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[RISCV] Support PreserveMost calling convention #148214

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[RISCV] Support PreserveMost calling convention #148214

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e3a2e57
[RISCV] Support PreserveMost calling convention
wangpc-pp Jul 11, 2025
a5812f1
Add E and getCallPreservedMask
wangpc-pp Jul 15, 2025
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2 changes: 2 additions & 0 deletions llvm/docs/LangRef.rst
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Expand Up @@ -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 registers.
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This isn't going to work as the PLT stub can/does clobber some temporary registers (and exactly which depend on a lot of things). This is why AArch64 does not preserve x16-x18.

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Looking at lld, t1, t2, and t3 are clobbered by the PLT resolver, so we need to account for that.

I think this means:

  • The function should be saving these three regs.
  • the mask used for preserved registers in calls should not include these three.


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
Expand Down
4 changes: 4 additions & 0 deletions llvm/lib/Target/RISCV/RISCVCallingConv.td
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Expand Up @@ -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<(add X1, (sequence "X%u", 5, 31))>;
def CSR_RT_MostRegs_RVE : CalleeSavedRegs<(sub CSR_RT_MostRegs,
(sequence "X%u", 16, 31))>;
1 change: 1 addition & 0 deletions llvm/lib/Target/RISCV/RISCVISelLowering.cpp
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Expand Up @@ -22205,6 +22205,7 @@ 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:
Expand Down
12 changes: 11 additions & 1 deletion llvm/lib/Target/RISCV/RISCVRegisterInfo.cpp
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Expand Up @@ -11,6 +11,7 @@
//===----------------------------------------------------------------------===//

#include "RISCVRegisterInfo.h"
#include "MCTargetDesc/RISCVBaseInfo.h"
#include "RISCV.h"
#include "RISCVSubtarget.h"
#include "llvm/ADT/SmallSet.h"
Expand Down Expand Up @@ -68,6 +69,9 @@ 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())
Expand Down Expand Up @@ -811,7 +815,13 @@ 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:
Expand Down
187 changes: 187 additions & 0 deletions llvm/test/CodeGen/RISCV/calling-conv-preserve-most.ll
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@@ -0,0 +1,187 @@
; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
; RUN: llc -mtriple=riscv32 < %s | FileCheck %s -check-prefix=RV32I
; RUN: llc -mtriple=riscv64 < %s | FileCheck %s -check-prefix=RV64I
; RUN: llc -mtriple=riscv32 -mattr=+e -target-abi ilp32e < %s | FileCheck %s -check-prefix=RV32E
; RUN: llc -mtriple=riscv64 -mattr=+e -target-abi lp64e < %s | FileCheck %s -check-prefix=RV64E

; Check the PreserveMost calling convention works.

declare void @standard_cc_func()
declare preserve_mostcc void @preserve_mostcc_func()

define preserve_mostcc void @preserve_mostcc1() nounwind {
; RV32I-LABEL: preserve_mostcc1:
; RV32I: # %bb.0: # %entry
; RV32I-NEXT: addi sp, sp, -64
; RV32I-NEXT: sw ra, 60(sp) # 4-byte Folded Spill
; RV32I-NEXT: sw t0, 56(sp) # 4-byte Folded Spill
; RV32I-NEXT: sw t1, 52(sp) # 4-byte Folded Spill
; RV32I-NEXT: sw t2, 48(sp) # 4-byte Folded Spill
; RV32I-NEXT: sw a0, 44(sp) # 4-byte Folded Spill
; RV32I-NEXT: sw a1, 40(sp) # 4-byte Folded Spill
; RV32I-NEXT: sw a2, 36(sp) # 4-byte Folded Spill
; RV32I-NEXT: sw a3, 32(sp) # 4-byte Folded Spill
; RV32I-NEXT: sw a4, 28(sp) # 4-byte Folded Spill
; RV32I-NEXT: sw a5, 24(sp) # 4-byte Folded Spill
; RV32I-NEXT: sw a6, 20(sp) # 4-byte Folded Spill
; RV32I-NEXT: sw a7, 16(sp) # 4-byte Folded Spill
; RV32I-NEXT: sw t3, 12(sp) # 4-byte Folded Spill
; RV32I-NEXT: sw t4, 8(sp) # 4-byte Folded Spill
; RV32I-NEXT: sw t5, 4(sp) # 4-byte Folded Spill
; RV32I-NEXT: sw t6, 0(sp) # 4-byte Folded Spill
; RV32I-NEXT: call standard_cc_func
; RV32I-NEXT: lw ra, 60(sp) # 4-byte Folded Reload
; RV32I-NEXT: lw t0, 56(sp) # 4-byte Folded Reload
; RV32I-NEXT: lw t1, 52(sp) # 4-byte Folded Reload
; RV32I-NEXT: lw t2, 48(sp) # 4-byte Folded Reload
; RV32I-NEXT: lw a0, 44(sp) # 4-byte Folded Reload
; RV32I-NEXT: lw a1, 40(sp) # 4-byte Folded Reload
; RV32I-NEXT: lw a2, 36(sp) # 4-byte Folded Reload
; RV32I-NEXT: lw a3, 32(sp) # 4-byte Folded Reload
; RV32I-NEXT: lw a4, 28(sp) # 4-byte Folded Reload
; RV32I-NEXT: lw a5, 24(sp) # 4-byte Folded Reload
; RV32I-NEXT: lw a6, 20(sp) # 4-byte Folded Reload
; RV32I-NEXT: lw a7, 16(sp) # 4-byte Folded Reload
; RV32I-NEXT: lw t3, 12(sp) # 4-byte Folded Reload
; RV32I-NEXT: lw t4, 8(sp) # 4-byte Folded Reload
; RV32I-NEXT: lw t5, 4(sp) # 4-byte Folded Reload
; RV32I-NEXT: lw t6, 0(sp) # 4-byte Folded Reload
; RV32I-NEXT: addi sp, sp, 64
; RV32I-NEXT: ret
;
; RV64I-LABEL: preserve_mostcc1:
; RV64I: # %bb.0: # %entry
; RV64I-NEXT: addi sp, sp, -128
; RV64I-NEXT: sd ra, 120(sp) # 8-byte Folded Spill
; RV64I-NEXT: sd t0, 112(sp) # 8-byte Folded Spill
; RV64I-NEXT: sd t1, 104(sp) # 8-byte Folded Spill
; RV64I-NEXT: sd t2, 96(sp) # 8-byte Folded Spill
; RV64I-NEXT: sd a0, 88(sp) # 8-byte Folded Spill
; RV64I-NEXT: sd a1, 80(sp) # 8-byte Folded Spill
; RV64I-NEXT: sd a2, 72(sp) # 8-byte Folded Spill
; RV64I-NEXT: sd a3, 64(sp) # 8-byte Folded Spill
; RV64I-NEXT: sd a4, 56(sp) # 8-byte Folded Spill
; RV64I-NEXT: sd a5, 48(sp) # 8-byte Folded Spill
; RV64I-NEXT: sd a6, 40(sp) # 8-byte Folded Spill
; RV64I-NEXT: sd a7, 32(sp) # 8-byte Folded Spill
; RV64I-NEXT: sd t3, 24(sp) # 8-byte Folded Spill
; RV64I-NEXT: sd t4, 16(sp) # 8-byte Folded Spill
; RV64I-NEXT: sd t5, 8(sp) # 8-byte Folded Spill
; RV64I-NEXT: sd t6, 0(sp) # 8-byte Folded Spill
; RV64I-NEXT: call standard_cc_func
; RV64I-NEXT: ld ra, 120(sp) # 8-byte Folded Reload
; RV64I-NEXT: ld t0, 112(sp) # 8-byte Folded Reload
; RV64I-NEXT: ld t1, 104(sp) # 8-byte Folded Reload
; RV64I-NEXT: ld t2, 96(sp) # 8-byte Folded Reload
; RV64I-NEXT: ld a0, 88(sp) # 8-byte Folded Reload
; RV64I-NEXT: ld a1, 80(sp) # 8-byte Folded Reload
; RV64I-NEXT: ld a2, 72(sp) # 8-byte Folded Reload
; RV64I-NEXT: ld a3, 64(sp) # 8-byte Folded Reload
; RV64I-NEXT: ld a4, 56(sp) # 8-byte Folded Reload
; RV64I-NEXT: ld a5, 48(sp) # 8-byte Folded Reload
; RV64I-NEXT: ld a6, 40(sp) # 8-byte Folded Reload
; RV64I-NEXT: ld a7, 32(sp) # 8-byte Folded Reload
; RV64I-NEXT: ld t3, 24(sp) # 8-byte Folded Reload
; RV64I-NEXT: ld t4, 16(sp) # 8-byte Folded Reload
; RV64I-NEXT: ld t5, 8(sp) # 8-byte Folded Reload
; RV64I-NEXT: ld t6, 0(sp) # 8-byte Folded Reload
; RV64I-NEXT: addi sp, sp, 128
; RV64I-NEXT: ret
;
; RV32E-LABEL: preserve_mostcc1:
; RV32E: # %bb.0: # %entry
; RV32E-NEXT: addi sp, sp, -40
; RV32E-NEXT: sw ra, 36(sp) # 4-byte Folded Spill
; RV32E-NEXT: sw t0, 32(sp) # 4-byte Folded Spill
; RV32E-NEXT: sw t1, 28(sp) # 4-byte Folded Spill
; RV32E-NEXT: sw t2, 24(sp) # 4-byte Folded Spill
; RV32E-NEXT: sw a0, 20(sp) # 4-byte Folded Spill
; RV32E-NEXT: sw a1, 16(sp) # 4-byte Folded Spill
; RV32E-NEXT: sw a2, 12(sp) # 4-byte Folded Spill
; RV32E-NEXT: sw a3, 8(sp) # 4-byte Folded Spill
; RV32E-NEXT: sw a4, 4(sp) # 4-byte Folded Spill
; RV32E-NEXT: sw a5, 0(sp) # 4-byte Folded Spill
; RV32E-NEXT: call standard_cc_func
; RV32E-NEXT: lw ra, 36(sp) # 4-byte Folded Reload
; RV32E-NEXT: lw t0, 32(sp) # 4-byte Folded Reload
; RV32E-NEXT: lw t1, 28(sp) # 4-byte Folded Reload
; RV32E-NEXT: lw t2, 24(sp) # 4-byte Folded Reload
; RV32E-NEXT: lw a0, 20(sp) # 4-byte Folded Reload
; RV32E-NEXT: lw a1, 16(sp) # 4-byte Folded Reload
; RV32E-NEXT: lw a2, 12(sp) # 4-byte Folded Reload
; RV32E-NEXT: lw a3, 8(sp) # 4-byte Folded Reload
; RV32E-NEXT: lw a4, 4(sp) # 4-byte Folded Reload
; RV32E-NEXT: lw a5, 0(sp) # 4-byte Folded Reload
; RV32E-NEXT: addi sp, sp, 40
; RV32E-NEXT: ret
;
; RV64E-LABEL: preserve_mostcc1:
; RV64E: # %bb.0: # %entry
; RV64E-NEXT: addi sp, sp, -80
; RV64E-NEXT: sd ra, 72(sp) # 8-byte Folded Spill
; RV64E-NEXT: sd t0, 64(sp) # 8-byte Folded Spill
; RV64E-NEXT: sd t1, 56(sp) # 8-byte Folded Spill
; RV64E-NEXT: sd t2, 48(sp) # 8-byte Folded Spill
; RV64E-NEXT: sd a0, 40(sp) # 8-byte Folded Spill
; RV64E-NEXT: sd a1, 32(sp) # 8-byte Folded Spill
; RV64E-NEXT: sd a2, 24(sp) # 8-byte Folded Spill
; RV64E-NEXT: sd a3, 16(sp) # 8-byte Folded Spill
; RV64E-NEXT: sd a4, 8(sp) # 8-byte Folded Spill
; RV64E-NEXT: sd a5, 0(sp) # 8-byte Folded Spill
; RV64E-NEXT: call standard_cc_func
; RV64E-NEXT: ld ra, 72(sp) # 8-byte Folded Reload
; RV64E-NEXT: ld t0, 64(sp) # 8-byte Folded Reload
; RV64E-NEXT: ld t1, 56(sp) # 8-byte Folded Reload
; RV64E-NEXT: ld t2, 48(sp) # 8-byte Folded Reload
; RV64E-NEXT: ld a0, 40(sp) # 8-byte Folded Reload
; RV64E-NEXT: ld a1, 32(sp) # 8-byte Folded Reload
; RV64E-NEXT: ld a2, 24(sp) # 8-byte Folded Reload
; RV64E-NEXT: ld a3, 16(sp) # 8-byte Folded Reload
; RV64E-NEXT: ld a4, 8(sp) # 8-byte Folded Reload
; RV64E-NEXT: ld a5, 0(sp) # 8-byte Folded Reload
; RV64E-NEXT: addi sp, sp, 80
; RV64E-NEXT: ret
entry:
call void @standard_cc_func()
ret void
}

define preserve_mostcc void @preserve_mostcc2() nounwind {
; RV32I-LABEL: preserve_mostcc2:
; RV32I: # %bb.0:
; RV32I-NEXT: addi sp, sp, -16
; RV32I-NEXT: sw ra, 12(sp) # 4-byte Folded Spill
; RV32I-NEXT: call preserve_mostcc_func
; RV32I-NEXT: lw ra, 12(sp) # 4-byte Folded Reload
; RV32I-NEXT: addi sp, sp, 16
; RV32I-NEXT: ret
;
; RV64I-LABEL: preserve_mostcc2:
; RV64I: # %bb.0:
; RV64I-NEXT: addi sp, sp, -16
; RV64I-NEXT: sd ra, 8(sp) # 8-byte Folded Spill
; RV64I-NEXT: call preserve_mostcc_func
; RV64I-NEXT: ld ra, 8(sp) # 8-byte Folded Reload
; RV64I-NEXT: addi sp, sp, 16
; RV64I-NEXT: ret
;
; RV32E-LABEL: preserve_mostcc2:
; RV32E: # %bb.0:
; RV32E-NEXT: addi sp, sp, -4
; RV32E-NEXT: sw ra, 0(sp) # 4-byte Folded Spill
; RV32E-NEXT: call preserve_mostcc_func
; RV32E-NEXT: lw ra, 0(sp) # 4-byte Folded Reload
; RV32E-NEXT: addi sp, sp, 4
; RV32E-NEXT: ret
;
; RV64E-LABEL: preserve_mostcc2:
; RV64E: # %bb.0:
; RV64E-NEXT: addi sp, sp, -8
; RV64E-NEXT: sd ra, 0(sp) # 8-byte Folded Spill
; RV64E-NEXT: call preserve_mostcc_func
; RV64E-NEXT: ld ra, 0(sp) # 8-byte Folded Reload
; RV64E-NEXT: addi sp, sp, 8
; RV64E-NEXT: ret
call preserve_mostcc void @preserve_mostcc_func()
ret void
}
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