[llvm] [X86] X86LowerTileCopy - Find dead register to use to prevent save-reload (PR #83628)

[Phoebe Wang via llvm-commits]

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@@ -73,10 +73,16 @@ FunctionPass *llvm::createX86LowerTileCopyPass() {
 bool X86LowerTileCopy::runOnMachineFunction(MachineFunction &MF) {
   const X86Subtarget &ST = MF.getSubtarget<X86Subtarget>();
   const X86InstrInfo *TII = ST.getInstrInfo();
+  const TargetRegisterInfo *TRI = ST.getRegisterInfo();
+  BitVector GR64Regs =
+      TRI->getAllocatableSet(MF, TRI->getRegClass(X86::GR64RegClassID));
   bool Changed = false;
 
   for (MachineBasicBlock &MBB : MF) {
-    for (MachineInstr &MI : llvm::make_early_inc_range(MBB)) {
+    LiveRegUnits UsedRegs(*TRI);
+    UsedRegs.addLiveOuts(MBB);
+    for (MachineInstr &MI : llvm::make_early_inc_range(reverse(MBB))) {
+      UsedRegs.stepBackward(MI);
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phoebewang wrote:

I assume the cost here is low, otherwise, it may less efficient that iterate after we find a tile copy instruction.

https://github.com/llvm/llvm-project/pull/83628