[llvm-dev] Unlikely branches can have expensive contents hoisted (original) (raw)
Neil Henning via llvm-dev llvm-dev at lists.llvm.org
Wed Jul 22 02:52:35 PDT 2020
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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 -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20200722/27617ff3/attachment.html>
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