[llvm] [AMDGPU] Add scheduling stage to rewrite MFMA from VGPR to AGPR (PR #149367)

[Lucas Ramirez via llvm-commits]

================
@@ -1036,6 +1041,110 @@ bool GCNSchedStage::initGCNSchedStage() {
   return true;
 }
 
+void RewriteScheduleStage::findReachingDefs(
+    MachineOperand &UseMO, LiveIntervals *LIS,
+    SmallVectorImpl<SlotIndex> &DefIdxs) {
+  assert(UseMO.isReg());
+  MachineInstr *UseMI = UseMO.getParent();
+  LiveInterval &UseLI = LIS->getInterval(UseMO.getReg());
+  VNInfo *VNI = UseLI.getVNInfoAt(LIS->getInstructionIndex(*UseMI));
+
+  SlotIndex DefMBBStart = LIS->getMBBStartIdx(LIS->getMBBFromIndex(VNI->def));
+
+  // If the def is in the block, then it must be the only reaching def.
+  if (DefMBBStart != VNI->def) {
+    DefIdxs.push_back(VNI->def);
+    return;
+  }
+
+  SmallPtrSet<MachineBasicBlock *, 8> Visited;
+  SmallVector<MachineBasicBlock *, 8> Worklist;
+
+  Visited.insert(UseMI->getParent());
+
+  // Mark the predecessor blocks for traversal
+  for (auto PredMBB : UseMI->getParent()->predecessors()) {
+    Worklist.push_back(PredMBB);
+    Visited.insert(PredMBB);
+  }
+
+  while (!Worklist.empty()) {
+    MachineBasicBlock *CurrMBB = Worklist.pop_back_val();
+
+    SlotIndex CurrMBBEnd = LIS->getMBBEndIdx(CurrMBB);
+    VNInfo *VNI = UseLI.getVNInfoAt(CurrMBBEnd.getPrevSlot());
+
+    MachineBasicBlock *DefMBB = LIS->getMBBFromIndex(VNI->def);
+    SlotIndex DefMBBStart = LIS->getMBBStartIdx(DefMBB);
+
+    // If there is a def in this block, then add it to the list. This is the
+    // reaching def of this path.
+    if (DefMBBStart != VNI->def) {
+      DefIdxs.push_back(VNI->def);
+      continue;
+    }
+
+    for (auto PredMBB : DefMBB->predecessors()) {
----------------
lucas-rami wrote:

```suggestion
    for (auto *PredMBB : DefMBB->predecessors()) {
```

https://github.com/llvm/llvm-project/pull/149367