[ValueTracking] When calculating known bits for integer abs, make sure we're looking...

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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@368195 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/Analysis/ValueTracking.cpp b/lib/Analysis/ValueTracking.cpp
index 343e738db813bf95b4ce81dbd0f41edb4839efb7..c09530cd697a14d4873f7eef29cb397786577cbf 100644 (file)
--- a/lib/Analysis/ValueTracking.cpp
+++ b/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);
 

diff --git a/test/Transforms/InstSimplify/icmp-abs-nabs.ll b/test/Transforms/InstSimplify/icmp-abs-nabs.ll
index 5f38194fe33c96cc7ecc182e937fadbf72a01b13..31f71cba84fd31642b8404a7e9e12553ab3017f0 100644 (file)
--- a/test/Transforms/InstSimplify/icmp-abs-nabs.ll
+++ b/test/Transforms/InstSimplify/icmp-abs-nabs.ll
@@ -404,7 +404,12 @@ define i1 @nabs_no_intersection(i32 %a) {
 ; We can't fold this to false unless both subs have nsw.
 define i1 @abs_sub_sub_missing_nsw(i32 %x, i32 %y) {
 ; CHECK-LABEL: @abs_sub_sub_missing_nsw(
-; CHECK-NEXT:    ret i1 false
+; CHECK-NEXT:    [[A:%.*]] = sub i32 [[X:%.*]], [[Y:%.*]]
+; CHECK-NEXT:    [[B:%.*]] = sub nsw i32 [[Y]], [[X]]
+; CHECK-NEXT:    [[C:%.*]] = icmp sgt i32 [[A]], -1
+; CHECK-NEXT:    [[D:%.*]] = select i1 [[C]], i32 [[A]], i32 [[B]]
+; CHECK-NEXT:    [[E:%.*]] = icmp slt i32 [[D]], 0
+; CHECK-NEXT:    ret i1 [[E]]
 ;
   %a = sub i32 %x, %y
   %b = sub nsw i32 %y, %x