[AArch64] Set COPY ZR isAsCheapAsAMove when needed.

If a subtarget has both ZCZeroing and CustomCheapAsMoveHandling features (now
only Kryo has both), set COPY (W|X)ZR isAsCheapAsAMove.

Differential Revision: http://reviews.llvm.org/D22360

llvm-svn: 275503

1 files changed, 6 insertions, 2 deletions

diff --git a/llvm/lib/Target/AArch64/AArch64InstrInfo.cpp b/llvm/lib/Target/AArch64/AArch64InstrInfo.cpp
index 90d7df07f00..04fe3af1681 100644
--- a/llvm/lib/Target/AArch64/AArch64InstrInfo.cpp
+++ b/llvm/lib/Target/AArch64/AArch64InstrInfo.cpp
@@ -624,11 +624,15 @@ bool AArch64InstrInfo::isAsCheapAsAMove(const MachineInstr &MI) const {
   case AArch64::MOVi64imm:
     return canBeExpandedToORR(MI, 64);
 
-  // It is cheap to move #0 to float registers if the subtarget has 
-  // ZeroCycleZeroing feature.
+  // It is cheap to zero out registers if the subtarget has ZeroCycleZeroing
+  // feature.
   case AArch64::FMOVS0:
   case AArch64::FMOVD0:
     return Subtarget.hasZeroCycleZeroing();
+  case TargetOpcode::COPY:
+    return (Subtarget.hasZeroCycleZeroing() &&
+            (MI.getOperand(1).getReg() == AArch64::WZR ||
+             MI.getOperand(1).getReg() == AArch64::XZR));
   }
 
   llvm_unreachable("Unknown opcode to check as cheap as a move!");