Use intersectForOffsetAdd for final GEP

Author: nikic

llvm/lib/Transforms/InstCombine/InstructionCombining.cpp

@@ -2681,7 +2681,7 @@ static Instruction *combineConstantOffsets(GetElementPtrInst &GEP,
   if (!GEP.hasAllConstantIndices())
     return nullptr;
 
-  GEPNoWrapFlags NW = GEP.getNoWrapFlags();
+  GEPNoWrapFlags NW = GEPNoWrapFlags::all();
   SmallVector<GetElementPtrInst *> Skipped;
   auto *InnerGEP = dyn_cast<GetElementPtrInst>(GEP.getPointerOperand());
   while (true) {
@@ -2725,8 +2725,9 @@ static Instruction *combineConstantOffsets(GetElementPtrInst &GEP,
     SkippedGEP->setNoWrapFlags(NW);
 
   return IC.replaceInstUsesWith(
-      GEP, IC.Builder.CreatePtrAdd(Skipped.front(), IC.Builder.getInt(Offset),
-                                   "", NW));
+      GEP,
+      IC.Builder.CreatePtrAdd(Skipped.front(), IC.Builder.getInt(Offset), "",
+                              NW.intersectForOffsetAdd(GEP.getNoWrapFlags())));
 }
 
 Instruction *InstCombinerImpl::visitGEPOfGEP(GetElementPtrInst &GEP,

llvm/test/Transforms/InstCombine/gep-canonicalize-constant-indices.ll

@@ -147,10 +147,12 @@ define ptr @merge_nuw_inbounds(ptr %p, i64 %a) {
   ret ptr %gep3
 }
 
+; It would be okay to preserve nusw here, as the constant addition does not
+; overflow.
 define ptr @merge_nuw_nusw(ptr %p, i64 %a) {
 ; CHECK-LABEL: @merge_nuw_nusw(
 ; CHECK-NEXT:    [[GEP2:%.*]] = getelementptr nusw nuw i32, ptr [[P:%.*]], i64 [[A:%.*]]
-; CHECK-NEXT:    [[GEP3:%.*]] = getelementptr nusw nuw i8, ptr [[GEP2]], i64 5
+; CHECK-NEXT:    [[GEP3:%.*]] = getelementptr nuw i8, ptr [[GEP2]], i64 5
 ; CHECK-NEXT:    ret ptr [[GEP3]]
 ;
   %gep1 = getelementptr nusw nuw i8, ptr %p, i64 1
@@ -159,6 +161,19 @@ define ptr @merge_nuw_nusw(ptr %p, i64 %a) {
   ret ptr %gep3
 }
 
+; Can't preserve nusw on the final GEP
+define ptr @merge_nuw_nusw_overflow(ptr %p, i64 %a) {
+; CHECK-LABEL: @merge_nuw_nusw_overflow(
+; CHECK-NEXT:    [[GEP2:%.*]] = getelementptr nusw nuw i32, ptr [[P:%.*]], i64 [[A:%.*]]
+; CHECK-NEXT:    [[GEP3:%.*]] = getelementptr nuw i8, ptr [[GEP2]], i64 -2305843009213693952
+; CHECK-NEXT:    ret ptr [[GEP3]]
+;
+  %gep1 = getelementptr nusw nuw i8, ptr %p, i64 u0x7000000000000000
+  %gep2 = getelementptr nusw nuw i32, ptr %gep1, i64 %a
+  %gep3 = getelementptr nusw nuw i8, ptr %gep2, i64 u0x7000000000000000
+  ret ptr %gep3
+}
+
 define ptr @merge_missing_nuw1(ptr %p, i64 %a) {
 ; CHECK-LABEL: @merge_missing_nuw1(
 ; CHECK-NEXT:    [[GEP2:%.*]] = getelementptr i32, ptr [[P:%.*]], i64 [[A:%.*]]
@@ -185,7 +200,7 @@ define ptr @merge_missing_nuw2(ptr %p, i64 %a) {
 
 define ptr @merge_missing_nuw3(ptr %p, i64 %a) {
 ; CHECK-LABEL: @merge_missing_nuw3(
-; CHECK-NEXT:    [[GEP2:%.*]] = getelementptr i32, ptr [[P:%.*]], i64 [[A:%.*]]
+; CHECK-NEXT:    [[GEP2:%.*]] = getelementptr nuw i32, ptr [[P:%.*]], i64 [[A:%.*]]
 ; CHECK-NEXT:    [[GEP3:%.*]] = getelementptr i8, ptr [[GEP2]], i64 5
 ; CHECK-NEXT:    ret ptr [[GEP3]]
 ;