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

[Neil Henning via llvm-dev]

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?

Thanks for any help,

Cheers,
-Neil.

-- 
Neil Henning
Senior Software Engineer Compiler
unity.com
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