[RISCV] Support PreserveMost calling convention (llvm#148214)

This adds the simplest implementation of `PreserveMost` calling
convention and we preserve `x5-x31` (except x6/x7/x28) registers.

Fixes llvm#148147.

Author: wangpc-pp

llvm/docs/LangRef.rst

@@ -413,6 +413,8 @@ added in the future:
     - On AArch64 the callee preserve all general purpose registers, except
       X0-X8 and X16-X18. Not allowed with ``nest``.
 
+    - On RISC-V the callee preserve x5-x31 except x6, x7 and x28 registers.
+
     The idea behind this convention is to support calls to runtime functions
     that have a hot path and a cold path. The hot path is usually a small piece
     of code that doesn't use many registers. The cold path might need to call out to

llvm/lib/Target/RISCV/RISCVCallingConv.td

@@ -93,3 +93,7 @@ def CSR_XLEN_F32_V_Interrupt_RVE: CalleeSavedRegs<(sub CSR_XLEN_F32_V_Interrupt,
 // Same as CSR_XLEN_F64_V_Interrupt, but excluding X16-X31.
 def CSR_XLEN_F64_V_Interrupt_RVE: CalleeSavedRegs<(sub CSR_XLEN_F64_V_Interrupt,
                                                    (sequence "X%u", 16, 31))>;
+
+def CSR_RT_MostRegs : CalleeSavedRegs<(sub CSR_Interrupt, X6, X7, X28)>;
+def CSR_RT_MostRegs_RVE : CalleeSavedRegs<(sub CSR_RT_MostRegs,
+                                               (sequence "X%u", 16, 31))>;

llvm/lib/Target/RISCV/RISCVISelLowering.cpp

@@ -22224,6 +22224,7 @@ SDValue RISCVTargetLowering::LowerFormalArguments(
   case CallingConv::C:
   case CallingConv::Fast:
   case CallingConv::SPIR_KERNEL:
+  case CallingConv::PreserveMost:
   case CallingConv::GRAAL:
   case CallingConv::RISCV_VectorCall:
 #define CC_VLS_CASE(ABI_VLEN) case CallingConv::RISCV_VLSCall_##ABI_VLEN:

llvm/lib/Target/RISCV/RISCVRegisterInfo.cpp

@@ -68,6 +68,9 @@ RISCVRegisterInfo::getCalleeSavedRegs(const MachineFunction *MF) const {
   auto &Subtarget = MF->getSubtarget<RISCVSubtarget>();
   if (MF->getFunction().getCallingConv() == CallingConv::GHC)
     return CSR_NoRegs_SaveList;
+  if (MF->getFunction().getCallingConv() == CallingConv::PreserveMost)
+    return Subtarget.hasStdExtE() ? CSR_RT_MostRegs_RVE_SaveList
+                                  : CSR_RT_MostRegs_SaveList;
   if (MF->getFunction().hasFnAttribute("interrupt")) {
     if (Subtarget.hasVInstructions()) {
       if (Subtarget.hasStdExtD())
@@ -811,7 +814,13 @@ RISCVRegisterInfo::getCallPreservedMask(const MachineFunction & MF,
 
   if (CC == CallingConv::GHC)
     return CSR_NoRegs_RegMask;
-  switch (Subtarget.getTargetABI()) {
+  RISCVABI::ABI ABI = Subtarget.getTargetABI();
+  if (CC == CallingConv::PreserveMost) {
+    if (ABI == RISCVABI::ABI_ILP32E || ABI == RISCVABI::ABI_LP64E)
+      return CSR_RT_MostRegs_RVE_RegMask;
+    return CSR_RT_MostRegs_RegMask;
+  }
+  switch (ABI) {
   default:
     llvm_unreachable("Unrecognized ABI");
   case RISCVABI::ABI_ILP32E: