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[InstCombine] Fix pointer replacement in foldSelectValueEquivalence #161701

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Merged
merged 5 commits into from
Oct 5, 2025
+52 −10
Merged

[InstCombine] Fix pointer replacement in foldSelectValueEquivalence #161701

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3349520
[InstCombine] Add pre-commit tests. NFC.
dtcxzyw Oct 2, 2025
18f21c6
[InstCombine] Fix pointer replacement in `foldSelectValueEquivalence`
dtcxzyw Oct 2, 2025
44f9269
[InstCombine] Cache query results
dtcxzyw Oct 2, 2025
9a0f030
[InstCombine] Address review comments. NFC.
dtcxzyw Oct 3, 2025
07a43ac
Remove unused variable
dtcxzyw Oct 4, 2025
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30 changes: 20 additions & 10 deletions llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -17,6 +17,7 @@
#include "llvm/Analysis/AssumptionCache.h"
#include "llvm/Analysis/CmpInstAnalysis.h"
#include "llvm/Analysis/InstructionSimplify.h"
#include "llvm/Analysis/Loads.h"
#include "llvm/Analysis/OverflowInstAnalysis.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/Analysis/VectorUtils.h"
Expand All @@ -42,6 +43,7 @@
#include "llvm/Support/KnownBits.h"
#include "llvm/Transforms/InstCombine/InstCombiner.h"
#include <cassert>
#include <optional>
#include <utility>

#define DEBUG_TYPE "instcombine"
Expand Down Expand Up @@ -1448,10 +1450,16 @@ Instruction *InstCombinerImpl::foldSelectValueEquivalence(SelectInst &Sel,
return nullptr;
};

if (Instruction *R = ReplaceOldOpWithNewOp(CmpLHS, CmpRHS))
return R;
if (Instruction *R = ReplaceOldOpWithNewOp(CmpRHS, CmpLHS))
return R;
bool CanReplaceCmpLHSWithRHS = canReplacePointersIfEqual(CmpLHS, CmpRHS, DL);
if (CanReplaceCmpLHSWithRHS) {
if (Instruction *R = ReplaceOldOpWithNewOp(CmpLHS, CmpRHS))
return R;
}
bool CanReplaceCmpRHSWithLHS = canReplacePointersIfEqual(CmpRHS, CmpLHS, DL);
if (CanReplaceCmpRHSWithLHS) {
if (Instruction *R = ReplaceOldOpWithNewOp(CmpRHS, CmpLHS))
return R;
}

auto *FalseInst = dyn_cast<Instruction>(FalseVal);
if (!FalseInst)
Expand All @@ -1466,12 +1474,14 @@ Instruction *InstCombinerImpl::foldSelectValueEquivalence(SelectInst &Sel,
// Example:
// (X == 42) ? 43 : (X + 1) --> (X == 42) ? (X + 1) : (X + 1) --> X + 1
SmallVector<Instruction *> DropFlags;
if (simplifyWithOpReplaced(FalseVal, CmpLHS, CmpRHS, SQ,
/* AllowRefinement */ false,
&DropFlags) == TrueVal ||
simplifyWithOpReplaced(FalseVal, CmpRHS, CmpLHS, SQ,
/* AllowRefinement */ false,
&DropFlags) == TrueVal) {
if ((CanReplaceCmpLHSWithRHS &&
simplifyWithOpReplaced(FalseVal, CmpLHS, CmpRHS, SQ,
/* AllowRefinement */ false,
&DropFlags) == TrueVal) ||
(CanReplaceCmpRHSWithLHS &&
simplifyWithOpReplaced(FalseVal, CmpRHS, CmpLHS, SQ,
/* AllowRefinement */ false,
&DropFlags) == TrueVal)) {
for (Instruction *I : DropFlags) {
I->dropPoisonGeneratingAnnotations();
Worklist.add(I);
Expand Down
32 changes: 32 additions & 0 deletions llvm/test/Transforms/InstCombine/select-gep.ll
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Original file line number Diff line number Diff line change
Expand Up @@ -286,3 +286,35 @@ define <2 x ptr> @test7(<2 x ptr> %p1, i64 %idx, <2 x i1> %cc) {
%select = select <2 x i1> %cc, <2 x ptr> %p1, <2 x ptr> %gep
ret <2 x ptr> %select
}

define ptr @ptr_eq_replace_freeze1(ptr %p, ptr %q) {
; CHECK-LABEL: @ptr_eq_replace_freeze1(
; CHECK-NEXT: [[Q_FR:%.*]] = freeze ptr [[Q:%.*]]
; CHECK-NEXT: [[Q_FR1:%.*]] = freeze ptr [[Q1:%.*]]
; CHECK-NEXT: [[CMP:%.*]] = icmp eq ptr [[Q_FR]], [[Q_FR1]]
; CHECK-NEXT: [[SELECT:%.*]] = select i1 [[CMP]], ptr [[Q_FR]], ptr [[Q_FR1]]
; CHECK-NEXT: ret ptr [[SELECT]]
;
%p.fr = freeze ptr %p
%q.fr = freeze ptr %q
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What is the significance of freeze here? Why does this issue not reproduce without it (or with just noundef)?

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It is guarded by

llvm-project/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp

Lines 1456 to 1458 in bd7e228

auto *FalseInst = dyn_cast<Instruction>(FalseVal);
if (!FalseInst)
return nullptr;

%cmp = icmp eq ptr %p.fr, %q.fr
%select = select i1 %cmp, ptr %p.fr, ptr %q.fr
ret ptr %select
}

define ptr @ptr_eq_replace_freeze2(ptr %p, ptr %q) {
; CHECK-LABEL: @ptr_eq_replace_freeze2(
; CHECK-NEXT: [[P_FR:%.*]] = freeze ptr [[P:%.*]]
; CHECK-NEXT: [[P_FR1:%.*]] = freeze ptr [[P1:%.*]]
; CHECK-NEXT: [[CMP:%.*]] = icmp eq ptr [[P_FR1]], [[P_FR]]
; CHECK-NEXT: [[SELECT_V:%.*]] = select i1 [[CMP]], ptr [[P_FR1]], ptr [[P_FR]]
; CHECK-NEXT: [[SELECT:%.*]] = getelementptr i8, ptr [[SELECT_V]], i64 16
; CHECK-NEXT: ret ptr [[SELECT]]
;
%gep1 = getelementptr i32, ptr %p, i64 4
%gep2 = getelementptr i32, ptr %q, i64 4
%cmp = icmp eq ptr %p, %q
%cmp.fr = freeze i1 %cmp
%select = select i1 %cmp.fr, ptr %gep1, ptr %gep2
ret ptr %select
}