[InstCombine] add overflow checking on AddSub C-(X+C2) --> (C-C2)-X

Differential Revision: https://reviews.llvm.org/D152068
llvm · Jun 5, 2023 · 4db8d4f · 4db8d4f
1 parent 41588b5
commit 4db8d4f
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Showing 2 changed files with 15 additions and 7 deletions.
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp b/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
@@ -1995,8 +1995,17 @@ Instruction *InstCombinerImpl::visitSub(BinaryOperator &I) {
     Constant *C2;
 
     // C-(X+C2) --> (C-C2)-X
-    if (match(Op1, m_Add(m_Value(X), m_ImmConstant(C2))))
-      return BinaryOperator::CreateSub(ConstantExpr::getSub(C, C2), X);
+    if (match(Op1, m_Add(m_Value(X), m_ImmConstant(C2)))) {
+      // C-C2 never overflow, and C-(X+C2), (X+C2) has NSW
+      // => (C-C2)-X can have NSW
+      bool WillNotSOV = willNotOverflowSignedSub(C, C2, I);
+      BinaryOperator *Res =
+          BinaryOperator::CreateSub(ConstantExpr::getSub(C, C2), X);
+      auto *OBO1 = cast<OverflowingBinaryOperator>(Op1);
+      Res->setHasNoSignedWrap(I.hasNoSignedWrap() && OBO1->hasNoSignedWrap() &&
+                              WillNotSOV);
+      return Res;
+    }
   }
 
   auto TryToNarrowDeduceFlags = [this, &I, &Op0, &Op1]() -> Instruction * {

diff --git a/llvm/test/Transforms/InstCombine/addsub-constant-folding.ll b/llvm/test/Transforms/InstCombine/addsub-constant-folding.ll
@@ -134,7 +134,7 @@ define i32 @add_const_const_sub(i32 %arg) {
 
 define i8 @add_nsw_const_const_sub_nsw(i8 %arg) {
 ; CHECK-LABEL: @add_nsw_const_const_sub_nsw(
-; CHECK-NEXT:    [[T1:%.*]] = sub i8 -128, [[ARG:%.*]]
+; CHECK-NEXT:    [[T1:%.*]] = sub nsw i8 -128, [[ARG:%.*]]
 ; CHECK-NEXT:    ret i8 [[T1]]
 ;
   %t0 = add nsw i8 %arg, 1
@@ -193,14 +193,13 @@ define i8 @add_const_const_sub_nuw(i8 %arg) {
   ret i8 %t1
 }
 
-
 define <2 x i8> @non_splat_vec_add_nsw_const_const_sub_nsw_not_ov1(<2 x i8> %arg) {
 ; CHECK-LABEL: @non_splat_vec_add_nsw_const_const_sub_nsw_not_ov1(
-; CHECK-NEXT:    [[T1:%.*]] = sub <2 x i8> <i8 -2, i8 -126>, [[ARG:%.*]]
+; CHECK-NEXT:    [[T1:%.*]] = sub nsw <2 x i8> <i8 -127, i8 -126>, [[ARG:%.*]]
 ; CHECK-NEXT:    ret <2 x i8> [[T1]]
 ;
-  %t0 = add nsw <2 x i8> %arg, <i8 1, i8 0>
-  %t1 = sub nsw <2 x i8> <i8 -1, i8 -126>, %t0
+  %t0 = add nsw <2 x i8> %arg, <i8 2, i8 0>
+  %t1 = sub nsw <2 x i8> <i8 -125, i8 -126>, %t0
   ret <2 x i8> %t1
 }