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

[Mikhail Gudim via llvm-commits]

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@@ -21874,6 +21874,108 @@ 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;
+  SmallVector<MachineBasicBlock *, 4> SaveMBBs;
+  SaveMBBs.push_back(&MF.front());
+  for (MachineBasicBlock &MBB : MF) {
+    if (MBB.isReturnBlock())
+      RestorePoints.push_back(&MBB.back());
+    if (MachineInstr *CallToCxaThrow = findInstrWhichNeedAllCSRs(MBB)) {
+      // MachineBasicBlock::iterator MII = CallToCxaThrow->getIterator();
+      //++MII;
+      // assert(MII->getOpcode() == RISCV::ADJCALLSTACKUP && "Unexpected
+      // instruction");
+      //++MII;
+      MachineBasicBlock::iterator MII = MBB.getFirstTerminator();
+      MachineInstr *NewRetMI = BuildMI(MBB, MII, CallToCxaThrow->getDebugLoc(),
----------------
mgudim wrote:

This is a hack! I just wanted to trigger CSR saves. I hope there are better ways to do this.

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