AArch64: use xzr/wzr for constant 0 in GlobalISel.

COPYs from xzr and wzr can often be folded away entirely during register
allocation, unlike a movz.

llvm-svn: 368003

1 files changed, 25 insertions, 0 deletions

diff --git a/llvm/lib/Target/AArch64/AArch64InstructionSelector.cpp b/llvm/lib/Target/AArch64/AArch64InstructionSelector.cpp
index a2f51415b75..59cfcca3157 100644
--- a/llvm/lib/Target/AArch64/AArch64InstructionSelector.cpp
+++ b/llvm/lib/Target/AArch64/AArch64InstructionSelector.cpp
@@ -1284,6 +1284,31 @@ bool AArch64InstructionSelector::earlySelect(MachineInstr &I) const {
     return earlySelectSHL(I, MRI);
   case TargetOpcode::G_LOAD:
     return earlySelectLoad(I, MRI);
+  case TargetOpcode::G_CONSTANT: {
+    bool IsZero = false;
+    if (I.getOperand(1).isCImm())
+      IsZero = I.getOperand(1).getCImm()->getZExtValue() == 0;
+    else if (I.getOperand(1).isImm())
+      IsZero = I.getOperand(1).getImm() == 0;
+
+    if (!IsZero)
+      return false;
+
+    Register DefReg = I.getOperand(0).getReg();
+    LLT Ty = MRI.getType(DefReg);
+    assert((Ty == LLT::scalar(64) || Ty == LLT::scalar(32)) &&
+           "Unexpected legal constant type");
+
+    if (Ty == LLT::scalar(64)) {
+      I.getOperand(1).ChangeToRegister(AArch64::XZR, false);
+      RBI.constrainGenericRegister(DefReg, AArch64::GPR64RegClass, MRI);
+    } else {
+      I.getOperand(1).ChangeToRegister(AArch64::WZR, false);
+      RBI.constrainGenericRegister(DefReg, AArch64::GPR32RegClass, MRI);
+    }
+    I.setDesc(TII.get(TargetOpcode::COPY));
+    return true;
+  }
   default:
     return false;
   }