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

[Matt Arsenault via llvm-commits]

================
@@ -2004,46 +2197,99 @@ bool PreRARematStage::isTriviallyReMaterializable(const MachineInstr &MI) {
   return true;
 }
 
-// When removing, we will have to check both beginning and ending of the region.
-// When inserting, we will only have to check if we are inserting NewMI in front
-// of a scheduling region and do not need to check the ending since we will only
-// ever be inserting before an already existing MI.
-void GCNScheduleDAGMILive::updateRegionBoundaries(
-    SmallVectorImpl<std::pair<MachineBasicBlock::iterator,
-                              MachineBasicBlock::iterator>> &RegionBoundaries,
-    MachineBasicBlock::iterator MI, MachineInstr *NewMI, bool Removing) {
-  unsigned I = 0, E = RegionBoundaries.size();
-  // Search for first region of the block where MI is located
-  while (I != E && MI->getParent() != RegionBoundaries[I].first->getParent())
-    ++I;
-
-  for (; I != E; ++I) {
-    if (MI->getParent() != RegionBoundaries[I].first->getParent())
-      return;
-
-    if (Removing && MI == RegionBoundaries[I].first &&
-        MI == RegionBoundaries[I].second) {
-      // MI is in a region with size 1, after removing, the region will be
-      // size 0, set RegionBegin and RegionEnd to pass end of block iterator.
-      RegionBoundaries[I] =
-          std::pair(MI->getParent()->end(), MI->getParent()->end());
-      return;
-    }
-    if (MI == RegionBoundaries[I].first) {
-      if (Removing)
-        RegionBoundaries[I] =
-            std::pair(std::next(MI), RegionBoundaries[I].second);
-      else
-        // Inserted NewMI in front of region, set new RegionBegin to NewMI
-        RegionBoundaries[I] = std::pair(MachineBasicBlock::iterator(NewMI),
-                                        RegionBoundaries[I].second);
-      return;
-    }
-    if (Removing && MI == RegionBoundaries[I].second) {
-      RegionBoundaries[I] = std::pair(RegionBoundaries[I].first, std::prev(MI));
-      return;
+/// Identifies the parent MBB of a \p Region. For an empty region, this runs in
+/// linear time in the number of MBBs in \p MF; otherwise it runs in constant
+/// time.
+static MachineBasicBlock *getRegionMBB(MachineFunction &MF,
+                                       RegionBoundaries &Region) {
+  if (Region.first != Region.second)
+    return Region.first->getParent();
+  // The boundaries of an empty region may not point to a valid MI from which we
+  // can get the parent MBB, so we have to iterate over all the MF's MBBs.
+  for (MachineBasicBlock &MBB : MF)
+    if (MBB.end() == Region.second)
+      return &MBB;
----------------
arsenm wrote:

```suggestion
  for (MachineBasicBlock &MBB : MF) {
    if (Region.second == MBB.end())
      return &MBB;
    }
```

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