[PATCH] D117015: [InstSimplify] Fold inbounds GEP to poison if base is undef.

[Florian Hahn via Phabricator via llvm-commits]

fhahn created this revision.
fhahn added reviewers: nikic, spatel, lebedev.ri, aqjune.
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D92270 <https://reviews.llvm.org/D92270> updated constant expression folding to fold inbounds GEP to
poison if the base is undef. Apply the same logic to SimplifyGEPInst.

The justification is that we can choose an out-of-bounds pointer as base
pointer.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D117015

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 i64* @undef_inbounds_var_idx(i64 %idx) {
 ; CHECK-LABEL: @undef_inbounds_var_idx(
-; CHECK-NEXT:    ret i64* undef
+; CHECK-NEXT:    ret i64* poison
 ;
   %el = getelementptr inbounds i64, i64* undef, i64 %idx
   ret i64* %el
@@ -176,7 +176,7 @@
 
 define <8 x i64*> @undef_vec1() {
 ; CHECK-LABEL: @undef_vec1(
-; CHECK-NEXT:    ret <8 x i64*> undef
+; CHECK-NEXT:    ret <8 x i64*> poison
 ;
   %el = getelementptr inbounds i64, i64* undef, <8 x i64> undef
   ret <8 x i64*> %el
Index: llvm/lib/Analysis/InstructionSimplify.cpp
===================================================================
--- llvm/lib/Analysis/InstructionSimplify.cpp
+++ llvm/lib/Analysis/InstructionSimplify.cpp
@@ -4457,7 +4457,8 @@
     return PoisonValue::get(GEPTy);
 
   if (Q.isUndefValue(Ops[0]))
-    return UndefValue::get(GEPTy);
+    // If inbounds, we can choose an out-of-bounds pointer as a base pointer.
+    return InBounds ? PoisonValue::get(GEPTy) : UndefValue::get(GEPTy);
 
   bool IsScalableVec =
       isa<ScalableVectorType>(SrcTy) || any_of(Ops, [](const Value *V) {


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