[llvm] [SeparateConstOffsetFromGEP] Check if non-extracted indices may be negative when preserving inbounds (PR #159515)

Thu Sep 18 00:09:09 PDT 2025

llvmbot wrote:




@llvm/pr-subscribers-backend-amdgpu

Author: Fabian Ritter (ritter-x2a)

<details>
<summary>Changes</summary>

If we know that the initial GEP was inbounds, and we change it to a sequence of
GEPs from the same base pointer where every offset is non-negative, then the
new GEPs are inbounds. So far, the implementation only checked if the extracted
offsets are non-negative. In cases where non-extracted offsets can be negative,
this would cause the inbounds flag to be wrongly preserved.

Fixes an issue in #130617 found by nikic.

---
Full diff: https://github.com/llvm/llvm-project/pull/159515.diff


4 Files Affected:

- (modified) llvm/lib/Transforms/Scalar/SeparateConstOffsetFromGEP.cpp (+7-6) 
- (modified) llvm/test/Transforms/SeparateConstOffsetFromGEP/AMDGPU/reorder-gep-inbounds.ll (+4-4) 
- (modified) llvm/test/Transforms/SeparateConstOffsetFromGEP/NVPTX/gep-chain.ll (+3-3) 
- (modified) llvm/test/Transforms/SeparateConstOffsetFromGEP/NVPTX/split-gep.ll (+4-4) 


``````````diff

diff --git a/llvm/lib/Transforms/Scalar/SeparateConstOffsetFromGEP.cpp b/llvm/lib/Transforms/Scalar/SeparateConstOffsetFromGEP.cpp
index 8a5569743ab44..333cbb6ed1384 100644
--- a/llvm/lib/Transforms/Scalar/SeparateConstOffsetFromGEP.cpp
+++ b/llvm/lib/Transforms/Scalar/SeparateConstOffsetFromGEP.cpp
@@ -1116,22 +1116,23 @@ bool SeparateConstOffsetFromGEP::splitGEP(GetElementPtrInst *GEP) {
 
       // Splits this GEP index into a variadic part and a constant offset, and
       // uses the variadic part as the new index.
-      Value *OldIdx = GEP->getOperand(I);
+      Value *Idx = GEP->getOperand(I);
       User *UserChainTail;
       bool PreservesNUW;
-      Value *NewIdx = ConstantOffsetExtractor::Extract(
-          OldIdx, GEP, UserChainTail, PreservesNUW);
+      Value *NewIdx = ConstantOffsetExtractor::Extract(Idx, GEP, UserChainTail,
+                                                       PreservesNUW);
       if (NewIdx != nullptr) {
         // Switches to the index with the constant offset removed.
         GEP->setOperand(I, NewIdx);
         // After switching to the new index, we can garbage-collect UserChain
         // and the old index if they are not used.
         RecursivelyDeleteTriviallyDeadInstructions(UserChainTail);
-        RecursivelyDeleteTriviallyDeadInstructions(OldIdx);
-        AllOffsetsNonNegative =
-            AllOffsetsNonNegative && isKnownNonNegative(NewIdx, *DL);
+        RecursivelyDeleteTriviallyDeadInstructions(Idx);
+        Idx = NewIdx;
         AllNUWPreserved &= PreservesNUW;
       }
+      AllOffsetsNonNegative =
+          AllOffsetsNonNegative && isKnownNonNegative(Idx, *DL);
     }
   }
   if (ExtractBase) {
diff --git a/llvm/test/Transforms/SeparateConstOffsetFromGEP/AMDGPU/reorder-gep-inbounds.ll b/llvm/test/Transforms/SeparateConstOffsetFromGEP/AMDGPU/reorder-gep-inbounds.ll
index 5cb3ee6f72e3b..f7c019bba5d1f 100644
--- a/llvm/test/Transforms/SeparateConstOffsetFromGEP/AMDGPU/reorder-gep-inbounds.ll
+++ b/llvm/test/Transforms/SeparateConstOffsetFromGEP/AMDGPU/reorder-gep-inbounds.ll
@@ -279,8 +279,8 @@ define void @addrspace3(ptr addrspace(3) %in.ptr, i64 %in.idx1) {
 ; CHECK-NEXT:  entry:
 ; CHECK-NEXT:    [[IN_IDX1_NNEG:%.*]] = and i64 [[IN_IDX1]], 9223372036854775807
 ; CHECK-NEXT:    [[IDXPROM:%.*]] = trunc i64 [[IN_IDX1_NNEG]] to i32
-; CHECK-NEXT:    [[TMP0:%.*]] = getelementptr inbounds i128, ptr addrspace(3) [[IN_PTR]], i32 [[IDXPROM]]
-; CHECK-NEXT:    [[IDX11:%.*]] = getelementptr inbounds i8, ptr addrspace(3) [[TMP0]], i32 1024
+; CHECK-NEXT:    [[TMP0:%.*]] = getelementptr i128, ptr addrspace(3) [[IN_PTR]], i32 [[IDXPROM]]
+; CHECK-NEXT:    [[IDX11:%.*]] = getelementptr i8, ptr addrspace(3) [[TMP0]], i32 1024
 ; CHECK-NEXT:    ret void
 ;
 entry:
@@ -296,8 +296,8 @@ define void @addrspace7(ptr addrspace(7) %in.ptr, i64 %in.idx1) {
 ; CHECK-NEXT:  entry:
 ; CHECK-NEXT:    [[IN_IDX1_NNEG:%.*]] = and i64 [[IN_IDX1]], 9223372036854775807
 ; CHECK-NEXT:    [[IDXPROM:%.*]] = trunc i64 [[IN_IDX1_NNEG]] to i32
-; CHECK-NEXT:    [[TMP0:%.*]] = getelementptr inbounds i128, ptr addrspace(7) [[IN_PTR]], i32 [[IDXPROM]]
-; CHECK-NEXT:    [[IDX11:%.*]] = getelementptr inbounds i8, ptr addrspace(7) [[TMP0]], i32 1024
+; CHECK-NEXT:    [[TMP0:%.*]] = getelementptr i128, ptr addrspace(7) [[IN_PTR]], i32 [[IDXPROM]]
+; CHECK-NEXT:    [[IDX11:%.*]] = getelementptr i8, ptr addrspace(7) [[TMP0]], i32 1024
 ; CHECK-NEXT:    ret void
 ;
 entry:
diff --git a/llvm/test/Transforms/SeparateConstOffsetFromGEP/NVPTX/gep-chain.ll b/llvm/test/Transforms/SeparateConstOffsetFromGEP/NVPTX/gep-chain.ll
index e4b9dac72e96c..bbfab0f96fc53 100644
--- a/llvm/test/Transforms/SeparateConstOffsetFromGEP/NVPTX/gep-chain.ll
+++ b/llvm/test/Transforms/SeparateConstOffsetFromGEP/NVPTX/gep-chain.ll
@@ -24,11 +24,11 @@ define i32 @more_interesting(ptr %ptr, i32 %offset1, i32 %offset2) {
 ; CHECK-NEXT:    [[IDXPROM:%.*]] = sext i32 [[OFFSET1]] to i64
 ; CHECK-NEXT:    [[GEP1:%.*]] = getelementptr inbounds [[STRUCT_UCHAR4:%.*]], ptr [[PTR]], i64 [[IDXPROM]]
 ; CHECK-NEXT:    [[IDXPROM1:%.*]] = sext i32 [[OFFSET2]] to i64
-; CHECK-NEXT:    [[TMP1:%.*]] = getelementptr inbounds [[STRUCT_UCHAR4]], ptr [[GEP1]], i64 [[IDXPROM1]]
-; CHECK-NEXT:    [[TMP2:%.*]] = getelementptr inbounds i8, ptr [[TMP1]], i64 8
+; CHECK-NEXT:    [[TMP1:%.*]] = getelementptr [[STRUCT_UCHAR4]], ptr [[GEP1]], i64 [[IDXPROM1]]
+; CHECK-NEXT:    [[TMP2:%.*]] = getelementptr i8, ptr [[TMP1]], i64 8
 ; CHECK-NEXT:    [[V1:%.*]] = load i32, ptr [[TMP2]], align 4
 ; CHECK-NEXT:    [[IDXPROM2:%.*]] = sext i32 [[OFFSET2]] to i64
-; CHECK-NEXT:    [[TMP4:%.*]] = getelementptr inbounds [[STRUCT_UCHAR4]], ptr [[TMP1]], i64 [[IDXPROM2]]
+; CHECK-NEXT:    [[TMP4:%.*]] = getelementptr [[STRUCT_UCHAR4]], ptr [[TMP1]], i64 [[IDXPROM2]]
 ; CHECK-NEXT:    [[V2:%.*]] = load i32, ptr [[TMP4]], align 4
 ; CHECK-NEXT:    [[R:%.*]] = add i32 [[V1]], [[V2]]
 ; CHECK-NEXT:    ret i32 [[R]]
diff --git a/llvm/test/Transforms/SeparateConstOffsetFromGEP/NVPTX/split-gep.ll b/llvm/test/Transforms/SeparateConstOffsetFromGEP/NVPTX/split-gep.ll
index da04a6e979425..77b3434f4f159 100644
--- a/llvm/test/Transforms/SeparateConstOffsetFromGEP/NVPTX/split-gep.ll
+++ b/llvm/test/Transforms/SeparateConstOffsetFromGEP/NVPTX/split-gep.ll
@@ -372,8 +372,8 @@ define ptr @trunk_explicit(ptr %ptr, i64 %idx) {
 ; CHECK-LABEL: define ptr @trunk_explicit(
 ; CHECK-SAME: ptr [[PTR:%.*]], i64 [[IDX:%.*]]) {
 ; CHECK-NEXT:  entry:
-; CHECK-NEXT:    [[TMP0:%.*]] = getelementptr inbounds [[STRUCT0:%.*]], ptr [[PTR]], i64 0, i32 3, i64 [[IDX]], i32 1
-; CHECK-NEXT:    [[PTR21:%.*]] = getelementptr inbounds i8, ptr [[TMP0]], i64 3216
+; CHECK-NEXT:    [[TMP0:%.*]] = getelementptr [[STRUCT0:%.*]], ptr [[PTR]], i64 0, i32 3, i64 [[IDX]], i32 1
+; CHECK-NEXT:    [[PTR21:%.*]] = getelementptr i8, ptr [[TMP0]], i64 3216
 ; CHECK-NEXT:    ret ptr [[PTR21]]
 ;
 entry:
@@ -389,8 +389,8 @@ define ptr @trunk_long_idx(ptr %ptr, i64 %idx) {
 ; CHECK-LABEL: define ptr @trunk_long_idx(
 ; CHECK-SAME: ptr [[PTR:%.*]], i64 [[IDX:%.*]]) {
 ; CHECK-NEXT:  entry:
-; CHECK-NEXT:    [[TMP0:%.*]] = getelementptr inbounds [[STRUCT0:%.*]], ptr [[PTR]], i64 0, i32 3, i64 [[IDX]], i32 1
-; CHECK-NEXT:    [[PTR21:%.*]] = getelementptr inbounds i8, ptr [[TMP0]], i64 3216
+; CHECK-NEXT:    [[TMP0:%.*]] = getelementptr [[STRUCT0:%.*]], ptr [[PTR]], i64 0, i32 3, i64 [[IDX]], i32 1
+; CHECK-NEXT:    [[PTR21:%.*]] = getelementptr i8, ptr [[TMP0]], i64 3216
 ; CHECK-NEXT:    ret ptr [[PTR21]]
 ;
 entry:

``````````

</details>


https://github.com/llvm/llvm-project/pull/159515
