier for people to start using GHC (by putting images on Docker.io) and, in the process, learning about GHC’s build process and trying to make things work for my own projects. The Docker image allows anyone with a Linux kernel to build and play with GHC HEAD. The information about building GHC yourself is difficult to approach and I found it hard to get started, and I want to improve that too, so I’m learning and asking questions.
From: Carter Schonwald Sent: Wednesday, January 1, 2014 5:54 PM To: Aaron Friel Cc: ghc-devs@haskell.org
7.8 should have working dylib support on the llvm backend. (i believe some of the relevant patches are in head already, though Ben Gamari can opine on that)
why do you want ghc to be built with llvm? (i know i've tried myself in the past, and it should be doable with 7.8 using 7.8 soon too)
On Wed, Jan 1, 2014 at 5:38 PM, Aaron Friel <aaron@frieltek.com> wrote:
Replying to include the email list. You’re right, the llvm backend and the gmp licensing issues are orthogonal - or should be. The problem is I get build errors when trying to build GHC with LLVM and dynamic libraries.
The result is that I get a few different choices when producing a platform image for development, with some uncomfortable tradeoffs:
LLVM-built GHC, dynamic libs - doesn’t build. LLVM-built GHC, static libs - potential licensing oddities with me shipping a statically linked ghc binary that is now gpled. I am not a lawyer, but the situation makes me uncomfortable. GCC/ASM-built GHC, dynamic libs - this is the *standard* for most platforms shipping ghc binaries, but it means that one of the biggest and most critical users of the LLVM backend is neglecting it. It also bifurcates development resources for GHC. Optimization work is duplicated and already devs are getting into the uncomfortable position of suggesting to users that they should trust GHC to build your programs in a particular way, but not itself. GCC/ASM-built GHC, static libs - worst of all possible worlds.
Because of this, the libgmp and llvm-backend issues aren’t entirely orthogonal. Trac ticket #7885 is exactly the issue I get when trying to compile #1.
From: Carter Schonwald Sent: Monday, December 30, 2013 1:05 PM To: Aaron Friel
Good question but you forgot to email the mailing list too :-)
Using llvm has nothing to do with Gmp. Use the native code gen (it's simper) and integer-simple.
That said, standard ghc dylinks to a system copy of Gmp anyways (I think ). Building ghc as a Dylib is orthogonal.
-Carter
On Dec 30, 2013, at 1:58 PM, Aaron Friel <aaron@frieltek.com> wrote:
Excellent research - I’m curious if this is the right thread to inquire about the status of trying to link GHC itself dynamically.
I’ve been attempting to do so with various LLVM versions (3.2, 3.3, 3.4) using snapshot builds of GHC (within the past week) from git, and I hit ticket #7885 [https://ghc.haskell.org/trac/ghc/ticket/7885] every time (even the exact same error message).
I’m interested in dynamically linking GHC with LLVM to avoid the entanglement with libgmp’s license.
If this is the wrong thread or if I should reply instead to the trac item, please let me know.
_______________________________________________ ghc-devs mailing list ghc-devs@haskell.org http://www.haskell.org/mailman/listinfo/ghc-devs
-- Regards, Austin Seipp, Haskell Consultant Well-Typed LLP, http://www.well-typed.com/
wrt We support a wide range of LLVM versions Why can't we stop doing that and only support one or two, e.g. GHC 7.8 would only support llvm 3.3 and perhaps 3.4? On Tue, Jan 7, 2014 at 4:54 PM, Austin Seipp <austin@well-typed.com> wrote:
Hi all,
Apologies for the late reply.
First off, one thing to note wrt GMP: GMP is an LGPL library which we link against. Technically, we need to allow relinking to be compliant and free of of the LGPL for our own executables, but this should be reasonably possible - on systems where there is a system-wide GMP installed, we use that copy (this occurs mostly on OSX and Linux.) And so do executables compiled by GHC. Even when GHC uses static linking or dynamic linking for haskell code in this case, it will still always dynamically link to libgmp - meaning replacing the shared object should be possible. This is just the way modern Linux/OSX systems distribute system-wide C libraries, as you expect.
In the case where we don't have this, we build our own copy of libgmp inside the source tree and use that instead. That said there are other reasons why we might want to be free of GMP entirely, but that's neither here nor there. In any case, the issue is pretty orthogonal to LLVM, dynamic haskell linking, etc - on a Linux system, you should reasonably be able to swap out a `libgmp.so` for another modified copy[1], and your Haskell programs should be compliant in this regard.[2]
Now, as for LLVM.
For one, LLVM actually is a 'relatively' cheap backend to have around. I say LLVM is 'relatively' cheap because All External Dependencies Have A Cost. The code is reasonably small, and in any case GHC still does most of the heavy lifting - the LLVM backend and native code generator share a very large amount of code. We don't really duplicate optimizations ourselves, for example, and some optimizations we do perform on our IR can't be done by LLVM anyway (it doesn't have enough information.)
But LLVM has some very notable costs for GHC developers:
* It's slower to compile with, because it tries to re-optimize the code we give it, but it mostly accomplishes nothing beyond advanced optimizations like vectorization/scalar evolution. * We support a wide range of LLVM versions (a nightmare IMO) which means pinning down specific versions and supporting them all is rather difficult. Combined with e.g. distro maintainers who may patch bugs themselves, and the things you're depending on in the wild (or what users might report bugs with) aren't as solid or well understood. * LLVM is extremely large, extremely complex, and the amount of people who can sensibly work on both GHC and LLVM are few and far inbetween. So fixing these issues is time consuming, difficult, and mostly tedious grunt work.
All this basically sums up to the fact that dealing with LLVM comes with complications all on its own that makes it a different kind of beast to handle.
So, the LLVM backend definitely needs some love. All of these things are solveable (and I have some ideas for solving most of them,) but none of them will quite come for free. But there are some real improvements that can be made here I think, and make LLVM much more smoothly supported for GHC itself. If you'd like to help it'd be really appreciated - I'd like to see LLVM have more love put forth, but it's a lot of work of course!.
(Finally, in reference to the last point: I am in the obvious minority, but I am favorable to having the native code generator around, even if it's a bit old and crufty these days - at least it's small, fast and simple enough to be grokked and hacked on, and I don't think it fragments development all that much. By comparison, LLVM is a mammoth beast of incredible size with a sizeable entry barrier IMO. I think there's merit to having both a simple, 'obviously working' option in addition to the heavy duty one.)
[1] Relevant tool: http://nixos.org/patchelf.html [2] Of course, IANAL, but there you go.
On Wed, Jan 1, 2014 at 9:03 PM, Aaron Friel <aaron@frieltek.com> wrote:
Because I think it’s going to be an organizational issue and a duplication of effort if GHC is built one way but the future direction of LLVM is another.
Imagine if GCC started developing a new engine and it didn’t work with one of the biggest, most regular consumers of GCC. Say, the Linux kernel, or itself. At first, the situation is optimistic - if this engine doesn’t work for the project that has the smartest, brightest GCC hackers potentially looking at it, then it should fix itself soon enough. Suppose the situation lingers though, and continues for months without fix. The new GCC backend starts to become the default, and the community around GCC advocates for end-users to use it to optimize code for their projects and it even becomes the default for some platforms, such as ARM.
What I’ve described is analogous to the GHC situation - and the result is that GHC isn’t self-hosting on some platforms and the inertia that used to be behind the LLVM backend seems to have stagnated. Whereas LLVM used to be the “new hotness”, I’ve noticed that issues like Trac #7787 no longer have a lot of eyes on them and externally it seems like GHC has accepted a bifurcated approach for development.
I dramatize the situation above, but there’s some truth to it. The LLVM backend needs some care and attention and if the majority of GHC devs can’t build GHC with LLVM, then that means the smartest, brightest GHC hackers won’t have their attention turned toward fixing those problems. If a patch to GHC-HEAD broke compilation for every backend, it would be fixed in short order. If a new version of GCC did not work with GHC, I can imagine it would be only hours before the first patches came in resolving the issue. On OS X Mavericks, an incompatibility with GHC has led to a swift reaction and strong support for resolving platform issues. The attention to the LLVM backend is visibly smaller, but I don’t know enough about the people working on GHC to know if it is actually smaller.
The way I am trying to change this is by making it easier for people to start using GHC (by putting images on Docker.io) and, in the process, learning about GHC’s build process and trying to make things work for my own projects. The Docker image allows anyone with a Linux kernel to build and play with GHC HEAD. The information about building GHC yourself is difficult to approach and I found it hard to get started, and I want to improve that too, so I’m learning and asking questions.
From: Carter Schonwald Sent: Wednesday, January 1, 2014 5:54 PM To: Aaron Friel Cc: ghc-devs@haskell.org
7.8 should have working dylib support on the llvm backend. (i believe some of the relevant patches are in head already, though Ben Gamari can opine on that)
why do you want ghc to be built with llvm? (i know i've tried myself in the past, and it should be doable with 7.8 using 7.8 soon too)
On Wed, Jan 1, 2014 at 5:38 PM, Aaron Friel <aaron@frieltek.com> wrote:
Replying to include the email list. You’re right, the llvm backend and
the
gmp licensing issues are orthogonal - or should be. The problem is I get build errors when trying to build GHC with LLVM and dynamic libraries.
The result is that I get a few different choices when producing a platform image for development, with some uncomfortable tradeoffs:
LLVM-built GHC, dynamic libs - doesn’t build. LLVM-built GHC, static libs - potential licensing oddities with me shipping a statically linked ghc binary that is now gpled. I am not a lawyer, but the situation makes me uncomfortable. GCC/ASM-built GHC, dynamic libs - this is the *standard* for most platforms shipping ghc binaries, but it means that one of the biggest and most critical users of the LLVM backend is neglecting it. It also bifurcates development resources for GHC. Optimization work is duplicated and already devs are getting into the uncomfortable position of suggesting to users that they should trust GHC to build your programs in a particular way, but not itself. GCC/ASM-built GHC, static libs - worst of all possible worlds.
Because of this, the libgmp and llvm-backend issues aren’t entirely orthogonal. Trac ticket #7885 is exactly the issue I get when trying to compile #1.
From: Carter Schonwald Sent: Monday, December 30, 2013 1:05 PM To: Aaron Friel
Good question but you forgot to email the mailing list too :-)
Using llvm has nothing to do with Gmp. Use the native code gen (it's simper) and integer-simple.
That said, standard ghc dylinks to a system copy of Gmp anyways (I think ). Building ghc as a Dylib is orthogonal.
-Carter
On Dec 30, 2013, at 1:58 PM, Aaron Friel <aaron@frieltek.com> wrote:
Excellent research - I’m curious if this is the right thread to inquire about the status of trying to link GHC itself dynamically.
I’ve been attempting to do so with various LLVM versions (3.2, 3.3, 3.4) using snapshot builds of GHC (within the past week) from git, and I hit ticket #7885 [https://ghc.haskell.org/trac/ghc/ticket/7885] every time (even the exact same error message).
I’m interested in dynamically linking GHC with LLVM to avoid the entanglement with libgmp’s license.
If this is the wrong thread or if I should reply instead to the trac item, please let me know.
_______________________________________________ ghc-devs mailing list ghc-devs@haskell.org http://www.haskell.org/mailman/listinfo/ghc-devs
-- Regards,
Austin Seipp, Haskell Consultant Well-Typed LLP, http://www.well-typed.com/ _______________________________________________ ghc-devs mailing list ghc-devs@haskell.org http://www.haskell.org/mailman/listinfo/ghc-devs
Personally I'd be in favor of that to keep it easy, but there hasn't really been any poll about what to do. For the most part it tends to work fine, but I think it's the wrong thing to do in any case. IMO the truly 'correct' thing to do, is not to rely on the system LLVM at all, but a version specifically tested with and distributed with GHC. This can be a private binary only we use. We already do this with MinGW on Windows actually, because in practice, relying on versions 'in the wild' is somewhat troublesome. In our case, we really just need the bitcode compiler and optimizer, which are pretty small pieces. Relying on a moving target like the system install or whatever possible random XYZ install from SVN or (some derivative forked toolchain!) is problematic for developers, and users invariably want to try new combinations, which can break in subtle or odd ways. I think it's more sensible and straightforward - for the vast majority of users and use-cases - for us to pick version that is tested, reliably works and optimizes code well, and ship that. Then users just know '-fasm is faster for compiling, -fllvm will optimize better for some code.' That's all they really need to know. If LLVM is to be considered 'stable' for Tier 1 GHC platforms, I'm sympathetic to Aaron's argument, and I'd say it should be held to the same standards as the NCG. That means it should be considered a reliable option and we should vet it to reasonable standards, even if it's a bit more work. It's just really hard to do that right now. But I think implementing this wouldn't be difficult, it just has some sticky bits about how to do it. We can of course upgrade it over time - but I think trying to hit moving targets in the wild is a bad long-term solution. On Tue, Jan 7, 2014 at 3:07 PM, George Colpitts <george.colpitts@gmail.com> wrote:
wrt
We support a wide range of LLVM versions
Why can't we stop doing that and only support one or two, e.g. GHC 7.8 would only support llvm 3.3 and perhaps 3.4?
On Tue, Jan 7, 2014 at 4:54 PM, Austin Seipp <austin@well-typed.com> wrote:
Hi all,
Apologies for the late reply.
First off, one thing to note wrt GMP: GMP is an LGPL library which we link against. Technically, we need to allow relinking to be compliant and free of of the LGPL for our own executables, but this should be reasonably possible - on systems where there is a system-wide GMP installed, we use that copy (this occurs mostly on OSX and Linux.) And so do executables compiled by GHC. Even when GHC uses static linking or dynamic linking for haskell code in this case, it will still always dynamically link to libgmp - meaning replacing the shared object should be possible. This is just the way modern Linux/OSX systems distribute system-wide C libraries, as you expect.
In the case where we don't have this, we build our own copy of libgmp inside the source tree and use that instead. That said there are other reasons why we might want to be free of GMP entirely, but that's neither here nor there. In any case, the issue is pretty orthogonal to LLVM, dynamic haskell linking, etc - on a Linux system, you should reasonably be able to swap out a `libgmp.so` for another modified copy[1], and your Haskell programs should be compliant in this regard.[2]
Now, as for LLVM.
For one, LLVM actually is a 'relatively' cheap backend to have around. I say LLVM is 'relatively' cheap because All External Dependencies Have A Cost. The code is reasonably small, and in any case GHC still does most of the heavy lifting - the LLVM backend and native code generator share a very large amount of code. We don't really duplicate optimizations ourselves, for example, and some optimizations we do perform on our IR can't be done by LLVM anyway (it doesn't have enough information.)
But LLVM has some very notable costs for GHC developers:
* It's slower to compile with, because it tries to re-optimize the code we give it, but it mostly accomplishes nothing beyond advanced optimizations like vectorization/scalar evolution. * We support a wide range of LLVM versions (a nightmare IMO) which means pinning down specific versions and supporting them all is rather difficult. Combined with e.g. distro maintainers who may patch bugs themselves, and the things you're depending on in the wild (or what users might report bugs with) aren't as solid or well understood. * LLVM is extremely large, extremely complex, and the amount of people who can sensibly work on both GHC and LLVM are few and far inbetween. So fixing these issues is time consuming, difficult, and mostly tedious grunt work.
All this basically sums up to the fact that dealing with LLVM comes with complications all on its own that makes it a different kind of beast to handle.
So, the LLVM backend definitely needs some love. All of these things are solveable (and I have some ideas for solving most of them,) but none of them will quite come for free. But there are some real improvements that can be made here I think, and make LLVM much more smoothly supported for GHC itself. If you'd like to help it'd be really appreciated - I'd like to see LLVM have more love put forth, but it's a lot of work of course!.
(Finally, in reference to the last point: I am in the obvious minority, but I am favorable to having the native code generator around, even if it's a bit old and crufty these days - at least it's small, fast and simple enough to be grokked and hacked on, and I don't think it fragments development all that much. By comparison, LLVM is a mammoth beast of incredible size with a sizeable entry barrier IMO. I think there's merit to having both a simple, 'obviously working' option in addition to the heavy duty one.)
[1] Relevant tool: http://nixos.org/patchelf.html [2] Of course, IANAL, but there you go.
On Wed, Jan 1, 2014 at 9:03 PM, Aaron Friel <aaron@frieltek.com> wrote:
Because I think it’s going to be an organizational issue and a duplication of effort if GHC is built one way but the future direction of LLVM is another.
Imagine if GCC started developing a new engine and it didn’t work with one of the biggest, most regular consumers of GCC. Say, the Linux kernel, or itself. At first, the situation is optimistic - if this engine doesn’t work for the project that has the smartest, brightest GCC hackers potentially looking at it, then it should fix itself soon enough. Suppose the situation lingers though, and continues for months without fix. The new GCC backend starts to become the default, and the community around GCC advocates for end-users to use it to optimize code for their projects and it even becomes the default for some platforms, such as ARM.
What I’ve described is analogous to the GHC situation - and the result is that GHC isn’t self-hosting on some platforms and the inertia that used to be behind the LLVM backend seems to have stagnated. Whereas LLVM used to be the “new hotness”, I’ve noticed that issues like Trac #7787 no longer have a lot of eyes on them and externally it seems like GHC has accepted a bifurcated approach for development.
I dramatize the situation above, but there’s some truth to it. The LLVM backend needs some care and attention and if the majority of GHC devs can’t build GHC with LLVM, then that means the smartest, brightest GHC hackers won’t have their attention turned toward fixing those problems. If a patch to GHC-HEAD broke compilation for every backend, it would be fixed in short order. If a new version of GCC did not work with GHC, I can imagine it would be only hours before the first patches came in resolving the issue. On OS X Mavericks, an incompatibility with GHC has led to a swift reaction and strong support for resolving platform issues. The attention to the LLVM backend is visibly smaller, but I don’t know enough about the people working on GHC to know if it is actually smaller.
The way I am trying to change this is by making it easier for people to start using GHC (by putting images on Docker.io) and, in the process, learning about GHC’s build process and trying to make things work for my own projects. The Docker image allows anyone with a Linux kernel to build and play with GHC HEAD. The information about building GHC yourself is difficult to approach and I found it hard to get started, and I want to improve that too, so I’m learning and asking questions.
From: Carter Schonwald Sent: Wednesday, January 1, 2014 5:54 PM
To: Aaron Friel Cc: ghc-devs@haskell.org
7.8 should have working dylib support on the llvm backend. (i believe some of the relevant patches are in head already, though Ben Gamari can opine on that)
why do you want ghc to be built with llvm? (i know i've tried myself in the past, and it should be doable with 7.8 using 7.8 soon too)
On Wed, Jan 1, 2014 at 5:38 PM, Aaron Friel <aaron@frieltek.com> wrote:
Replying to include the email list. You’re right, the llvm backend and the gmp licensing issues are orthogonal - or should be. The problem is I get build errors when trying to build GHC with LLVM and dynamic libraries.
The result is that I get a few different choices when producing a platform image for development, with some uncomfortable tradeoffs:
LLVM-built GHC, dynamic libs - doesn’t build. LLVM-built GHC, static libs - potential licensing oddities with me shipping a statically linked ghc binary that is now gpled. I am not a lawyer, but the situation makes me uncomfortable. GCC/ASM-built GHC, dynamic libs - this is the *standard* for most platforms shipping ghc binaries, but it means that one of the biggest and most critical users of the LLVM backend is neglecting it. It also bifurcates development resources for GHC. Optimization work is duplicated and already devs are getting into the uncomfortable position of suggesting to users that they should trust GHC to build your programs in a particular way, but not itself. GCC/ASM-built GHC, static libs - worst of all possible worlds.
Because of this, the libgmp and llvm-backend issues aren’t entirely orthogonal. Trac ticket #7885 is exactly the issue I get when trying to compile #1.
From: Carter Schonwald Sent: Monday, December 30, 2013 1:05 PM
To: Aaron Friel
Good question but you forgot to email the mailing list too :-)
Using llvm has nothing to do with Gmp. Use the native code gen (it's simper) and integer-simple.
That said, standard ghc dylinks to a system copy of Gmp anyways (I think ). Building ghc as a Dylib is orthogonal.
-Carter
On Dec 30, 2013, at 1:58 PM, Aaron Friel <aaron@frieltek.com> wrote:
Excellent research - I’m curious if this is the right thread to inquire about the status of trying to link GHC itself dynamically.
I’ve been attempting to do so with various LLVM versions (3.2, 3.3, 3.4) using snapshot builds of GHC (within the past week) from git, and I hit ticket #7885 [https://ghc.haskell.org/trac/ghc/ticket/7885] every time (even the exact same error message).
I’m interested in dynamically linking GHC with LLVM to avoid the entanglement with libgmp’s license.
If this is the wrong thread or if I should reply instead to the trac item, please let me know.
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-- Regards,
Austin Seipp, Haskell Consultant Well-Typed LLP, http://www.well-typed.com/ _______________________________________________ ghc-devs mailing list ghc-devs@haskell.org http://www.haskell.org/mailman/listinfo/ghc-devs
-- Regards, Austin Seipp, Haskell Consultant Well-Typed LLP, http://www.well-typed.com/
well said points. Theres a lot we can do, and i think I many of those active in GHC have discussed various ideas to explore in this area for after the ghc 7.8 release. I believe someone did an experiment with llvm-general as an alternative ghc backend a few months back, who was it who did that? (llvm-general only makes sense for stage-2 ghc, but it does provide the advantage of statically linking LLVM as a haskell lib.) On Wed, Jan 8, 2014 at 3:01 AM, Austin Seipp <austin@well-typed.com> wrote:
Personally I'd be in favor of that to keep it easy, but there hasn't really been any poll about what to do. For the most part it tends to work fine, but I think it's the wrong thing to do in any case.
IMO the truly 'correct' thing to do, is not to rely on the system LLVM at all, but a version specifically tested with and distributed with GHC. This can be a private binary only we use. We already do this with MinGW on Windows actually, because in practice, relying on versions 'in the wild' is somewhat troublesome. In our case, we really just need the bitcode compiler and optimizer, which are pretty small pieces.
Relying on a moving target like the system install or whatever possible random XYZ install from SVN or (some derivative forked toolchain!) is problematic for developers, and users invariably want to try new combinations, which can break in subtle or odd ways.
I think it's more sensible and straightforward - for the vast majority of users and use-cases - for us to pick version that is tested, reliably works and optimizes code well, and ship that. Then users just know '-fasm is faster for compiling, -fllvm will optimize better for some code.' That's all they really need to know.
If LLVM is to be considered 'stable' for Tier 1 GHC platforms, I'm sympathetic to Aaron's argument, and I'd say it should be held to the same standards as the NCG. That means it should be considered a reliable option and we should vet it to reasonable standards, even if it's a bit more work.
It's just really hard to do that right now. But I think implementing this wouldn't be difficult, it just has some sticky bits about how to do it.
We can of course upgrade it over time - but I think trying to hit moving targets in the wild is a bad long-term solution.
On Tue, Jan 7, 2014 at 3:07 PM, George Colpitts <george.colpitts@gmail.com> wrote:
wrt
We support a wide range of LLVM versions
Why can't we stop doing that and only support one or two, e.g. GHC 7.8 would only support llvm 3.3 and perhaps 3.4?
On Tue, Jan 7, 2014 at 4:54 PM, Austin Seipp <austin@well-typed.com> wrote:
Hi all,
Apologies for the late reply.
First off, one thing to note wrt GMP: GMP is an LGPL library which we link against. Technically, we need to allow relinking to be compliant and free of of the LGPL for our own executables, but this should be reasonably possible - on systems where there is a system-wide GMP installed, we use that copy (this occurs mostly on OSX and Linux.) And so do executables compiled by GHC. Even when GHC uses static linking or dynamic linking for haskell code in this case, it will still always dynamically link to libgmp - meaning replacing the shared object should be possible. This is just the way modern Linux/OSX systems distribute system-wide C libraries, as you expect.
In the case where we don't have this, we build our own copy of libgmp inside the source tree and use that instead. That said there are other reasons why we might want to be free of GMP entirely, but that's neither here nor there. In any case, the issue is pretty orthogonal to LLVM, dynamic haskell linking, etc - on a Linux system, you should reasonably be able to swap out a `libgmp.so` for another modified copy[1], and your Haskell programs should be compliant in this regard.[2]
Now, as for LLVM.
For one, LLVM actually is a 'relatively' cheap backend to have around. I say LLVM is 'relatively' cheap because All External Dependencies Have A Cost. The code is reasonably small, and in any case GHC still does most of the heavy lifting - the LLVM backend and native code generator share a very large amount of code. We don't really duplicate optimizations ourselves, for example, and some optimizations we do perform on our IR can't be done by LLVM anyway (it doesn't have enough information.)
But LLVM has some very notable costs for GHC developers:
* It's slower to compile with, because it tries to re-optimize the code we give it, but it mostly accomplishes nothing beyond advanced optimizations like vectorization/scalar evolution. * We support a wide range of LLVM versions (a nightmare IMO) which means pinning down specific versions and supporting them all is rather difficult. Combined with e.g. distro maintainers who may patch bugs themselves, and the things you're depending on in the wild (or what users might report bugs with) aren't as solid or well understood. * LLVM is extremely large, extremely complex, and the amount of people who can sensibly work on both GHC and LLVM are few and far inbetween. So fixing these issues is time consuming, difficult, and mostly tedious grunt work.
All this basically sums up to the fact that dealing with LLVM comes with complications all on its own that makes it a different kind of beast to handle.
So, the LLVM backend definitely needs some love. All of these things are solveable (and I have some ideas for solving most of them,) but none of them will quite come for free. But there are some real improvements that can be made here I think, and make LLVM much more smoothly supported for GHC itself. If you'd like to help it'd be really appreciated - I'd like to see LLVM have more love put forth, but it's a lot of work of course!.
(Finally, in reference to the last point: I am in the obvious minority, but I am favorable to having the native code generator around, even if it's a bit old and crufty these days - at least it's small, fast and simple enough to be grokked and hacked on, and I don't think it fragments development all that much. By comparison, LLVM is a mammoth beast of incredible size with a sizeable entry barrier IMO. I think there's merit to having both a simple, 'obviously working' option in addition to the heavy duty one.)
[1] Relevant tool: http://nixos.org/patchelf.html [2] Of course, IANAL, but there you go.
On Wed, Jan 1, 2014 at 9:03 PM, Aaron Friel <aaron@frieltek.com> wrote:
Because I think it’s going to be an organizational issue and a duplication of effort if GHC is built one way but the future direction of LLVM is another.
Imagine if GCC started developing a new engine and it didn’t work with one of the biggest, most regular consumers of GCC. Say, the Linux kernel,
or
itself. At first, the situation is optimistic - if this engine doesn’t work for the project that has the smartest, brightest GCC hackers potentially looking at it, then it should fix itself soon enough. Suppose the situation lingers though, and continues for months without fix. The new GCC backend starts to become the default, and the community around GCC advocates for end-users to use it to optimize code for their projects and it even becomes the default for some platforms, such as ARM.
What I’ve described is analogous to the GHC situation - and the result is that GHC isn’t self-hosting on some platforms and the inertia that used to be behind the LLVM backend seems to have stagnated. Whereas LLVM used to be the “new hotness”, I’ve noticed that issues like Trac #7787 no longer have a lot of eyes on them and externally it seems like GHC has accepted a bifurcated approach for development.
I dramatize the situation above, but there’s some truth to it. The LLVM backend needs some care and attention and if the majority of GHC devs can’t build GHC with LLVM, then that means the smartest, brightest GHC hackers won’t have their attention turned toward fixing those problems. If a patch to GHC-HEAD broke compilation for every backend, it would be fixed in short order. If a new version of GCC did not work with GHC, I can imagine it would be only hours before the first patches came in resolving the issue. On OS X Mavericks, an incompatibility with GHC has led to a swift reaction and strong support for resolving platform issues. The attention to the LLVM backend is visibly smaller, but I don’t know enough about the people working on GHC to know if it is actually smaller.
The way I am trying to change this is by making it easier for people to start using GHC (by putting images on Docker.io) and, in the process, learning about GHC’s build process and trying to make things work for my own projects. The Docker image allows anyone with a Linux kernel to build and play with GHC HEAD. The information about building GHC yourself is difficult to approach and I found it hard to get started, and I want to improve that too, so I’m learning and asking questions.
From: Carter Schonwald Sent: Wednesday, January 1, 2014 5:54 PM
To: Aaron Friel Cc: ghc-devs@haskell.org
7.8 should have working dylib support on the llvm backend. (i believe some of the relevant patches are in head already, though Ben Gamari can opine on that)
why do you want ghc to be built with llvm? (i know i've tried myself in the past, and it should be doable with 7.8 using 7.8 soon too)
On Wed, Jan 1, 2014 at 5:38 PM, Aaron Friel <aaron@frieltek.com> wrote:
Replying to include the email list. You’re right, the llvm backend
and
the gmp licensing issues are orthogonal - or should be. The problem is I get build errors when trying to build GHC with LLVM and dynamic libraries.
The result is that I get a few different choices when producing a platform image for development, with some uncomfortable tradeoffs:
LLVM-built GHC, dynamic libs - doesn’t build. LLVM-built GHC, static libs - potential licensing oddities with me shipping a statically linked ghc binary that is now gpled. I am not a lawyer, but the situation makes me uncomfortable. GCC/ASM-built GHC, dynamic libs - this is the *standard* for most platforms shipping ghc binaries, but it means that one of the biggest and most critical users of the LLVM backend is neglecting it. It also bifurcates development resources for GHC. Optimization work is duplicated and already devs are getting into the uncomfortable position of suggesting to users that they should trust GHC to build your programs in a particular way, but not itself. GCC/ASM-built GHC, static libs - worst of all possible worlds.
Because of this, the libgmp and llvm-backend issues aren’t entirely orthogonal. Trac ticket #7885 is exactly the issue I get when trying to compile #1.
From: Carter Schonwald Sent: Monday, December 30, 2013 1:05 PM
To: Aaron Friel
Good question but you forgot to email the mailing list too :-)
Using llvm has nothing to do with Gmp. Use the native code gen (it's simper) and integer-simple.
That said, standard ghc dylinks to a system copy of Gmp anyways (I think ). Building ghc as a Dylib is orthogonal.
-Carter
On Dec 30, 2013, at 1:58 PM, Aaron Friel <aaron@frieltek.com> wrote:
Excellent research - I’m curious if this is the right thread to inquire about the status of trying to link GHC itself dynamically.
I’ve been attempting to do so with various LLVM versions (3.2, 3.3, 3.4) using snapshot builds of GHC (within the past week) from git, and I hit ticket #7885 [https://ghc.haskell.org/trac/ghc/ticket/7885] every time (even the exact same error message).
I’m interested in dynamically linking GHC with LLVM to avoid the entanglement with libgmp’s license.
If this is the wrong thread or if I should reply instead to the trac item, please let me know.
_______________________________________________ ghc-devs mailing list ghc-devs@haskell.org http://www.haskell.org/mailman/listinfo/ghc-devs
-- Regards,
Austin Seipp, Haskell Consultant Well-Typed LLP, http://www.well-typed.com/ _______________________________________________ ghc-devs mailing list ghc-devs@haskell.org http://www.haskell.org/mailman/listinfo/ghc-devs
-- Regards,
Austin Seipp, Haskell Consultant Well-Typed LLP, http://www.well-typed.com/ _______________________________________________ ghc-devs mailing list ghc-devs@haskell.org http://www.haskell.org/mailman/listinfo/ghc-devs
participants (5)
-
Aaron Friel -
Austin Seipp -
Carter Schonwald -
George Colpitts -
Simon Peyton-Jones