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[profcheck] Add unknown branch weights to expanded cmpxchg loop. #165841

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Nov 5, 2025
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[profcheck] Add unknown branch weights to expanded cmpxchg loop. #165841

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8 changes: 7 additions & 1 deletion llvm/lib/CodeGen/AtomicExpandPass.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -38,6 +38,7 @@
#include "llvm/IR/MDBuilder.h"
#include "llvm/IR/MemoryModelRelaxationAnnotations.h"
#include "llvm/IR/Module.h"
#include "llvm/IR/ProfDataUtils.h"
#include "llvm/IR/Type.h"
#include "llvm/IR/User.h"
#include "llvm/IR/Value.h"
Expand Down Expand Up @@ -1680,7 +1681,12 @@ Value *AtomicExpandImpl::insertRMWCmpXchgLoop(

Loaded->addIncoming(NewLoaded, LoopBB);

Builder.CreateCondBr(Success, ExitBB, LoopBB);
Instruction *CondBr = Builder.CreateCondBr(Success, ExitBB, LoopBB);

// Atomic RMW expands to a cmpxchg loop, Since precise branch weights
// cannot be easily determined here, we mark the branch as "unknown" (50/50)
// to prevent misleading optimizations.
setExplicitlyUnknownBranchWeightsIfProfiled(*CondBr, *F, DEBUG_TYPE);
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@tgymnich tgymnich Nov 5, 2025 •
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Suggested change
setExplicitlyUnknownBranchWeightsIfProfiled(*CondBr, *F, DEBUG_TYPE);
setExplicitlyUnknownBranchWeightsIfProfiled(*CondBr, DEBUG_TYPE, *F);

The argument order seems wrong. (was changed 2h ago #166032)

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@tgymnich tgymnich Nov 5, 2025

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#166601

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@jinhuang1102 jinhuang1102 Nov 5, 2025

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Thank you Tim!


Builder.SetInsertPoint(ExitBB, ExitBB->begin());
return NewLoaded;
Expand Down
14 changes: 11 additions & 3 deletions llvm/test/Transforms/AtomicExpand/AArch64/atomicrmw-fp.ll
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@@ -1,7 +1,7 @@
; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --check-globals
; RUN: opt -S -mtriple=aarch64-linux-gnu -passes=atomic-expand %s | FileCheck %s

define float @test_atomicrmw_fadd_f32(ptr %ptr, float %value) {
define float @test_atomicrmw_fadd_f32(ptr %ptr, float %value) !prof !0 {
; CHECK-LABEL: @test_atomicrmw_fadd_f32(
; CHECK-NEXT: [[TMP1:%.*]] = load float, ptr [[PTR:%.*]], align 4
; CHECK-NEXT: br label [[ATOMICRMW_START:%.*]]
Expand All @@ -14,7 +14,7 @@ define float @test_atomicrmw_fadd_f32(ptr %ptr, float %value) {
; CHECK-NEXT: [[SUCCESS:%.*]] = extractvalue { i32, i1 } [[TMP4]], 1
; CHECK-NEXT: [[NEWLOADED:%.*]] = extractvalue { i32, i1 } [[TMP4]], 0
; CHECK-NEXT: [[TMP5]] = bitcast i32 [[NEWLOADED]] to float
; CHECK-NEXT: br i1 [[SUCCESS]], label [[ATOMICRMW_END:%.*]], label [[ATOMICRMW_START]]
; CHECK-NEXT: br i1 [[SUCCESS]], label [[ATOMICRMW_END:%.*]], label [[ATOMICRMW_START]], !prof [[PROF1:![0-9]+]]
; CHECK: atomicrmw.end:
; CHECK-NEXT: ret float [[TMP5]]
;
Expand Down Expand Up @@ -336,3 +336,11 @@ define <2 x half> @atomicrmw_fminimum_2_x_half(ptr %ptr, <2 x half> %val) {
%res = atomicrmw fminimum ptr %ptr, <2 x half> %val seq_cst
ret <2 x half> %res
}

!0 = !{!"function_entry_count", i64 1000}
;.
; CHECK: attributes #[[ATTR0:[0-9]+]] = { nocallback nocreateundeforpoison nofree nosync nounwind speculatable willreturn memory(none) }
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@jinhuang1102 jinhuang1102 Nov 5, 2025

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Manually add nocreateundeforpoison to pass the upstream test checks.

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@boomanaiden154 boomanaiden154 Nov 5, 2025

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I'm guessing something else was changed to infer the new attribute and your local opt binary was too old.

;.
; CHECK: [[META0:![0-9]+]] = !{!"function_entry_count", i64 1000}
; CHECK: [[PROF1]] = !{!"unknown", !"atomic-expand"}
;.