[ValueTracking] Use ConstantRange based overflow check for signed sub

This is D59450, but for signed sub. This case is not NFC, because
the overflow logic in ConstantRange is more powerful than the existing
check. This resolves the TODO in the function.

I've added two tests to show that this indeed catches more cases than
the previous logic, but the main correctness test coverage here is in
the existing ConstantRange unit tests.

Differential Revision: https://reviews.llvm.org/D59617

llvm-svn: 356685

1 files changed, 5 insertions, 10 deletions

diff --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp
index 52328b1..e31ddf0 100644
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -4190,17 +4190,12 @@ OverflowResult llvm::computeOverflowForSignedSub(const Value *LHS,
     return OverflowResult::NeverOverflows;
 
   KnownBits LHSKnown = computeKnownBits(LHS, DL, 0, AC, CxtI, DT);
-
   KnownBits RHSKnown = computeKnownBits(RHS, DL, 0, AC, CxtI, DT);
-
-  // Subtraction of two 2's complement numbers having identical signs will
-  // never overflow.
-  if ((LHSKnown.isNegative() && RHSKnown.isNegative()) ||
-      (LHSKnown.isNonNegative() && RHSKnown.isNonNegative()))
-    return OverflowResult::NeverOverflows;
-
-  // TODO: implement logic similar to checkRippleForAdd
-  return OverflowResult::MayOverflow;
+  ConstantRange LHSRange =
+      ConstantRange::fromKnownBits(LHSKnown, /*signed*/ true);
+  ConstantRange RHSRange =
+      ConstantRange::fromKnownBits(RHSKnown, /*signed*/ true);
+  return mapOverflowResult(LHSRange.signedSubMayOverflow(RHSRange));
 }
 
 bool llvm::isOverflowIntrinsicNoWrap(const IntrinsicInst *II,