[AArch64] Refactor cheap cost model

Refactor the order in `TII::isAsCheapAsAMove()` to ease future development
and maintenance.  Practically NFC.

llvm-svn: 343489

1 files changed, 23 insertions, 12 deletions

diff --git a/llvm/lib/Target/AArch64/AArch64InstrInfo.cpp b/llvm/lib/Target/AArch64/AArch64InstrInfo.cpp
index 5c097be67c3..da555665a75 100644
--- a/llvm/lib/Target/AArch64/AArch64InstrInfo.cpp
+++ b/llvm/lib/Target/AArch64/AArch64InstrInfo.cpp
@@ -675,6 +675,26 @@ bool AArch64InstrInfo::isAsCheapAsAMove(const MachineInstr &MI) const {
   if (!Subtarget.hasCustomCheapAsMoveHandling())
     return MI.isAsCheapAsAMove();
 
+  const unsigned Opcode = MI.getOpcode();
+
+  // Firstly, check cases gated by features.
+
+  if (Subtarget.hasZeroCycleZeroingFP()) {
+    if (Opcode == AArch64::FMOVH0 ||
+        Opcode == AArch64::FMOVS0 ||
+        Opcode == AArch64::FMOVD0)
+      return true;
+  }
+
+  if (Subtarget.hasZeroCycleZeroingGP()) {
+    if (Opcode == TargetOpcode::COPY &&
+        (MI.getOperand(1).getReg() == AArch64::WZR ||
+         MI.getOperand(1).getReg() == AArch64::XZR))
+      return true;
+  }
+
+  // Secondly, check cases specific to sub-targets.
+
   if (Subtarget.hasExynosCheapAsMoveHandling()) {
     if (isExynosResetFast(MI) || isExynosShiftLeftFast(MI))
       return true;
@@ -682,7 +702,9 @@ bool AArch64InstrInfo::isAsCheapAsAMove(const MachineInstr &MI) const {
       return MI.isAsCheapAsAMove();
   }
 
-  switch (MI.getOpcode()) {
+  // Finally, check generic cases.
+
+  switch (Opcode) {
   default:
     return false;
 
@@ -723,17 +745,6 @@ bool AArch64InstrInfo::isAsCheapAsAMove(const MachineInstr &MI) const {
     return canBeExpandedToORR(MI, 32);
   case AArch64::MOVi64imm:
     return canBeExpandedToORR(MI, 64);
-
-  // It is cheap to zero out registers if the subtarget has ZeroCycleZeroing
-  // feature.
-  case AArch64::FMOVH0:
-  case AArch64::FMOVS0:
-  case AArch64::FMOVD0:
-    return Subtarget.hasZeroCycleZeroingFP();
-  case TargetOpcode::COPY:
-    return (Subtarget.hasZeroCycleZeroingGP() &&
-            (MI.getOperand(1).getReg() == AArch64::WZR ||
-             MI.getOperand(1).getReg() == AArch64::XZR));
   }
 
   llvm_unreachable("Unknown opcode to check as cheap as a move!");