[InstSimplify] Fold gep inbounds undef to undef instead of poison

With the semantics change from D154051, it is no longer valid to
fold gep inbounds undef to poison (unless we know the index is
non-zero). Fold it to undef instead.

Differential Revision: https://reviews.llvm.org/D154215

diff --git a/llvm/lib/Analysis/InstructionSimplify.cpp b/llvm/lib/Analysis/InstructionSimplify.cpp
index a32f688..8599847 100644 (file)
--- a/llvm/lib/Analysis/InstructionSimplify.cpp
+++ b/llvm/lib/Analysis/InstructionSimplify.cpp
@@ -4896,9 +4896,9 @@ static Value *simplifyGEPInst(Type *SrcTy, Value *Ptr,
       any_of(Indices, [](const auto *V) { return isa<PoisonValue>(V); }))
     return PoisonValue::get(GEPTy);
 
+  // getelementptr undef, idx -> undef
   if (Q.isUndefValue(Ptr))
-    // If inbounds, we can choose an out-of-bounds pointer as a base pointer.
-    return InBounds ? PoisonValue::get(GEPTy) : UndefValue::get(GEPTy);
+    return UndefValue::get(GEPTy);
 
   bool IsScalableVec =
       isa<ScalableVectorType>(SrcTy) || any_of(Indices, [](const Value *V) {

diff --git a/llvm/test/Transforms/InstSimplify/gep.ll b/llvm/test/Transforms/InstSimplify/gep.ll
index 529fc5e..5e70c2c 100644 (file)
--- a/llvm/test/Transforms/InstSimplify/gep.ll
+++ b/llvm/test/Transforms/InstSimplify/gep.ll
@@ -160,7 +160,7 @@ define ptr @test7(ptr %b, ptr %e) {
 
 define ptr @undef_inbounds_var_idx(i64 %idx) {
 ; CHECK-LABEL: @undef_inbounds_var_idx(
-; CHECK-NEXT:    ret ptr poison
+; CHECK-NEXT:    ret ptr undef
 ;
   %el = getelementptr inbounds i64, ptr undef, i64 %idx
   ret ptr %el
@@ -176,7 +176,7 @@ define ptr @undef_no_inbounds_var_idx(i64 %idx) {
 
 define <8 x ptr> @undef_vec1() {
 ; CHECK-LABEL: @undef_vec1(
-; CHECK-NEXT:    ret <8 x ptr> poison
+; CHECK-NEXT:    ret <8 x ptr> undef
 ;
   %el = getelementptr inbounds i64, ptr undef, <8 x i64> undef
   ret <8 x ptr> %el