Fix a bug in the code which determines when it's safe to use the

bt instruction, which was exposed by r106263.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@106718 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 13 insertions, 7 deletions

diff --git a/lib/Target/X86/X86ISelLowering.cpp b/lib/Target/X86/X86ISelLowering.cpp
index bd95c85629..d64ac6ca22 100644
--- a/lib/Target/X86/X86ISelLowering.cpp
+++ b/lib/Target/X86/X86ISelLowering.cpp
@@ -6175,15 +6175,21 @@ SDValue X86TargetLowering::LowerToBT(SDValue And, ISD::CondCode CC,
     Op1 = Op1.getOperand(0);
 
   SDValue LHS, RHS;
-  if (Op1.getOpcode() == ISD::SHL) {
-    if (ConstantSDNode *And10C = dyn_cast<ConstantSDNode>(Op1.getOperand(0)))
-      if (And10C->getZExtValue() == 1) {
-        LHS = Op0;
-        RHS = Op1.getOperand(1);
-      }
-  } else if (Op0.getOpcode() == ISD::SHL) {
+  if (Op1.getOpcode() == ISD::SHL)
+    std::swap(Op0, Op1);
+  if (Op0.getOpcode() == ISD::SHL) {
     if (ConstantSDNode *And00C = dyn_cast<ConstantSDNode>(Op0.getOperand(0)))
       if (And00C->getZExtValue() == 1) {
+        // If we looked past a truncate, check that it's only truncating away
+        // known zeros.
+        unsigned BitWidth = Op0.getValueSizeInBits();
+        unsigned AndBitWidth = And.getValueSizeInBits();
+        if (BitWidth > AndBitWidth) {
+          APInt Mask = APInt::getAllOnesValue(BitWidth), Zeros, Ones;
+          DAG.ComputeMaskedBits(Op0, Mask, Zeros, Ones);
+          if (Zeros.countLeadingOnes() < BitWidth - AndBitWidth)
+            return SDValue();
+        }
         LHS = Op1;
         RHS = Op0.getOperand(1);
       }