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

[Kohei Asano via Phabricator via llvm-commits]

khei4 created this revision.
khei4 added reviewers: nikic, goldstein.w.n, spatel.
Herald added subscribers: StephenFan, hiraditya.
Herald added a project: All.
khei4 requested review of this revision.
Herald added a project: LLVM.
Herald added a subscriber: llvm-commits.

On transform `~X + C --> (C-1) - X`, we can safely attach NSW to (C-1) - X if the 1,2 are satisfied

1. `~X + C` has NSW
2. `C-1` won't signed overflow

proof: https://alive2.llvm.org/ce/z/DXXaoC

This will resolve https://reviews.llvm.org/D146903 's InstCombine part. https://alive2.llvm.org/ce/z/-_FdJU


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
@@ -357,6 +357,28 @@
   ret i8 %C
 }
 
+; ~X + -127 and (-128) - X with nsw are equally poisonous
+define i8 @test18_nsw(i8 %A) {
+; CHECK-LABEL: @test18_nsw(
+; CHECK-NEXT:    [[C:%.*]] = sub nsw i8 -128, [[A:%.*]]
+; CHECK-NEXT:    ret i8 [[C]]
+;
+  %B = xor i8 %A, -1
+  %C = add nsw i8 %B, -127
+  ret i8 %C
+}
+
+; 127 - X with nsw will be more poisonous than ~X + -128 with nsw. (see X = -1)
+define i8 @test18_nsw_overflow(i8 %A) {
+; CHECK-LABEL: @test18_nsw_overflow(
+; CHECK-NEXT:    [[C:%.*]] = sub i8 127, [[A:%.*]]
+; CHECK-NEXT:    ret i8 [[C]]
+;
+  %B = xor i8 %A, -1
+  %C = add nsw i8 %B, -128
+  ret i8 %C
+}
+
 define <2 x i64> @test18vec(<2 x i64> %A) {
 ; CHECK-LABEL: @test18vec(
 ; CHECK-NEXT:    [[ADD:%.*]] = sub <2 x i64> <i64 1, i64 2>, [[A:%.*]]
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;


-------------- next part --------------
A non-text attachment was scrubbed...
Name: D152088.528178.patch
Type: text/x-patch
Size: 1941 bytes
Desc: not available
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20230604/1e318f58/attachment.bin>