[llvm-dev] LLVM and heap-allocated thread stacks

[TB Schardl via llvm-dev]

Speaking as a bystander just trying to understand the problem: Are you
looking for a way to ensure that "setHasOpaqueSPAdjustment(true)" is
executed on appropriate MachineFrameInfo objects during CodeGen for GHC and
Go?  Or do you need something more than that?

Cheers,
TB

On Thu, Aug 23, 2018 at 2:08 PM Demi M. Obenour via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> In some language implementations, such as the Glasgow Haskell Compiler
> (GHC) and the reference implementation of Go, a thread’s stack is allocated
> as a data structure on the garbage-collected heap.  The garbage collector
> is free to move this data structure whenever it is invoked.
>
> Currently, GHC’s LLVM backend does not use the C stack.  However, there
> have been discussions about whether using the C stack could lead to a
> performance gain.  I think it could.  The elephant in the room, however, is
> that **any call into the RTS may then change the stack pointer*.*  LLVM
> presumably has no support for this.  Without such support, however, GHC
> must spill all locals to memory at every call into the RTS.  It seems to me
> that this is why GHC cannot transform its output into SSA form: GHC must
> reify its stack.
>
> It would be nice (both for GHC and for llgo) if LLVM could be made to
> treat the stack pointer as a volatile register that may be changed by any
> function call.  In this model, the stack pointer needs to be treated the
> same as any other GC’d object — stack maps need to be emitted for it, and
> the RTS is allowed to relocate it.
>
> Would this be practical?  If so, it would be a major boon to the
> implementation of lightweight threading in languages that compile to LLVM
> IR.
>
> Sincerely,
>
> Demi Obenour
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20180823/f3ae64a6/attachment.html>