[X86] Don't select (cmp (and, imm), 0) to testw

Summary:
X86ISelDAGToDAG tries to analyze ANDs compared with 0 to optimize to narrower immediates using subregisters.

I don't think we should be optimizing to 16-bit test instructions. It goes against our normal behavior of promoting i16 operations to i32. It only saves one byte due to the need to add a 0x66 prefix. I think it would also be subject to a length changing prefix penalty in the decoders on Intel CPUs.

Reviewers: RKSimon, zvi, spatel

Reviewed By: spatel

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D38273

llvm-svn: 314474

1 files changed, 4 insertions, 1 deletions

diff --git a/llvm/lib/Target/X86/X86ISelDAGToDAG.cpp b/llvm/lib/Target/X86/X86ISelDAGToDAG.cpp
index 84b1f9624ad..eb0107a5fa4 100644
--- a/llvm/lib/Target/X86/X86ISelDAGToDAG.cpp
+++ b/llvm/lib/Target/X86/X86ISelDAGToDAG.cpp
@@ -3025,7 +3025,10 @@ void X86DAGToDAGISel::Select(SDNode *Node) {
       }
 
       // For example, "testl %eax, $32776" to "testw %ax, $32776".
-      if (isUInt<16>(Mask) && N0.getValueType() != MVT::i16 &&
+      // NOTE: We only want to form TESTW instructions if optimizing for
+      // min size. Otherwise we only save one byte and possibly get a length
+      // changing prefix penalty in the decoders.
+      if (OptForMinSize && isUInt<16>(Mask) && N0.getValueType() != MVT::i16 &&
           (!(Mask & 0x8000) || hasNoSignedComparisonUses(Node))) {
         SDValue Imm = CurDAG->getTargetConstant(Mask, dl, MVT::i16);
         SDValue Reg = N0.getOperand(0);