[llvm] r321055 - [X86] Use ZERO_EXTEND instead of ANY_EXTEND when extending the shift amount for a non-uniform shift.

[Craig Topper via llvm-commits]

Author: ctopper
Date: Mon Dec 18 20:52:04 2017
New Revision: 321055

URL: http://llvm.org/viewvc/llvm-project?rev=321055&view=rev
Log:
[X86] Use ZERO_EXTEND instead of ANY_EXTEND when extending the shift amount for a non-uniform shift.

My reading of the SDM says that all bits of the shift amount are used. If the value of the element is larger than the number of bits the result the shift result is zero. So I think we need to zero_extend here to avoid garbage in the upper bits.

In reality we lower any_extend as zero_extend so in most cases it would be hard to hit this.

Modified:
    llvm/trunk/lib/Target/X86/X86ISelLowering.cpp

Modified: llvm/trunk/lib/Target/X86/X86ISelLowering.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Target/X86/X86ISelLowering.cpp?rev=321055&r1=321054&r2=321055&view=diff
==============================================================================
--- llvm/trunk/lib/Target/X86/X86ISelLowering.cpp (original)
+++ llvm/trunk/lib/Target/X86/X86ISelLowering.cpp Mon Dec 18 20:52:04 2017
@@ -22989,7 +22989,7 @@ static SDValue LowerShift(SDValue Op, co
     unsigned ExtOpc =
         Op.getOpcode() == ISD::SRA ? ISD::SIGN_EXTEND : ISD::ZERO_EXTEND;
     R = DAG.getNode(ExtOpc, dl, ExtVT, R);
-    Amt = DAG.getNode(ISD::ANY_EXTEND, dl, ExtVT, Amt);
+    Amt = DAG.getNode(ISD::ZERO_EXTEND, dl, ExtVT, Amt);
     return DAG.getNode(ISD::TRUNCATE, dl, VT,
                        DAG.getNode(Op.getOpcode(), dl, ExtVT, R, Amt));
   }