[llvm] Rebased save csr in ra (PR #131845)

[Michael Maitland via llvm-commits]

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@@ -786,6 +788,153 @@ void RISCVFrameLowering::allocateStack(MachineBasicBlock &MBB,
   }
 }
 
+struct CFIBuildInfo {
+  MachineBasicBlock *MBB;
+  MachineInstr *InsertAfterMI; // nullptr means insert at MBB.begin()
+  DebugLoc DL;
+  unsigned CFIIndex;
+};
+
+static void trackRegisterAndEmitCFIs(
+    MachineFunction &MF, MachineInstr &MI, MCRegister Reg, int DwarfEHRegNum,
+    const ReachingDefAnalysis &RDA, const TargetInstrInfo &TII,
+    const MachineFrameInfo &MFI, const RISCVRegisterInfo &TRI,
+    std::vector<CFIBuildInfo> &CFIBuildInfos,
+    std::unordered_set<MachineInstr *> &VisitedRestorePoints,
+    std::unordered_set<MachineInstr *> &VisitedDefs) {
+
+  if (VisitedRestorePoints.find(&MI) != VisitedRestorePoints.end()) {
+    return;
+  }
+  VisitedRestorePoints.insert(&MI);
+  SmallPtrSet<MachineInstr *, 2> Defs;
+  RDA.getGlobalReachingDefs(&MI, Reg, Defs);
+  MachineBasicBlock &EntryMBB = MF.front();
+  if (Defs.empty()) {
+    // it's a live-in register at the entry block.
+    // unsigned CFIIndex =
+    // MF.addFrameInst(MCCFIInstruction::createSameValue(nullptr,
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michaelmaitland wrote:

Why is this commented out? Do we need it?

https://github.com/llvm/llvm-project/pull/131845