[RISCV][MRI] Account for fixed registers when determining callee saved regs (#115756)

This fixes
https://discourse.llvm.org/t/fixed-register-being-spill-and-restored-in-clang/83058.

We need to do it in `MachineRegisterInfo::getCalleeSavedRegs` instead of
`RISCVRegisterInfo::getCalleeSavedRegs` since the MF argument of
`TargetRegisterInfo:::getCalleeSavedRegs` is `const`, so we can't call
`MF->getRegInfo().disableCalleeSavedRegister` there.

So to put it in `MachineRegisterInfo::getCalleeSavedRegs`, we move
`isRegisterReservedByUser` into `TargetSubtargetInfo`.

Author: michaelmaitland

llvm/include/llvm/CodeGen/TargetSubtargetInfo.h

@@ -349,6 +349,8 @@ class TargetSubtargetInfo : public MCSubtargetInfo {
     // Conservatively assume such instructions exist by default.
     return true;
   }
+
+  virtual bool isRegisterReservedByUser(Register R) const { return false; }
 };
 } // end namespace llvm
 

llvm/lib/CodeGen/MachineRegisterInfo.cpp

@@ -635,7 +635,13 @@ const MCPhysReg *MachineRegisterInfo::getCalleeSavedRegs() const {
   if (IsUpdatedCSRsInitialized)
     return UpdatedCSRs.data();
 
-  return getTargetRegisterInfo()->getCalleeSavedRegs(MF);
+  const MCPhysReg *Regs = getTargetRegisterInfo()->getCalleeSavedRegs(MF);
+
+  for (unsigned I = 0; Regs[I]; ++I)
+    if (MF->getSubtarget().isRegisterReservedByUser(Regs[I]))
+      MF->getRegInfo().disableCalleeSavedRegister(Regs[I]);
+
+  return Regs;
 }
 
 void MachineRegisterInfo::setCalleeSavedRegs(ArrayRef<MCPhysReg> CSRs) {

llvm/lib/Target/RISCV/RISCVISelLowering.cpp

@@ -20076,8 +20076,7 @@ SDValue RISCVTargetLowering::LowerCall(CallLoweringInfo &CLI,
   // reserved, if so report an error. Do the same for the return address if this
   // is not a tailcall.
   validateCCReservedRegs(RegsToPass, MF);
-  if (!IsTailCall &&
-      MF.getSubtarget<RISCVSubtarget>().isRegisterReservedByUser(RISCV::X1))
+  if (!IsTailCall && MF.getSubtarget().isRegisterReservedByUser(RISCV::X1))
     MF.getFunction().getContext().diagnose(DiagnosticInfoUnsupported{
         MF.getFunction(),
         "Return address register required, but has been reserved."});

llvm/lib/Target/RISCV/RISCVRegisterInfo.cpp

@@ -162,7 +162,7 @@ BitVector RISCVRegisterInfo::getReservedRegs(const MachineFunction &MF) const {
 
 bool RISCVRegisterInfo::isAsmClobberable(const MachineFunction &MF,
                                          MCRegister PhysReg) const {
-  return !MF.getSubtarget<RISCVSubtarget>().isRegisterReservedByUser(PhysReg);
+  return !MF.getSubtarget().isRegisterReservedByUser(PhysReg);
 }
 
 const uint32_t *RISCVRegisterInfo::getNoPreservedMask() const {

llvm/lib/Target/RISCV/RISCVSubtarget.h

@@ -231,7 +231,7 @@ class RISCVSubtarget : public RISCVGenSubtargetInfo {
            TargetABI == RISCVABI::ABI_ILP32 ||
            TargetABI == RISCVABI::ABI_ILP32E;
   }
-  bool isRegisterReservedByUser(Register i) const {
+  bool isRegisterReservedByUser(Register i) const override {
     assert(i < RISCV::NUM_TARGET_REGS && "Register out of range");
     return UserReservedRegister[i];
   }

llvm/test/CodeGen/RISCV/fixed-csr.ll

@@ -0,0 +1,35 @@
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py UTC_ARGS: --version 5
+; RUN: llc -mtriple=riscv64 -mattr=+reserve-x24 < %s | FileCheck %s
+
+define noundef signext i32 @foo() {
+; CHECK-LABEL: foo:
+; CHECK:       # %bb.0:
+; CHECK-NEXT:    li s8, 321
+; CHECK-NEXT:    li a0, 0
+; CHECK-NEXT:    ret
+  tail call void @llvm.write_register.i64(metadata !0, i64 321)
+  ret i32 0
+}
+
+declare void @llvm.write_register.i64(metadata, i64)
+
+define noundef signext i32 @bar() nounwind {
+; CHECK-LABEL: bar:
+; CHECK:       # %bb.0:
+; CHECK-NEXT:    addi sp, sp, -16
+; CHECK-NEXT:    sd s9, 8(sp) # 8-byte Folded Spill
+; CHECK-NEXT:    #APP
+; CHECK-NEXT:    #NO_APP
+; CHECK-NEXT:    li s8, 321
+; CHECK-NEXT:    li a0, 0
+; CHECK-NEXT:    ld s9, 8(sp) # 8-byte Folded Reload
+; CHECK-NEXT:    addi sp, sp, 16
+; CHECK-NEXT:    ret
+  tail call void asm sideeffect "", "~{x25}"() #3
+  tail call void @llvm.write_register.i64(metadata !0, i64 321)
+  ret i32 0
+}
+
+!llvm.named.register.x24 = !{!0}
+!0 = !{!"x24"}
+