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

[fhahn]

Summary

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.

[lebedev.ri]

LG

[nikic]

LGTM https://alive2.llvm.org/ce/z/kAJxYC

llvm/lib/Analysis/InstructionSimplify.cpp

static Value *SimplifyGEPInst(Type *SrcTy, ArrayRef<Value *> Ops, bool InBounds,
   }
 
   // getelementptr poison, idx -> poison
   // getelementptr baseptr, poison -> poison
   if (any_of(Ops, [](const auto *V) { return isa<PoisonValue>(V); }))
     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) {
         return isa<ScalableVectorType>(V->getType());
       });
 
   if (Ops.size() == 2) {
     // getelementptr P, 0 -> P.

llvm/test/Transforms/InstSimplify/gep.ll

;
   %b_ptr = ptrtoint i8* %b to i64
   %sub = sub i64 %e_ptr, %b_ptr
   %gep = getelementptr inbounds i8, i8* %b, i64 %sub
   ret i8* %gep
 }
 
 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
 }
 
 define i64* @undef_no_inbounds_var_idx(i64 %idx) {
 ; CHECK-LABEL: @undef_no_inbounds_var_idx(
 ; CHECK-NEXT:    ret i64* undef
 ;
   %el = getelementptr i64, i64* undef, i64 %idx
   ret i64* %el
 }
 
 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
 }
 
 define <8 x i64*> @undef_vec2() {
 ; CHECK-LABEL: @undef_vec2(
 ; CHECK-NEXT:    ret <8 x i64*> undef