[PATCH] D152088: [InstCombine] add nsw on InstCombineAdd ~X + C --> (C-1) - X

[Kohei Asano via Phabricator via llvm-commits]

khei4 updated this revision to Diff 528180.
khei4 added a comment.

Rebase for pre-commit test


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D152088/new/

https://reviews.llvm.org/D152088

Files:
  llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
  llvm/test/Transforms/InstCombine/add.ll


Index: llvm/test/Transforms/InstCombine/add.ll
===================================================================
--- llvm/test/Transforms/InstCombine/add.ll
+++ llvm/test/Transforms/InstCombine/add.ll
@@ -360,7 +360,7 @@
 ; ~X + -127 and (-128) - X with nsw are equally poisonous
 define i8 @test18_nsw(i8 %A) {
 ; CHECK-LABEL: @test18_nsw(
-; CHECK-NEXT:    [[C:%.*]] = sub i8 -128, [[A:%.*]]
+; CHECK-NEXT:    [[C:%.*]] = sub nsw i8 -128, [[A:%.*]]
 ; CHECK-NEXT:    ret i8 [[C]]
 ;
   %B = xor i8 %A, -1
Index: llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
===================================================================
--- llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
+++ llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
@@ -880,8 +880,15 @@
     return SelectInst::Create(X, InstCombiner::SubOne(Op1C), Op1);
 
   // ~X + C --> (C-1) - X
-  if (match(Op0, m_Not(m_Value(X))))
-    return BinaryOperator::CreateSub(InstCombiner::SubOne(Op1C), X);
+  if (match(Op0, m_Not(m_Value(X)))) {
+    // ~X + C has NSW and (C-1) won't oveflow => (C-1)-X can have NSW
+    auto *COne = ConstantInt::get(Op1C->getType(), 1);
+    bool WillNotSOV = willNotOverflowSignedSub(Op1C, COne, Add);
+    BinaryOperator *Res =
+        BinaryOperator::CreateSub(ConstantExpr::getSub(Op1C, COne), X);
+    Res->setHasNoSignedWrap(Add.hasNoSignedWrap() && WillNotSOV);
+    return Res;
+  }
 
   // (iN X s>> (N - 1)) + 1 --> zext (X > -1)
   const APInt *C;


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