[InstCombine] Fold getelementptr inbounds null, idx -> null

Author: dtcxzyw

llvm/lib/Transforms/InstCombine/InstructionCombining.cpp

@@ -2911,10 +2911,12 @@ Instruction *InstCombinerImpl::visitGetElementPtrInst(GetElementPtrInst &GEP) {
                           SQ.getWithInstruction(&GEP)))
     return replaceInstUsesWith(GEP, V);
 
-  // // getelementptr inbounds null, idx -> null
-  // if (auto *BaseC = dyn_cast<Constant>(PtrOp))
-  //   if (GEP.isInBounds() && BaseC->isNullValue() && !NullPointerIsDefined(GEP.getFunction(), GEPType->getPointerAddressSpace()))
-  //     return replaceInstUsesWith(GEP, PtrOp);
+  // getelementptr inbounds null, idx -> null
+  if (auto *BaseC = dyn_cast<Constant>(PtrOp))
+    if (GEP.isInBounds() && BaseC->isNullValue() &&
+        !NullPointerIsDefined(GEP.getFunction(),
+                              GEPType->getPointerAddressSpace()))
+      return replaceInstUsesWith(GEP, Constant::getNullValue(GEPType));
 
   // For vector geps, use the generic demanded vector support.
   // Skip if GEP return type is scalable. The number of elements is unknown at

llvm/test/Transforms/InstCombine/getelementptr.ll

@@ -1328,8 +1328,7 @@ define ptr @PR45084_extra_use(i1 %cond, ptr %p) {
 
 define ptr @gep_null_inbounds(i64 %idx) {
 ; CHECK-LABEL: @gep_null_inbounds(
-; CHECK-NEXT:    [[GEP:%.*]] = getelementptr inbounds i8, ptr null, i64 [[IDX:%.*]]
-; CHECK-NEXT:    ret ptr [[GEP]]
+; CHECK-NEXT:    ret ptr null
 ;
   %gep = getelementptr inbounds i8, ptr null, i64 %idx
   ret ptr %gep
@@ -1355,8 +1354,7 @@ define ptr @gep_null_defined(i64 %idx) null_pointer_is_valid {
 
 define ptr @gep_null_inbounds_different_type(i64 %idx1, i64 %idx2) {
 ; CHECK-LABEL: @gep_null_inbounds_different_type(
-; CHECK-NEXT:    [[GEP:%.*]] = getelementptr inbounds [0 x i8], ptr null, i64 0, i64 [[IDX2:%.*]]
-; CHECK-NEXT:    ret ptr [[GEP]]
+; CHECK-NEXT:    ret ptr null
 ;
   %gep = getelementptr inbounds [0 x i8], ptr null, i64 %idx1, i64 %idx2
   ret ptr %gep
@@ -2021,17 +2019,15 @@ define ptr @gep_merge_nusw_const(ptr %p, i64 %idx, i64 %idx2) {
 
 define <2 x ptr> @gep_inbounds_null_vec(i64 %idx) {
 ; CHECK-LABEL: @gep_inbounds_null_vec(
-; CHECK-NEXT:    [[P:%.*]] = getelementptr inbounds i8, <2 x ptr> zeroinitializer, i64 [[IDX:%.*]]
-; CHECK-NEXT:    ret <2 x ptr> [[P]]
+; CHECK-NEXT:    ret <2 x ptr> zeroinitializer
 ;
   %p = getelementptr inbounds i8, <2 x ptr> zeroinitializer, i64 %idx
   ret <2 x ptr> %p
 }
 
 define <2 x ptr> @gep_inbounds_null_vec_broadcast(<2 x i64> %idx) {
 ; CHECK-LABEL: @gep_inbounds_null_vec_broadcast(
-; CHECK-NEXT:    [[P:%.*]] = getelementptr inbounds i8, ptr null, <2 x i64> [[IDX:%.*]]
-; CHECK-NEXT:    ret <2 x ptr> [[P]]
+; CHECK-NEXT:    ret <2 x ptr> zeroinitializer
 ;
   %p = getelementptr inbounds i8, ptr null, <2 x i64> %idx
   ret <2 x ptr> %p

llvm/test/Transforms/InstCombine/store.ll

@@ -49,8 +49,7 @@ define void @test2(ptr %P) {
 
 define void @store_at_gep_off_null_inbounds(i64 %offset) {
 ; CHECK-LABEL: @store_at_gep_off_null_inbounds(
-; CHECK-NEXT:    [[PTR:%.*]] = getelementptr inbounds i32, ptr null, i64 [[OFFSET:%.*]]
-; CHECK-NEXT:    store i32 poison, ptr [[PTR]], align 4
+; CHECK-NEXT:    store i32 poison, ptr null, align 4
 ; CHECK-NEXT:    ret void
 ;
   %ptr = getelementptr inbounds i32, ptr null, i64 %offset