[RISCV] Remove CalleeSavedInfo for Zcmp/save-restore-libcalls registers

Registers that are pushed/popped by Zcmp or libcalls have pre-defined
frame indices that are never allocated in MachineFrameInfo. They're
being used throughout PEI, but the rest of codegen doesn't work that
way and expects each frame index to be a valid index in MFI.

This patch keeps it local to PEI and removes them from the
CalleeSavedInfo list at the end of the pass.

Before this pass, any MIR testing post-PEI is broken and asserts (see
issue #79491).

Author: Francis Visoiu Mistrih

llvm/lib/Target/RISCV/RISCVFrameLowering.cpp

@@ -1252,6 +1252,30 @@ void RISCVFrameLowering::processFunctionBeforeFrameFinalized(
   RVFI->setCalleeSavedStackSize(Size);
 }
 
+void RISCVFrameLowering::processFunctionBeforeFrameIndicesReplaced(
+    MachineFunction &MF, RegScavenger *RS) const {
+  // Remove CalleeSavedInfo for registers saved by Zcmp or save/restore
+  // libcalls.
+  MachineFrameInfo &MFI = MF.getFrameInfo();
+  const TargetRegisterInfo *TRI = MF.getSubtarget().getRegisterInfo();
+  const auto *RVFI = MF.getInfo<RISCVMachineFunctionInfo>();
+  if (!RVFI->isPushable(MF) && !RVFI->useSaveRestoreLibCalls(MF))
+    return;
+  const std::vector<CalleeSavedInfo> &CSIs = MFI.getCalleeSavedInfo();
+  std::vector<CalleeSavedInfo> NewCSIs;
+  for (const auto &CSI : CSIs) {
+    // Skip CSRs that have fake a frame index.
+    int ReservedFI = 0;
+    if (TRI->hasReservedSpillSlot(MF, CSI.getReg(), ReservedFI)) {
+      assert(CSI.getFrameIdx() == ReservedFI &&
+             "Reserved CSR spill slot frame index mismatch in CSI");
+      continue;
+    }
+    NewCSIs.push_back(CSI);
+  }
+  MFI.setCalleeSavedInfo(std::move(NewCSIs));
+}
+
 // Not preserve stack space within prologue for outgoing variables when the
 // function contains variable size objects or there are vector objects accessed
 // by the frame pointer.

llvm/lib/Target/RISCV/RISCVFrameLowering.h

@@ -37,6 +37,10 @@ class RISCVFrameLowering : public TargetFrameLowering {
   void processFunctionBeforeFrameFinalized(MachineFunction &MF,
                                            RegScavenger *RS) const override;
 
+  void
+  processFunctionBeforeFrameIndicesReplaced(MachineFunction &MF,
+                                            RegScavenger *RS) const override;
+
   bool hasFP(const MachineFunction &MF) const override;
 
   bool hasBP(const MachineFunction &MF) const;