[llvm] [RISCV][WIP] Let RA do the CSR saves. (PR #90819)

[Michael Maitland via llvm-commits]

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@@ -48,12 +50,28 @@ static bool isValidRegDefOf(const MachineOperand &MO, MCRegister PhysReg,
   return TRI->regsOverlap(MO.getReg(), PhysReg);
 }
 
+static bool isFIDef(const MachineInstr &MI, int FrameIndex,
+                    const TargetInstrInfo *TII) {
+  int DefFrameIndex = 0;
+  int SrcFrameIndex = 0;
+  if (TII->isStoreToStackSlot(MI, DefFrameIndex) ||
+      TII->isStackSlotCopy(MI, DefFrameIndex, SrcFrameIndex)) {
+    return DefFrameIndex == FrameIndex;
+  }
+  return false;
+}
+
 void ReachingDefAnalysis::enterBasicBlock(MachineBasicBlock *MBB) {
   unsigned MBBNumber = MBB->getNumber();
   assert(MBBNumber < MBBReachingDefs.numBlockIDs() &&
          "Unexpected basic block number.");
   MBBReachingDefs.startBasicBlock(MBBNumber, NumRegUnits);
 
+  MBBFrameObjsReachingDefs[MBBNumber].resize(NumStackObjects);
+  for (unsigned FOIdx = 0; FOIdx < NumStackObjects; ++FOIdx) {
+    MBBFrameObjsReachingDefs[MBBNumber][FOIdx].push_back(-1);
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michaelmaitland wrote:

Avoid initializing with `-1` for all elements here by returning `LatestDef` when `MBBFrameObjsReachingDefs[MBBNumber][FOIdx].empty()` below?

https://github.com/llvm/llvm-project/pull/90819