[ConstantFold] Fix GEP of GEP fold with opaque pointers

This was previously combining indices even though they operate on different types. For non-opaque pointers, the condition is automatically satisfied based on the pointer types being equal.

Author: nikic

llvm/lib/IR/ConstantFold.cpp

@@ -2347,8 +2347,11 @@ static bool isIndexInRangeOfArrayType(uint64_t NumElements,
 // Combine Indices - If the source pointer to this getelementptr instruction
 // is a getelementptr instruction, combine the indices of the two
 // getelementptr instructions into a single instruction.
-static Constant *foldGEPOfGEP(GEPOperator *GEP, bool InBounds,
+static Constant *foldGEPOfGEP(GEPOperator *GEP, Type *PointeeTy, bool InBounds,
  ArrayRef<Value *> Idxs) {
+ if (PointeeTy != GEP->getResultElementType())
+ return nullptr;
+
  Constant *Idx0 = cast<Constant>(Idxs[0]);
  if (Idx0->isNullValue()) {
  // Handle the simple case of a zero index.
@@ -2491,7 +2494,7 @@ Constant *llvm::ConstantFoldGetElementPtr(Type *PointeeTy, Constant *C,
 
  if (ConstantExpr *CE = dyn_cast<ConstantExpr>(C)) {
  if (auto *GEP = dyn_cast<GEPOperator>(CE))
- if (Constant *C = foldGEPOfGEP(GEP, InBounds, Idxs))
+ if (Constant *C = foldGEPOfGEP(GEP, PointeeTy, InBounds, Idxs))
  return C;
 
  // Attempt to fold casts to the same type away. For example, folding:

llvm/test/Other/force-opaque-ptrs.ll

@@ -64,6 +64,13 @@ define void @remangle_intrinsic() {
  ret void
 }
 
+define i32* @constexpr_gep() {
+; CHECK-LABEL: define {{[^@]+}}@constexpr_gep() {
+; CHECK-NEXT: ret ptr getelementptr (i32, ptr getelementptr (i8, ptr null, i64 4), i64 1)
+;
+ ret i32* getelementptr(i32, i32* bitcast (i8* getelementptr (i8, i8* null, i64 4) to i32*), i64 1)
+}
+
 declare i8* @llvm.stacksave()
 declare void @llvm.stackprotector(i8*, i8**)
 declare <2 x i64> @llvm.masked.expandload.v2i64(i64*, <2 x i1>, <2 x i64>)