[ValueTracking] Use computeConstantRange() for signed sub overflow determination

This is the same change as D60420 but for signed sub rather than
signed add: Range information is intersected into the known bits
result, allows to detect more no/always overflow conditions.

Differential Revision: https://reviews.llvm.org/D60469

llvm-svn: 358020

1 files changed, 4 insertions, 6 deletions

diff --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp
index 1407f24e93b..944de24a5cb 100644
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -4192,12 +4192,10 @@ OverflowResult llvm::computeOverflowForSignedSub(const Value *LHS,
       ComputeNumSignBits(RHS, DL, 0, AC, CxtI, DT) > 1)
     return OverflowResult::NeverOverflows;
 
-  KnownBits LHSKnown = computeKnownBits(LHS, DL, 0, AC, CxtI, DT);
-  KnownBits RHSKnown = computeKnownBits(RHS, DL, 0, AC, CxtI, DT);
-  ConstantRange LHSRange =
-      ConstantRange::fromKnownBits(LHSKnown, /*signed*/ true);
-  ConstantRange RHSRange =
-      ConstantRange::fromKnownBits(RHSKnown, /*signed*/ true);
+  ConstantRange LHSRange = computeConstantRangeIncludingKnownBits(
+      LHS, /*ForSigned=*/true, DL, /*Depth=*/0, AC, CxtI, DT);
+  ConstantRange RHSRange = computeConstantRangeIncludingKnownBits(
+      RHS, /*ForSigned=*/true, DL, /*Depth=*/0, AC, CxtI, DT);
   return mapOverflowResult(LHSRange.signedSubMayOverflow(RHSRange));
 }