[LLVMdev] LICM for function calls

[Hal Finkel]

----- Original Message -----
> From: "Thomas F Raoux" <thomas.f.raoux at intel.com>
> To: llvmdev at cs.uiuc.edu
> Sent: Tuesday, July 14, 2015 9:45:20 AM
> Subject: [LLVMdev] LICM for function calls
> 
> Hi,
> 
> Right now in LICM (and many other transformations) we always assume
> it is never safe to speculatively execute a function call.
> 
> The following function always return false for all function calls
> except for few intrinsics:
> bool llvm::isSafeToSpeculativelyExecute(const Value *V,
>                                         const DataLayout *TD) {
> ...
>   case Instruction::Call: {
>    if (const IntrinsicInst *II = dyn_cast<IntrinsicInst>(Inst)) {
> ...
>    }
>     return false; // The called function could have undefined
>     behavior or
>                   // side-effects, even if marked readnone nounwind.
> 
> In some cases this could have an important performance impact so I'm
> looking for a potential way to improve it.
> 
> Is there any combination of attributes which would guarantee that a
> function is safe to speculatively execute a function call? (As far
> as I can tell there isn't.)
> 
> Does anybody have a suggestion on what would be the best way to
> implement such functionality? Or do you think such optimization is
> undesirable?

I think we'd need some kind of 'safe_to_speculate' attribute on the function. Regarding the optimization, under what circumstances would you want to speculate a function? I can imagine doing this only if I knew the function would be lowered in the backend to some simple set of instructions.

 -Hal

> 
> Cheers,
> Thomas
> 
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-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory