[ValueTracking] Improve ValueTracking on left shift with nsw flag

Patch by Li Huang

Differential Revison: https://reviews.llvm.org/D23296

llvm-svn: 278172

1 files changed, 13 insertions, 4 deletions

diff --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp
index c3a5e6af62b..59c39e5da9f 100644
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -1069,13 +1069,22 @@ static void computeKnownBitsFromOperator(Operator *I, APInt &KnownZero,
   }
   case Instruction::Shl: {
     // (shl X, C1) & C2 == 0   iff   (X & C2 >>u C1) == 0
-    auto KZF = [BitWidth](const APInt &KnownZero, unsigned ShiftAmt) {
-      return (KnownZero << ShiftAmt) |
+    bool NSW = cast<OverflowingBinaryOperator>(I)->hasNoSignedWrap();
+    auto KZF = [BitWidth, NSW](const APInt &KnownZero, unsigned ShiftAmt) {
+      APInt KZResult = (KnownZero << ShiftAmt) |
              APInt::getLowBitsSet(BitWidth, ShiftAmt); // Low bits known 0.
+      // If this shift has "nsw" keyword, then the result is either a poison 
+      // value or has the same sign bit as the first operand.
+      if (NSW && KnownZero.isNegative())
+        KZResult.setBit(BitWidth - 1);
+      return KZResult;
     };
 
-    auto KOF = [BitWidth](const APInt &KnownOne, unsigned ShiftAmt) {
-      return KnownOne << ShiftAmt;
+    auto KOF = [BitWidth, NSW](const APInt &KnownOne, unsigned ShiftAmt) {
+      APInt KOResult = KnownOne << ShiftAmt;
+      if (NSW && KnownOne.isNegative())
+        KOResult.setBit(BitWidth - 1);
+      return KOResult;
     };
 
     computeKnownBitsFromShiftOperator(I, KnownZero, KnownOne,