[PATCH] D154215: [InstSimplify] Fold gep inbounds undef to undef instead of poison

[Nikita Popov via Phabricator via llvm-commits]

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With the semantics change from D154051 <https://reviews.llvm.org/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, it is no longer valid to fold gep inbounds undef to poison (unless we know the index is non-zero).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.

Depends on D154051 <https://reviews.llvm.org/D154051>.


https://reviews.llvm.org/D154215

Files:
  llvm/lib/Analysis/InstructionSimplify.cpp
  llvm/test/Transforms/InstSimplify/gep.ll


Index: llvm/test/Transforms/InstSimplify/gep.ll
===================================================================
--- llvm/test/Transforms/InstSimplify/gep.ll
+++ llvm/test/Transforms/InstSimplify/gep.ll
@@ -160,7 +160,7 @@
 
 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 <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
Index: llvm/lib/Analysis/InstructionSimplify.cpp
===================================================================
--- llvm/lib/Analysis/InstructionSimplify.cpp
+++ llvm/lib/Analysis/InstructionSimplify.cpp
@@ -4896,9 +4896,9 @@
       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) {


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