[InstCombine] Check inbounds in load/store of gep null transform (PR48577)

If the GEP isn't inbounds, then accessing a GEP of null location
is generally not UB.

While this is a minimal fix, the GEP of null handling should
probably be its own fold.

Author: nikic

llvm/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp

@@ -908,15 +908,16 @@ static bool canSimplifyNullStoreOrGEP(StoreInst &SI) {
 
   auto *Ptr = SI.getPointerOperand();
   if (GetElementPtrInst *GEPI = dyn_cast<GetElementPtrInst>(Ptr))
-    Ptr = GEPI->getOperand(0);
+    if (GEPI->isInBounds())
+      Ptr = GEPI->getOperand(0);
   return (isa<ConstantPointerNull>(Ptr) &&
           !NullPointerIsDefined(SI.getFunction(), SI.getPointerAddressSpace()));
 }
 
 static bool canSimplifyNullLoadOrGEP(LoadInst &LI, Value *Op) {
   if (GetElementPtrInst *GEPI = dyn_cast<GetElementPtrInst>(Op)) {
     const Value *GEPI0 = GEPI->getOperand(0);
-    if (isa<ConstantPointerNull>(GEPI0) &&
+    if (isa<ConstantPointerNull>(GEPI0) && GEPI->isInBounds() &&
         !NullPointerIsDefined(LI.getFunction(), GEPI->getPointerAddressSpace()))
       return true;
   }

llvm/test/Transforms/InstCombine/load.ll

@@ -69,8 +69,9 @@ define i32 @load_gep_null_inbounds(i64 %X) {
 
 define i32 @load_gep_null_not_inbounds(i64 %X) {
 ; CHECK-LABEL: @load_gep_null_not_inbounds(
-; CHECK-NEXT:    store i32 undef, i32* null, align 536870912
-; CHECK-NEXT:    ret i32 undef
+; CHECK-NEXT:    [[V:%.*]] = getelementptr i32, i32* null, i64 [[X:%.*]]
+; CHECK-NEXT:    [[R:%.*]] = load i32, i32* [[V]], align 4
+; CHECK-NEXT:    ret i32 [[R]]
 ;
   %V = getelementptr i32, i32* null, i64 %X
   %R = load i32, i32* %V

llvm/test/Transforms/InstCombine/store.ll

@@ -37,7 +37,7 @@ define void @store_at_gep_off_null_inbounds(i64 %offset) {
 define void @store_at_gep_off_null_not_inbounds(i64 %offset) {
 ; CHECK-LABEL: @store_at_gep_off_null_not_inbounds(
 ; CHECK-NEXT:    [[PTR:%.*]] = getelementptr i32, i32* null, i64 [[OFFSET:%.*]]
-; CHECK-NEXT:    store i32 undef, i32* [[PTR]], align 4
+; CHECK-NEXT:    store i32 24, i32* [[PTR]], align 4
 ; CHECK-NEXT:    ret void
 ;
   %ptr = getelementptr i32, i32 *null, i64 %offset