[llvm] [AMDGPU][True16] added Pre-RA hint to improve copy elimination (PR #103366)

[Matt Arsenault via llvm-commits]

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@@ -253,5 +263,38 @@ bool GCNPreRAOptimizationsImpl::run(MachineFunction &MF) {
     Changed |= processReg(Reg);
   }
 
+  if (!ST.useRealTrue16Insts())
+    return Changed;
+
+  // Add RA hints to improve True16 COPY elimination.
+  for (const MachineBasicBlock &MBB : MF) {
+    for (const MachineInstr &MI : MBB) {
+      if (MI.getOpcode() != AMDGPU::COPY)
+        continue;
+      Register Dst = MI.getOperand(0).getReg();
+      Register Src = MI.getOperand(1).getReg();
+      if (Dst.isVirtual() &&
+          MRI->getRegClass(Dst) == &AMDGPU::VGPR_16RegClass &&
+          Src.isPhysical() &&
+          TRI->getRegClassForReg(*MRI, Src) == &AMDGPU::VGPR_32RegClass)
+        MRI->setRegAllocationHint(Dst, 0, TRI->getSubReg(Src, AMDGPU::lo16));
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arsenm wrote:

I think this situation should be a hard machine verifier error 

https://github.com/llvm/llvm-project/pull/103366