[ValueTracking] When calculating known bits for integer abs, make sure we're looking at a negate and not just any instruction with the nsw flag set.

The matchSelectPattern code can match patterns like (x >= 0) ? x : -x
for absolute value. But it can also match ((x-y) >= 0) ? (x-y) : (y-x).
If the latter form was matched we can only use the nsw flag if its
set on both subtracts.

This match makes sure we're looking at the former case only.

Differential Revision: https://reviews.llvm.org/D65692

llvm-svn: 368195

1 files changed, 6 insertions, 4 deletions

diff --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp
index 343e738db81..c09530cd697 100644
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -1095,7 +1095,8 @@ static void computeKnownBitsFromOperator(const Operator *I, KnownBits &Known,
       // RHS from matchSelectPattern returns the negation part of abs pattern.
       // If the negate has an NSW flag we can assume the sign bit of the result
       // will be 0 because that makes abs(INT_MIN) undefined.
-      if (Q.IIQ.hasNoSignedWrap(cast<Instruction>(RHS)))
+      if (match(RHS, m_Neg(m_Specific(LHS))) &&
+          Q.IIQ.hasNoSignedWrap(cast<Instruction>(RHS)))
         MaxHighZeros = 1;
     }
 
@@ -5611,7 +5612,7 @@ static void setLimitsForIntrinsic(const IntrinsicInst &II, APInt &Lower,
 }
 
 static void setLimitsForSelectPattern(const SelectInst &SI, APInt &Lower,
-                                      APInt &Upper) {
+                                      APInt &Upper, const InstrInfoQuery &IIQ) {
   const Value *LHS, *RHS;
   SelectPatternResult R = matchSelectPattern(&SI, LHS, RHS);
   if (R.Flavor == SPF_UNKNOWN)
@@ -5624,7 +5625,8 @@ static void setLimitsForSelectPattern(const SelectInst &SI, APInt &Lower,
     // then the result of abs(X) is [0..SIGNED_MAX],
     // otherwise it is [0..SIGNED_MIN], as -SIGNED_MIN == SIGNED_MIN.
     Lower = APInt::getNullValue(BitWidth);
-    if (cast<Instruction>(RHS)->hasNoSignedWrap())
+    if (match(RHS, m_Neg(m_Specific(LHS))) &&
+        IIQ.hasNoSignedWrap(cast<Instruction>(RHS)))
       Upper = APInt::getSignedMaxValue(BitWidth) + 1;
     else
       Upper = APInt::getSignedMinValue(BitWidth) + 1;
@@ -5678,7 +5680,7 @@ ConstantRange llvm::computeConstantRange(const Value *V, bool UseInstrInfo) {
   else if (auto *II = dyn_cast<IntrinsicInst>(V))
     setLimitsForIntrinsic(*II, Lower, Upper);
   else if (auto *SI = dyn_cast<SelectInst>(V))
-    setLimitsForSelectPattern(*SI, Lower, Upper);
+    setLimitsForSelectPattern(*SI, Lower, Upper, IIQ);
 
   ConstantRange CR = ConstantRange::getNonEmpty(Lower, Upper);