Re: LLVM and dynamic linking

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 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 *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 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 *Sent:* ‎Friday‎, ‎December‎ ‎27‎, ‎2013 ‎2‎:‎41‎ ‎PM *To:* Ben Gamari *Cc:* ghc-devs@haskell.org

great work! :)

On Fri, Dec 27, 2013 at 3:21 PM, Ben Gamari wrote:

...

Simon Marlow 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 : ef8: 48 8b 45 00 mov 0x0(%rbp),%rax efc: 48 8d 1d cd 11 20 00 lea 0x2011cd(%rip),%rbx # 2020d0 f03: ff e0 jmpq *%rax

0000000000000f28 : f28: eb ce jmp ef8 f2a: 66 0f 1f 44 00 00 nopw 0x0(%rax,%rax,1)

### NCG ###

0000000000000d58 : d58: 48 8d 1d 71 13 20 00 lea 0x201371(%rip),%rbx # 2020d0 d5f: ff 65 00 jmpq *0x0(%rbp)

0000000000000d88 : d88: eb ce jmp d58

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 : fb0: 48 8b 45 00 mov 0x0(%rbp),%rax fb4: 48 8d 1d a5 10 20 00 lea 0x2010a5(%rip),%rbx # 202060 fbb: ff e0 jmpq *%rax

0000000000000fe0 : fe0: eb ce jmp fb0

### NCG ###

0000000000000e10 : e10: 48 8d 1d 51 12 20 00 lea 0x201251(%rip),%rbx # 202068 e17: ff 65 00 jmpq *0x0(%rbp)

0000000000000e40 : e40: eb ce jmp e10

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 : 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 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 10c4: 48 85 c0 test %rax,%rax 10c7: 74 22 je 10eb 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 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 10eb: 48 8b 03 mov (%rbx),%rax 10ee: 5a pop %rdx 10ef: ff e0 jmpq *%rax

NCG:

0000000000000ef8 : ef8: 48 8d 45 f0 lea -0x10(%rbp),%rax efc: 4c 39 f8 cmp %r15,%rax eff: 72 3f jb f40 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 f15: 48 83 c4 08 add $0x8,%rsp f19: 48 85 c0 test %rax,%rax f1c: 74 20 je f3e 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 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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