Teach ValueTracking to look at the dividend when determining the sign bit of an

srem instruction.

llvm-svn: 126637

1 files changed, 12 insertions, 0 deletions

diff --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp
index 1060bc5349e..fac0407da52 100644
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -460,6 +460,18 @@ void llvm::ComputeMaskedBits(Value *V, const APInt &Mask,
         assert((KnownZero & KnownOne) == 0 && "Bits known to be one AND zero?"); 
       }
     }
+    if (Mask.isNegative()) {  // We're looking for the sign bit.
+      APInt Mask2 = APInt::getSignBit(BitWidth);
+      KnownZero2 = 0;
+      KnownOne2 = 0;
+      ComputeMaskedBits(I->getOperand(0), Mask2, KnownZero2, KnownOne2, TD, 
+                        Depth+1);
+      if (KnownOne2[BitWidth-1])
+        KnownOne |= Mask2;
+      if (KnownZero2[BitWidth-1])
+        KnownZero |= Mask2;
+      assert((KnownZero & KnownOne) == 0 && "Bits known to be one AND zero?");
+    }
     break;
   case Instruction::URem: {
     if (ConstantInt *Rem = dyn_cast<ConstantInt>(I->getOperand(1))) {