Re: Questions concerning the LLVM backend - i.e. 'proc point splitting'

Hello Simon, hello Ben, thank you very much to both of you for your encouraging answers! First of all, I've two questions concerning your answer, Ben. a)

The jump issue aside, I don't know how you would deal with tables-next-to-code. The prefix data support that currently available in LLVM is attached to functions and I unfortunately don't see that changing any time soon.

Looks like I messed things up with those info-tables. As far as I understood [1], these are part of the STG. Therefore, every function present at the STG-level needs a info-table. The code generator does generate info-tables for every proc-point after splitting them. But to me, this was a consequence of making them stand-alone functions. Could you explain the need of further info-tables for 'inner' proc-points (those, which are not the entry-block of a function) to me, please? b)

That being said, it sounds as though eliminating proc-point splitting would make for quite a nice project in the native code generator.

The documentation states, that proc-point splitting is only required by the LLVM backend (see [2]). Does the cmm-pipeline perform proc-point splitting even when using the native code generator? Second, some background information concerning my situation. As a student assistant, I'm part of a work group at university. They're doing research on loop-optimization (which is the reason why I'm doing stencil calculations in Haskell). The group uses LLVM (which is the reason why I'm using the LLVM backend). Recently I talked to them, trying to explain the reasons for GHC emitting code containing mutual recursion instead of loops under certain circumstances. They told me, that LLVM's cross-procedure optimization fails to merge the mutual recursion into a single loop because of the following. When pushing a continuation defined in the local LLVM module onto the stack and calling into a method defined externally, the analysis can't assure, that control eventually returns to the local continuation. To make control flow more apparent to LLVM, they came up with the following idea. Remark before showing the idea: I know that it greatly interferes with CPS. It leaves parts of the continuation-handling to the implicit LLVM call-stack. Further, this does not solve the problem of proc-point splitting at all. It is more a workaround than a solution. But it would greatly increase LLVM's possibilities to optimize code later on. That's why I found this idea worth mentioning. Especially because after reading your answer, Ben, I'm somewhat convinced that solving the problem of proc-point splitting is nearly impossible with the capabilities of LLVM IR available at the moment. Now back to the idea. As stated above, the major problem of LLVM seems to be inferring control flow when it comes to call proc-points. As it's hard to avoid proc-point splitting, we leave the cmm-pipeline 'as-is'. We modify the code, llvmGen produces when it comes to call proc-points instead. All other types of proc-points remain unchanged. Initial situation in pseudo cmm: foo() { ... call bar() returns to cont // bar defined externally cont: ... } After proc-point splitting, this is converted into LLVM IR similar to the following: @foo() { ... store @cont(), %Sp ; push continuation on the stack tail call @bar() ; @bar defined externally } @cont() { ... } To fix the above issue, we introduce a method, which restores the pointers Sp, Hp and the register R1 (among everything else, what has to be restored) and 'fakes' the continuation. Note, that '@restore' returns without calling into the continuation. This way, the call into the continuation can be made direct. The above code would be transformed into something similar to the following: @foo() { ... store @restore(), %Sp ; 'fake' continuation call @bar() ; br label restore restore: %Sp = load (getelementptr @environment 0, 0) ; restore what has to be ... ; restored tail call @cont() } @cont() { ... } @restore(%Sp, %Hp, %R1, ...) { store %Sp, (getelementptr @environment 0, 0) ; store everything into ... ; global variable ret void } @environment ; structure of type similar to {i64*, i64*, i64*, i64, i64, ..} As an alternative, the above transformation could be done by a LLVM pass. llvmGen could then annotate the continuation of calls call cite. This would greatly simplify the detection of the correct continuation. What do you think about this idea? Especially when thinking about the interference with CPS? Regards, David [1] https://ghc.haskell.org/trac/ghc/wiki/Commentary/Compiler/GeneratedCode#Impo... [2] https://ghc.haskell.org/trac/ghc/wiki/Commentary/Compiler/CodeGen#Secondstag... On 11/21/2015 12:00 AM, Ben Gamari wrote:

Simon Peyton Jones writes:

...

David

All this stuff is terribly paged-out for me. But I'd love someone to pay attention to some of this back-end stuff, so if you'd like to work there, I'd like to help you.

David Terei was also thinking of getting rid of proc point splitting for LLVM (see attached email and the notes attached to it)

Simon Marlow also was thinking about this; see his attached email.

The main *reason* for establishing a proc-point is so that, when generating C code (which we don't do any more) you can take its address. E.g.

foo() { ... Push &bar onto the stack (effectively a return address) Jump to thingumpy }

bar() { ... }

Really bar is part of foo, but we have make it a separate top level thing so that we can take the address of bar and push it on the stack.

The trouble is that if there is a label inside bar that foo wants to jump to (i.e. without pushing &bar etc) then we have to make that label too into a proc-point, so that both foo and bar can see it. Urgh.

In LLVM this probably is not true; presumably you can take the address of any label?

This is true. While labels themselves have function-local scope in LLVM, there is an expression, `blockaddress`, which allows you to take an address to a label in another function [1]. That being said, in reading through the documentation it's not at all clear to me that it would be safe to jump to such an address. In fact, given that the instruction that this expression is supposed to be used with, `indirectbr`, can only be used for local blocks, I suspect it is not. More information about this feature can be found here [2].

The jump issue aside, I don't know how you would deal with tables-next-to-code. The prefix data support that currently available in LLVM is attached to functions and I unfortunately don't see that changing any time soon.

Ultimately it seems that trying to refer to labels defined in other functions is using LLVM against the way it was intended. One alternative would be to teach llvmGen to merge mutually recusive top-level functions into a single LLVM function during code generation. Otherwise I'm afraid you are stuck with either the status quo or attempting to improve on LLVM's own cross-procedure optimization ability.

That being said, it sounds as though eliminating proc-point splitting would make for quite a nice project in the native code generator.

Cheers,

- Ben

[1] http://llvm.org/docs/LangRef.html#addresses-of-basic-blocks [2] http://blog.llvm.org/2010/01/address-of-label-and-indirect-branches.html [3] http://llvm.org/docs/LangRef.html#prefix-data