[llvm] [AMDGPU] Compiler should synthesize private buffer resource descriptor from flat_scratch_init (PR #79586)

[Scott Linder via llvm-commits]

================
@@ -398,6 +398,7 @@ void SIFrameLowering::emitEntryFunctionFlatScratchInit(
 
   Register FlatScrInitLo;
   Register FlatScrInitHi;
+  Register FlatScratchInitReg;
----------------
slinder1 wrote:

Nit: I'd prefer these variables agree on `Scr` vs `Scratch`

The same code is also repeated across some paths, including asserting `FlatScratchInitReg` is valid and initializing `FlatScrInit{Lo,Hi}`; could some of the code be extracted and do this setup first so it can share as much as possible? For example:

```cpp
Register FlatScratchInit;
if (ST.isAmdPalOS()) {
  // Extract the scratch offset from the descriptor in the GIT
  LiveRegUnits LiveUnits;
  LiveUnits.init(*TRI);
  LiveUnits.addLiveIns(MBB);
  // Find unused reg to load flat scratch init into
  ArrayRef<MCPhysReg> AllSGPR64s = TRI->getAllSGPR64(MF);
  unsigned NumPreloaded = (MFI->getNumPreloadedSGPRs() + 1) / 2;
  AllSGPR64s = AllSGPR64s.slice(
      std::min(static_cast<unsigned>(AllSGPR64s.size()), NumPreloaded));
  Register GITPtrLoReg = MFI->getGITPtrLoReg(MF);
  for (MCPhysReg Reg : AllSGPR64s) {
    if (LiveUnits.available(Reg) && !MRI.isReserved(Reg) &&
        MRI.isAllocatable(Reg) && !TRI->isSubRegisterEq(Reg, GITPtrLoReg)) {
      FlatScratchInit = Reg;
      break;
    }
  }
} else {
  FlatScratchInit =
      MFI->getPreloadedReg(AMDGPUFunctionArgInfo::FLAT_SCRATCH_INIT);
  MRI.addLiveIn(FlatScratchInit);
  MBB.addLiveIn(FlatScratchInit);
}
assert(FlatScratchInit && "Failed to find free register for scratch init");
Register FlatScratchInitLo = TRI->getSubReg(FlatScratchInit, AMDGPU::sub0);
Register FlatScratchInitHi = TRI->getSubReg(FlatScratchInit, AMDGPU::sub1);

if (ST.isAmdPalOS()) {
  buildGitPtr(MBB, I, DL, TII, FlatScratchInit);

```

https://github.com/llvm/llvm-project/pull/79586