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

[Michael Maitland via llvm-commits]

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@@ -22601,6 +22601,108 @@ bool RISCVTargetLowering::fallBackToDAGISel(const Instruction &Inst) const {
   return false;
 }
 
+static MachineInstr *findInstrWhichNeedAllCSRs(MachineBasicBlock &MBB) {
+  // Some instructions may require (implicitly) all CSRs to be saved.
+  // For example, call to __cxa_throw is noreturn, but expects that all CSRs are
+  // taken care of.
+  // TODO: try to speedup this?
+  for (MachineInstr &MI : MBB) {
+    unsigned Opc = MI.getOpcode();
+    if (Opc != RISCV::PseudoCALL && Opc != RISCV::PseudoTAIL)
+      continue;
+    MachineOperand &MO = MI.getOperand(0);
+    StringRef Name = "";
+    if (MO.isSymbol()) {
+      Name = MO.getSymbolName();
+    } else if (MO.isGlobal()) {
+      Name = MO.getGlobal()->getName();
+    } else {
+      llvm_unreachable("Unexpected operand type.");
+    }
+    if (Name == "__cxa_throw" || Name == "__cxa_rethrow" ||
+        Name == "_Unwind_Resume")
+      return &MI;
+  }
+  return nullptr;
+}
+
+void RISCVTargetLowering::finalizeLowering(MachineFunction &MF) const {
+  if (!Subtarget.doCSRSavesInRA()) {
+    TargetLoweringBase::finalizeLowering(MF);
+    return;
+  }
+
+  MachineRegisterInfo &MRI = MF.getRegInfo();
+  const TargetInstrInfo &TII = *MF.getSubtarget().getInstrInfo();
+  const RISCVRegisterInfo &TRI = *Subtarget.getRegisterInfo();
+  const RISCVFrameLowering &TFI = *Subtarget.getFrameLowering();
+
+  SmallVector<MachineInstr *, 4> RestorePoints;
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michaelmaitland wrote:

Why didn't we do this in prologepilog when we inserted the CFI instructions?

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