[llvm-dev] Unlikely branches can have expensive contents hoisted

[Hiroshi Yamauchi via llvm-dev]

On Wed, Jul 22, 2020 at 2:52 AM Neil Henning via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hey all - me again,
>
> So I'm looking at llvm.expect specifically for branch hints. In the
> following example LLVM will hoist the pow/cos calls into the entry block
> even though I've used the llvm.expect intrinsic to make it clear that one
> of the calls is unlikely to occur.
>
> target datalayout =
> "e-m:w-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
> target triple = "x86_64-pc-windows-msvc-coff"
>
> define dllexport double @foo(i32 %val) {
> entry:
>   %0 = icmp slt i32 %val, 42
>   %1 = call i1 @llvm.expect.i1(i1 %0, i1 false)
>   %2 = sitofp i32 %val to double
>   br i1 %1, label %true, label %false
>
> true:
>   %3 = call double @llvm.cos.f64(double %2)
>   br label %merge
>
> false:
>   %4 = call double @llvm.pow.f64(double %2, double 4.242000e+01)
>   br label %merge
>
> merge:
>   %var.1.0 = phi double [ %4, %false ], [ %3, %true ]
>   ret double %var.1.0
> }
>
> declare i1 @llvm.expect.i1(i1, i1)
> declare double @llvm.cos.f64(double)
> declare double @llvm.pow.f64(double, double)
>
> This seems counter intuitive to me - I've told LLVM that one of the calls
> will probably not happen, and I expected it to preserve the call in the
> unlikely branch so we don't pay the cost for something unlikely to be used.
>
> I also injected a pass locally that adds, for branches whose condition is
> llvm.expect, the branch weight metadata - but LLVM will still always fold
> the branch away ensuring that the expensive call is always called.
>
> The part of SimplifyCFG that does this is FoldTwoEntryPHINode from what I
> can tell.
>
> So is there anything I can do here without modifying LLVM? Have I missed
> something?
>
> Otherwise I guess I'd have to change FoldTwoEntryPHINode to not do this in
> the presence of branch weights / expect?
>

Passing -two-entry-phi-node-folding-threshold=1 seems to prevent this
folding, but that may not be what you need.

As it doesn't look like FoldTwoEntryPHINode checks for branch hints, it may
make sense to change FoldTwoEntryPHINode.


> Thanks for any help,
>
> Cheers,
> -Neil.
>
> --
> Neil Henning
> Senior Software Engineer Compiler
> unity.com
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> llvm-dev at lists.llvm.org
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>
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