[llvm] Co-issue packed instructions by unpacking (PR #151704)

[Akash Dutta via llvm-commits]

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@@ -260,38 +589,46 @@ 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));
-      if (Src.isVirtual() &&
-          MRI->getRegClass(Src) == &AMDGPU::VGPR_16RegClass &&
-          Dst.isPhysical() &&
-          TRI->getRegClassForReg(*MRI, Dst) == &AMDGPU::VGPR_32RegClass)
-        MRI->setRegAllocationHint(Src, 0, TRI->getSubReg(Dst, AMDGPU::lo16));
-      if (!Dst.isVirtual() || !Src.isVirtual())
-        continue;
-      if (MRI->getRegClass(Dst) == &AMDGPU::VGPR_32RegClass &&
-          MRI->getRegClass(Src) == &AMDGPU::VGPR_16RegClass) {
-        MRI->setRegAllocationHint(Dst, AMDGPURI::Size32, Src);
-        MRI->setRegAllocationHint(Src, AMDGPURI::Size16, Dst);
+  // Unpack packed instructions to overlap MFMAs. This allows the compiler to co-issue unpacked instructions with MFMA
+  for (MachineBasicBlock &MBB : MF) {
+    DenseSet<MachineInstr *> instrsToUnpack;
+    for (MachineInstr &MI : MBB) {
+      if (SIInstrInfo::isMFMA(MI)){
+        createListOfPackedInstr(MI, instrsToUnpack);
+      }
+      if (ST.useRealTrue16Insts()){
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akadutta wrote:

There was an early exit there for True16. However, leaving that there would mean code bloat, as the MF scan code for detecting the appropriate packed instructions needs to run anyhow. This way, it's just one block of code.

https://github.com/llvm/llvm-project/pull/151704