[PATCH] D152068: [InstCombine] add overflow checking on AddSub `C-(X+C2) --> (C-C2)-X`

[Kohei Asano via Phabricator via llvm-commits]

khei4 updated this revision to Diff 528184.
khei4 edited the summary of this revision.
khei4 added a comment.

Apply feedback from @goldstein.w.n


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D152068/new/

https://reviews.llvm.org/D152068

Files:
  llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
  llvm/test/Transforms/InstCombine/addsub-constant-folding.ll


Index: llvm/test/Transforms/InstCombine/addsub-constant-folding.ll
===================================================================
--- llvm/test/Transforms/InstCombine/addsub-constant-folding.ll
+++ llvm/test/Transforms/InstCombine/addsub-constant-folding.ll
@@ -134,7 +134,7 @@
 
 define i32 @add_nsw_const_const_sub_nsw(i32 %arg) {
 ; CHECK-LABEL: @add_nsw_const_const_sub_nsw(
-; CHECK-NEXT:    [[T1:%.*]] = sub i32 -2147483648, [[ARG:%.*]]
+; CHECK-NEXT:    [[T1:%.*]] = sub nsw i32 -2147483648, [[ARG:%.*]]
 ; CHECK-NEXT:    ret i32 [[T1]]
 ;
   %t0 = add nsw i32 %arg, 1
Index: llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
===================================================================
--- llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
+++ llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
@@ -1995,8 +1995,17 @@
     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 * {


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