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Do not treat llvm.fake.use as a debug instruction #128684

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Merged
merged 2 commits into from
Feb 25, 2025
Merged

Do not treat llvm.fake.use as a debug instruction #128684

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06f5b36
[LLVM] Do not treat llvm.fake.use as a debug instruction
SLTozer Feb 25, 2025
3b5863f
Remove unused test param
SLTozer Feb 25, 2025
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5 changes: 1 addition & 4 deletions llvm/lib/IR/Instruction.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -1233,10 +1233,7 @@ Instruction::getNextNonDebugInstruction(bool SkipPseudoOp) const {
const Instruction *
Instruction::getPrevNonDebugInstruction(bool SkipPseudoOp) const {
for (const Instruction *I = getPrevNode(); I; I = I->getPrevNode())
if (!isa<DbgInfoIntrinsic>(I) &&
!(SkipPseudoOp && isa<PseudoProbeInst>(I)) &&
!(isa<IntrinsicInst>(I) &&
cast<IntrinsicInst>(I)->getIntrinsicID() == Intrinsic::fake_use))
if (!isa<DbgInfoIntrinsic>(I) && !(SkipPseudoOp && isa<PseudoProbeInst>(I)))
return I;
return nullptr;
}
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67 changes: 67 additions & 0 deletions llvm/test/Transforms/SimplifyCFG/X86/fake-use-considered-when-sinking.ll
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@@ -0,0 +1,67 @@
; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 5
; RUN: opt -p="simplifycfg<sink-common-insts>" -S < %s | FileCheck %s

;; Verify that fake uses are not ignored when sinking instructions in
;; SimplifyCFG; when a fake use appears in only one incoming block they prevent
;; further sinking, and when identical fake uses appear on both sides they
;; are sunk normally.
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@OCHyams OCHyams Feb 25, 2025

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Seems a bit fuzzy to me whether fake.use should block this optimisation or ignore the intrinsics and sink them into the suc. I don't find the docs particularly clarifying for this specific case.

You've been close to this work and have upstreamed it, so this is less me contesting the correct behaviour, more hoping for a bit more of rationale/explanation?

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@SLTozer SLTozer Feb 25, 2025

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The basic reason here is that fake uses must be treated as any other instruction by default. As an optimization we can create specific exceptions for fake uses in cases where we know it's either safe or beneficial to treat them differently, but we wouldn't want to do so "generally" as we do by skipping them in getPrevNonDebugInstruction.

For sinking common instructions specifically, I imagine it's safe to sink fake uses separate to the other instructions in their block, but that specialization should exist in the sinking code (and would be beyond the scope of this patch).


target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-i128:128-f80:128-n8:16:32:64-S128"
target triple = "x86_64-unknown-linux-gnu"

define void @foo(i1 %bool, ptr %p) {
; CHECK-LABEL: define void @foo(
; CHECK-SAME: i1 [[BOOL:%.*]], ptr [[P:%.*]]) {
; CHECK-NEXT: [[ENTRY:.*:]]
; CHECK-NEXT: br i1 [[BOOL]], label %[[IF_ELSE:.*]], label %[[IF_THEN:.*]]
; CHECK: [[COMMON_RET:.*]]:
; CHECK-NEXT: ret void
; CHECK: [[IF_THEN]]:
; CHECK-NEXT: store ptr [[P]], ptr [[P]], align 8
; CHECK-NEXT: br label %[[COMMON_RET]]
; CHECK: [[IF_ELSE]]:
; CHECK-NEXT: store ptr [[P]], ptr [[P]], align 8
; CHECK-NEXT: notail call void (...) @llvm.fake.use(ptr [[P]])
; CHECK-NEXT: br label %[[COMMON_RET]]
;
entry:
br i1 %bool, label %if.else, label %if.then

common.ret: ; preds = %if.else, %if.then
ret void

if.then: ; preds = %entry
store ptr %p, ptr %p, align 8
br label %common.ret

if.else: ; preds = %entry
store ptr %p, ptr %p, align 8
notail call void (...) @llvm.fake.use(ptr %p)
br label %common.ret
}

define void @bar(i1 %bool, ptr %p) {
; CHECK-LABEL: define void @bar(
; CHECK-SAME: i1 [[BOOL:%.*]], ptr [[P:%.*]]) {
; CHECK-NEXT: [[ENTRY:.*:]]
; CHECK-NEXT: store ptr [[P]], ptr [[P]], align 8
; CHECK-NEXT: notail call void (...) @llvm.fake.use(ptr [[P]])
; CHECK-NEXT: ret void
;
entry:
br i1 %bool, label %if.else, label %if.then

common.ret: ; preds = %if.else, %if.then
ret void

if.then: ; preds = %entry
store ptr %p, ptr %p, align 8
notail call void (...) @llvm.fake.use(ptr %p)
br label %common.ret

if.else: ; preds = %entry
store ptr %p, ptr %p, align 8
notail call void (...) @llvm.fake.use(ptr %p)
br label %common.ret
}