[llvm] [AMDGPU][Scheduler] Refactor ArchVGPR rematerialization during scheduling (PR #125885)

[Matt Arsenault via llvm-commits]

================
@@ -1688,174 +1696,420 @@ bool PreRARematStage::allUsesAvailableAt(const MachineInstr *InstToRemat,
   return true;
 }
 
-void PreRARematStage::collectRematerializableInstructions() {
-  const SIRegisterInfo *SRI = static_cast<const SIRegisterInfo *>(DAG.TRI);
-  for (unsigned I = 0, E = DAG.MRI.getNumVirtRegs(); I != E; ++I) {
-    Register Reg = Register::index2VirtReg(I);
-    if (!DAG.LIS->hasInterval(Reg))
-      continue;
+namespace {
+/// Models excess register pressure in a region and tracks our progress as we
+/// identify rematerialization opportunities.
+struct ExcessRP {
+  /// Number of excess ArchVGPRs.
+  unsigned ArchVGPRs = 0;
+  /// Number of excess AGPRs.
+  unsigned AGPRs = 0;
+  /// For unified register files, number of excess VGPRs.
+  unsigned VGPRs = 0;
+  /// For unified register files with AGPR usage, number of excess ArchVGPRs to
+  /// save before we are able to save a whole allocation granule.
+  unsigned ArchVGPRsToAlignment = 0;
+  /// Whether the region uses AGPRs.
+  bool HasAGPRs = false;
+  /// Whether the subtarget has a unified RF.
+  bool UnifiedRF;
+
+  /// Constructs the excess RP model; determines the excess pressure w.r.t. a
+  /// maximum number of allowed VGPRs.
+  ExcessRP(const GCNSubtarget &ST, const GCNRegPressure &RP, unsigned MaxVGPRs);
+
+  /// Accounts for \p NumRegs saved ArchVGPRs in the model. If \p
+  /// UseArchVGPRForAGPRSpill is true, saved ArchVGPRs are used to save excess
+  /// AGPRs once excess ArchVGPR pressure has been eliminated. Returns whether
+  /// saving these ArchVGPRs helped reduce excess pressure.
+  bool saveArchVGPRs(unsigned NumRegs, bool UseArchVGPRForAGPRSpill);
+
+  /// Accounts for \p NumRegs saved AGPRS in the model. Returns whether saving
+  /// these ArchVGPRs helped reduce excess pressure.
+  bool saveAGPRs(unsigned NumRegs);
+
+  /// Returns whether there is any excess register pressure.
+  operator bool() const { return ArchVGPRs != 0 || AGPRs != 0 || VGPRs != 0; }
 
-    // TODO: Handle AGPR and SGPR rematerialization
-    if (!SRI->isVGPRClass(DAG.MRI.getRegClass(Reg)) ||
-        !DAG.MRI.hasOneDef(Reg) || !DAG.MRI.hasOneNonDBGUse(Reg))
-      continue;
+#ifndef NDEBUG
----------------
arsenm wrote:

```suggestion
#if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
```

https://github.com/llvm/llvm-project/pull/125885