[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 528171.
khei4 added a comment.

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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
@@ -637,3 +637,13 @@
   %r = add i8 %sub, %y
   ret i8 %r
 }
+
+define i8 @add_sub_nsw(i8 %x, i8 %y) {
+; CHECK-LABEL: @add_sub_nsw(
+; CHECK-NEXT:    [[B:%.*]] = sub nsw i8 -11, [[X:%.*]]
+; CHECK-NEXT:    ret i8 [[B]]
+;
+  %a = add nsw i8 %x, 12; a = x + 12
+  %b = sub nsw i8 1, %a; b = 0 - a = (12 - 1 -  x)
+  ret i8 %b
+}
Index: llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
===================================================================
--- llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
+++ llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
@@ -1995,8 +1995,16 @@
     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, C-(X+C2), (X+C2) never overflow=> (C-C2)-X never overflow
+      bool HasNSW = willNotOverflowSignedSub(C, C2, I);
+      BinaryOperator *Res = BinaryOperator::CreateSub(
+          ConstantExpr::getSub(C, C2, /*HasNUW=*/false, HasNSW), X);
+      auto *OBO1 = cast<OverflowingBinaryOperator>(Op1);
+      if (I.hasNoSignedWrap() && OBO1->hasNoSignedWrap() && HasNSW)
+        Res->setHasNoSignedWrap(true);
+      return Res;
+    }
   }
 
   auto TryToNarrowDeduceFlags = [this, &I, &Op0, &Op1]() -> Instruction * {


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