[llvm] [InstCombine] Rebuild type-changing PHIs in PointerReplacer (PR #212429)
Keshav Vinayak Jha via llvm-commits
llvm-commits at lists.llvm.org
Tue Jul 28 01:30:43 PDT 2026
https://github.com/keshavvinayak01 created https://github.com/llvm/llvm-project/pull/212429
PointerReplacer mutates PHI types when replacing an alloca with a pointer in another address space. Mutating a value type in place can invalidate existing users whose result types or operand constraints were formed from the original pointer type.
For example:
```
%p = phi ptr addrspace(5) [ %a, %bb0 ], [ %b, %bb1 ]
%g = getelementptr i8, ptr addrspace(5) %p, i64 1
```
Changing `%p` to `ptr addrspace(4)` leaves the existing GEP result in `AS5` while its pointer operand is now in `AS4`. (This is what exposed the bug). This triggered:
https://github.com/llvm/llvm-project/blob/0bcff14b1740cf32f9e0983726238dcf353c6ac8/llvm/lib/IR/Operator.cpp#L129-L131
Create a new PHI when the replacement changes type so existing users remain attached to the original well-typed graph while PointerReplacer builds their replacements. Preserve in-place operand updates when the PHI type is unchanged.
Assisted-by: Codex
>From 91207aed4613de0142ce295b2715afb56a21f510 Mon Sep 17 00:00:00 2001
From: Keshav Vinayak Jha <keshavvinayakjha at gmail.com>
Date: Tue, 28 Jul 2026 13:29:39 +0530
Subject: [PATCH] [InstCombine] Rebuild type-changing PHIs in PointerReplacer
PointerReplacer mutates PHI types when replacing an alloca with a pointer in another address space. Mutating a value type in place can invalidate existing users whose result types or operand constraints were formed from the original pointer type.
For example:
%p = phi ptr addrspace(5) [ %a, %bb0 ], [ %b, %bb1 ]
%g = getelementptr i8, ptr addrspace(5) %p, i64 1
Changing %p to ptr addrspace(4) leaves the existing GEP result in addrspace(5) while its pointer operand is now in addrspace(4). The invalid mutation is target-independent. AMDGPU exposes it as an offset-width assertion because these address spaces use different index widths:
https://github.com/llvm/llvm-project/blob/0bcff14b1740cf32f9e0983726238dcf353c6ac8/llvm/lib/IR/Operator.cpp#L129-L131
Create a new PHI when the replacement changes type so existing users remain attached to the original well-typed graph while PointerReplacer builds their replacements. Preserve in-place operand updates when the PHI type is unchanged. Add a target-independent regression test.
Co-authored-by: GPT-5 <noreply at openai.com>
Signed-off-by: Keshav Vinayak Jha <keshavvinayakjha at gmail.com>
---
.../InstCombineLoadStoreAlloca.cpp | 31 +++++++++++-----
.../InstCombine/ptr-replace-alloca.ll | 36 +++++++++++++++++++
2 files changed, 59 insertions(+), 8 deletions(-)
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp b/llvm/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp
index e6481c89c0265..41da23c14bdb8 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp
@@ -425,15 +425,30 @@ void PointerReplacer::replace(Instruction *I) {
// replacement (new value).
WorkMap[NewI] = NewI;
} else if (auto *PHI = dyn_cast<PHINode>(I)) {
- // Create a new PHI by replacing any incoming value that is a user of the
- // root pointer and has a replacement.
- Value *V = WorkMap.lookup(PHI->getIncomingValue(0));
- PHI->mutateType(V ? V->getType() : PHI->getIncomingValue(0)->getType());
- for (unsigned int I = 0; I < PHI->getNumIncomingValues(); ++I) {
- Value *V = WorkMap.lookup(PHI->getIncomingValue(I));
- PHI->setIncomingValue(I, V ? V : PHI->getIncomingValue(I));
+ Value *FirstIncoming = PHI->getIncomingValue(0);
+ Value *V = WorkMap.lookup(FirstIncoming);
+ Type *NewType = V ? V->getType() : FirstIncoming->getType();
+ if (PHI->getType() == NewType) {
+ for (unsigned I = 0; I < PHI->getNumIncomingValues(); ++I) {
+ Value *V = WorkMap.lookup(PHI->getIncomingValue(I));
+ PHI->setIncomingValue(I, V ? V : PHI->getIncomingValue(I));
+ }
+ WorkMap[PHI] = PHI;
+ return;
+ }
+
+ auto *NewPHI = PHINode::Create(NewType, PHI->getNumIncomingValues(), "");
+ IC.InsertNewInstWith(NewPHI, PHI->getIterator());
+ NewPHI->takeName(PHI);
+ NewPHI->copyMetadata(*PHI);
+ WorkMap[PHI] = NewPHI;
+ for (unsigned I = 0; I < PHI->getNumIncomingValues(); ++I) {
+ Value *IncomingValue = PHI->getIncomingValue(I);
+ Value *V = WorkMap.lookup(IncomingValue);
+ assert(V && V->getType() == NewType &&
+ "Type-changing PHI incoming value was not replaced");
+ NewPHI->addIncoming(V, PHI->getIncomingBlock(I));
}
- WorkMap[PHI] = PHI;
} else if (auto *GEP = dyn_cast<GetElementPtrInst>(I)) {
auto *V = getReplacement(GEP->getPointerOperand());
assert(V && "Operand not replaced");
diff --git a/llvm/test/Transforms/InstCombine/ptr-replace-alloca.ll b/llvm/test/Transforms/InstCombine/ptr-replace-alloca.ll
index 3490773b03b82..38957cc72ad7f 100644
--- a/llvm/test/Transforms/InstCombine/ptr-replace-alloca.ll
+++ b/llvm/test/Transforms/InstCombine/ptr-replace-alloca.ll
@@ -38,6 +38,41 @@ sink:
ret i8 %load
}
+define i8 @phi_addrspace_change(i1 %cond) {
+; CHECK-LABEL: @phi_addrspace_change(
+; CHECK-NEXT: entry:
+; CHECK-NEXT: br i1 [[COND:%.*]], label [[IF:%.*]], label [[ELSE:%.*]]
+; CHECK: if:
+; CHECK-NEXT: br label [[SINK:%.*]]
+; CHECK: else:
+; CHECK-NEXT: br label [[SINK]]
+; CHECK: sink:
+; CHECK-NEXT: [[PTR:%.*]] = phi ptr [ getelementptr inbounds nuw (i8, ptr @g1, i64 1), [[IF]] ], [ getelementptr inbounds nuw (i8, ptr @g1, i64 2), [[ELSE]] ]
+; CHECK-NEXT: [[GEP:%.*]] = getelementptr i8, ptr [[PTR]], i64 1
+; CHECK-NEXT: [[LOAD:%.*]] = load i8, ptr [[GEP]], align 1
+; CHECK-NEXT: ret i8 [[LOAD]]
+;
+entry:
+ %alloca = alloca [32 x i8], align 4, addrspace(5)
+ call void @llvm.memcpy.p5.p0.i64(ptr addrspace(5) %alloca, ptr @g1, i64 32, i1 false)
+ %base = getelementptr [32 x i8], ptr addrspace(5) %alloca, i32 0, i32 0
+ br i1 %cond, label %if, label %else
+
+if:
+ %val.if = getelementptr i8, ptr addrspace(5) %base, i32 1
+ br label %sink
+
+else:
+ %val.else = getelementptr i8, ptr addrspace(5) %base, i32 2
+ br label %sink
+
+sink:
+ %ptr = phi ptr addrspace(5) [ %val.if, %if ], [ %val.else, %else ]
+ %gep = getelementptr i8, ptr addrspace(5) %ptr, i32 1
+ %load = load i8, ptr addrspace(5) %gep
+ ret i8 %load
+}
+
define i8 @volatile_load_keep_alloca(i1 %cond) {
; CHECK-LABEL: @volatile_load_keep_alloca(
; CHECK-NEXT: entry:
@@ -502,6 +537,7 @@ define i8 @preserve_load_metadata(i64 %idx) {
!0 = !{!"my-annotation"}
declare void @llvm.memcpy.p1.p0.i64(ptr addrspace(1), ptr, i64, i1)
+declare void @llvm.memcpy.p5.p0.i64(ptr addrspace(5), ptr, i64, i1)
declare void @llvm.memcpy.p0.p0.i64(ptr, ptr, i64, i1)
declare void @llvm.memcpy.p0.p1.i64(ptr, ptr addrspace(1), i64, i1)
declare void @llvm.memcpy.p1.p1.i64(ptr addrspace(1), ptr addrspace(1), i64, i1)
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