Call Convention

CC_Mips 
 -> CC_Mips_VarArg
    -> CC_MipsCheriPureCap_VarArg
    -> CC_Mips_ByVal
    -> CC_Mips_N_VarArg
 -> CC_Mips_FixedArg
    -> CC_CHERI_CCall
        -> CC_MipsN
    -> CC_Mips_ByVal
    -> CC_Mips_FastCC
    -> CC_MIpsN

// lib/Target/Mips/MipsCallingConv.td

def CC_CHERI_CCall : CallingConv<[
  CCIfType<[iFATPTR64, iFATPTR128, iFATPTR256, iFATPTR512], CCAssignToReg<[C1, C2, C3, C4, C5, C6, C7, C8, C9, C10]>>,
  CCIfType<[i64], CCAssignToReg<[V0_64]>>,
  CCDelegateTo<CC_MipsN>
]>;

// lib/Target/Mips/MipsCallingConv.td

def CC_MipsN : CallingConv<[
  ...
  // All integers (except soft-float integers) are promoted to 64-bit.
  CCIfType<[i8, i16, i32], CCIfOrigArgWasNotFloat<CCPromoteToType<i64>>>,

  // The only i32's we have left are soft-float arguments.
  CCIfSubtarget<"useSoftFloat()", CCIfType<[i32],
                CCDelegateTo<CC_MipsN_SoftFloat>>>,
  ...
  // All stack parameter slots become 64-bit doublewords and are 8-byte aligned.
  CCIfType<[f32], CCAssignToStack<4, 8>>,
  CCIfType<[i64, f64], CCAssignToStack<8, 8>>,
  CCIfType<[iFATPTR64, iFATPTR128, iFATPTR256, iFATPTR512], CCAssignToReg<[C3, C4, C5, C6, C7, C8, C9, C10]>>,
  CCIfType<[iFATPTR64, iFATPTR128, iFATPTR256, iFATPTR512], CCDelegateTo<CC_MipsCheriCapOnStack>>
]>;

// lib/Target/Mips/MipsCallingConv.td

def CC_MipsCheriCapOnStack : CallingConv<[
  CCIfSubtarget<"isCheri128()", CCAssignToStack<16, 16>>,
  CCIfSubtargetNot<"isCheri128()", CCAssignToStack<32, 32>>
]>;

// In purecap mode, all variadic arguments are passed on the stack.
def CC_MipsCheriPureCap_VarArg : CallingConv<[
  // Promote i8/i16 arguments to i32.
  CCIfType<[i8, i16, i32], CCPromoteToType<i64>>,
  // All stack parameter slots become 64-bit doublewords and are 8-byte aligned.
  CCIfType<[f32], CCAssignToStack<4, 8>>,
  CCIfType<[i64, f64], CCAssignToStack<8, 8>>,
  CCIfType<[iFATPTR64, iFATPTR128, iFATPTR256, iFATPTR512], CCDelegateTo<CC_MipsCheriCapOnStack>>
]>;

def CC_Mips_FixedArg : CallingConv<[
  ...
  CCIf<"State.getCallingConv() == CallingConv::CHERI_CCall",
       CCDelegateTo<CC_CHERI_CCall>>,
  ...
]>;

def CC_Mips_VarArg : CallingConv<[
  // If this is the CHERI purecap ABI then we need to pass everything on the
  // stack for variadic functions.
  CCIfSubtarget<"isABI_CheriPureCap()", CCDelegateTo<CC_MipsCheriPureCap_VarArg>>,
  CCIfByVal<CCDelegateTo<CC_Mips_ByVal>>,
  ...
]>;

def CSR_N64_Cheri :
    CalleeSavedRegs<(add (sequence "D%u_64", 31, 24), RA_64, FP_64,
                                   GP_64, (sequence "S%u_64", 7, 0),
                                   (sequence "C%u", 23, 17))>;
def CSR_Cheri_Purecap :
    CalleeSavedRegs<(add (sequence "D%u_64", 31, 24), RA_64, FP_64,
                                   GP_64, (sequence "S%u_64", 7, 0),
                                   (sequence "C%u", 25, 17))>;