isa/aarch64/abi.rs

//! Implementation of a standard AArch64 ABI.

use crate::ir;
use crate::ir::types;
use crate::ir::types::*;
use crate::ir::MemFlags;
use crate::ir::Opcode;
use crate::ir::{ExternalName, LibCall};
use crate::isa;
use crate::isa::aarch64::{inst::EmitState, inst::*};
use crate::isa::unwind::UnwindInst;
use crate::machinst::*;
use crate::settings;
use crate::{CodegenError, CodegenResult};
use alloc::boxed::Box;
use alloc::vec::Vec;
use regalloc::{RealReg, Reg, RegClass, Set, Writable};
use smallvec::{smallvec, SmallVec};

// We use a generic implementation that factors out AArch64 and x64 ABI commonalities, because
// these ABIs are very similar.

/// Support for the AArch64 ABI from the callee side (within a function body).
pub(crate) type AArch64ABICallee = ABICalleeImpl<AArch64MachineDeps>;

/// Support for the AArch64 ABI from the caller side (at a callsite).
pub(crate) type AArch64ABICaller = ABICallerImpl<AArch64MachineDeps>;

// Spidermonkey specific ABI convention.

/// This is SpiderMonkey's `WasmTableCallSigReg`.
static BALDRDASH_SIG_REG: u8 = 10;

/// This is SpiderMonkey's `WasmTlsReg`.
static BALDRDASH_TLS_REG: u8 = 23;

/// Offset in stack-arg area to callee-TLS slot in Baldrdash-2020 calling convention.
static BALDRDASH_CALLEE_TLS_OFFSET: i64 = 0;
/// Offset in stack-arg area to caller-TLS slot in Baldrdash-2020 calling convention.
static BALDRDASH_CALLER_TLS_OFFSET: i64 = 8;

// These two lists represent the registers the JIT may *not* use at any point in generated code.
//
// So these are callee-preserved from the JIT's point of view, and every register not in this list
// has to be caller-preserved by definition.
//
// Keep these lists in sync with the NonAllocatableMask set in Spidermonkey's
// Architecture-arm64.cpp.

// Indexed by physical register number.
#[rustfmt::skip]
static BALDRDASH_JIT_CALLEE_SAVED_GPR: &[bool] = &[
    /* 0 = */ false, false, false, false, false, false, false, false,
    /* 8 = */ false, false, false, false, false, false, false, false,
    /* 16 = */ true /* x16 / ip1 */, true /* x17 / ip2 */, true /* x18 / TLS */, false,
    /* 20 = */ false, false, false, false,
    /* 24 = */ false, false, false, false,
    // There should be 28, the pseudo stack pointer in this list, however the wasm stubs trash it
    // gladly right now.
    /* 28 = */ false, false, true /* x30 = FP */, false /* x31 = SP */
];

#[rustfmt::skip]
static BALDRDASH_JIT_CALLEE_SAVED_FPU: &[bool] = &[
    /* 0 = */ false, false, false, false, false, false, false, false,
    /* 8 = */ false, false, false, false, false, false, false, false,
    /* 16 = */ false, false, false, false, false, false, false, false,
    /* 24 = */ false, false, false, false, false, false, false, true /* v31 / d31 */
];

/// This is the limit for the size of argument and return-value areas on the
/// stack. We place a reasonable limit here to avoid integer overflow issues
/// with 32-bit arithmetic: for now, 128 MB.
static STACK_ARG_RET_SIZE_LIMIT: u64 = 128 * 1024 * 1024;

/// Try to fill a Baldrdash register, returning it if it was found.
fn try_fill_baldrdash_reg(call_conv: isa::CallConv, param: &ir::AbiParam) -> Option<ABIArg> {
    if call_conv.extends_baldrdash() {
        match &param.purpose {
            &ir::ArgumentPurpose::VMContext => {
                // This is SpiderMonkey's `WasmTlsReg`.
                Some(ABIArg::reg(
                    xreg(BALDRDASH_TLS_REG).to_real_reg(),
                    ir::types::I64,
                    param.extension,
                    param.purpose,
                ))
            }
            &ir::ArgumentPurpose::SignatureId => {
                // This is SpiderMonkey's `WasmTableCallSigReg`.
                Some(ABIArg::reg(
                    xreg(BALDRDASH_SIG_REG).to_real_reg(),
                    ir::types::I64,
                    param.extension,
                    param.purpose,
                ))
            }
            &ir::ArgumentPurpose::CalleeTLS => {
                // This is SpiderMonkey's callee TLS slot in the extended frame of Wasm's ABI-2020.
                assert!(call_conv == isa::CallConv::Baldrdash2020);
                Some(ABIArg::stack(
                    BALDRDASH_CALLEE_TLS_OFFSET,
                    ir::types::I64,
                    ir::ArgumentExtension::None,
                    param.purpose,
                ))
            }
            &ir::ArgumentPurpose::CallerTLS => {
                // This is SpiderMonkey's caller TLS slot in the extended frame of Wasm's ABI-2020.
                assert!(call_conv == isa::CallConv::Baldrdash2020);
                Some(ABIArg::stack(
                    BALDRDASH_CALLER_TLS_OFFSET,
                    ir::types::I64,
                    ir::ArgumentExtension::None,
                    param.purpose,
                ))
            }
            _ => None,
        }
    } else {
        None
    }
}

impl Into<AMode> for StackAMode {
    fn into(self) -> AMode {
        match self {
            StackAMode::FPOffset(off, ty) => AMode::FPOffset(off, ty),
            StackAMode::NominalSPOffset(off, ty) => AMode::NominalSPOffset(off, ty),
            StackAMode::SPOffset(off, ty) => AMode::SPOffset(off, ty),
        }
    }
}

// Returns the size of stack space needed to store the
// `int_reg` and `vec_reg`.
fn saved_reg_stack_size(
    call_conv: isa::CallConv,
    int_reg: &[Writable<RealReg>],
    vec_reg: &[Writable<RealReg>],
) -> (usize, usize) {
    // Round up to multiple of 2, to keep 16-byte stack alignment.
    let int_save_bytes = (int_reg.len() + (int_reg.len() & 1)) * 8;
    // The Baldrdash ABIs require saving and restoring the whole 16-byte
    // SIMD & FP registers, so the necessary stack space is always a
    // multiple of the mandatory 16-byte stack alignment. However, the
    // Procedure Call Standard for the Arm 64-bit Architecture (AAPCS64,
    // including several related ABIs such as the one used by Windows)
    // mandates saving only the bottom 8 bytes of the vector registers,
    // so in that case we round up the number of registers to ensure proper
    // stack alignment (similarly to the situation with `int_reg`).
    let vec_reg_size = if call_conv.extends_baldrdash() { 16 } else { 8 };
    let vec_save_padding = if call_conv.extends_baldrdash() {
        0
    } else {
        vec_reg.len() & 1
    };
    let vec_save_bytes = (vec_reg.len() + vec_save_padding) * vec_reg_size;

    (int_save_bytes, vec_save_bytes)
}

/// AArch64-specific ABI behavior. This struct just serves as an implementation
/// point for the trait; it is never actually instantiated.
pub(crate) struct AArch64MachineDeps;

impl ABIMachineSpec for AArch64MachineDeps {
    type I = Inst;

    fn word_bits() -> u32 {
        64
    }

    /// Return required stack alignment in bytes.
    fn stack_align(_call_conv: isa::CallConv) -> u32 {
        16
    }

    fn compute_arg_locs(
        call_conv: isa::CallConv,
        _flags: &settings::Flags,
        params: &[ir::AbiParam],
        args_or_rets: ArgsOrRets,
        add_ret_area_ptr: bool,
    ) -> CodegenResult<(Vec<ABIArg>, i64, Option<usize>)> {
        let is_apple_cc = call_conv.extends_apple_aarch64();
        let is_baldrdash = call_conv.extends_baldrdash();
        let has_baldrdash_tls = call_conv == isa::CallConv::Baldrdash2020;

        // See AArch64 ABI (https://github.com/ARM-software/abi-aa/blob/2021Q1/aapcs64/aapcs64.rst#64parameter-passing), sections 6.4.
        //
        // MacOS aarch64 is slightly different, see also
        // https://developer.apple.com/documentation/xcode/writing_arm64_code_for_apple_platforms.
        // We are diverging from the MacOS aarch64 implementation in the
        // following ways:
        // - sign- and zero- extensions of data types less than 32 bits are not
        // implemented yet.
        // - we align the arguments stack space to a 16-bytes boundary, while
        // the MacOS allows aligning only on 8 bytes. In practice it means we're
        // slightly overallocating when calling, which is fine, and doesn't
        // break our other invariants that the stack is always allocated in
        // 16-bytes chunks.

        let mut next_xreg = 0;
        let mut next_vreg = 0;
        let mut next_stack: u64 = 0;
        let mut ret = vec![];

        if args_or_rets == ArgsOrRets::Args && has_baldrdash_tls {
            // Baldrdash ABI-2020 always has two stack-arg slots reserved, for the callee and
            // caller TLS-register values, respectively.
            next_stack = 16;
        }

        let (max_per_class_reg_vals, mut remaining_reg_vals) = match args_or_rets {
            ArgsOrRets::Args => (8, 16), // x0-x7 and v0-v7

            // Note on return values: on the regular ABI, we may return values
            // in 8 registers for V128 and I64 registers independently of the
            // number of register values returned in the other class. That is,
            // we can return values in up to 8 integer and
            // 8 vector registers at once.
            //
            // In Baldrdash and Wasmtime, we can only use one register for
            // return value for all the register classes. That is, we can't
            // return values in both one integer and one vector register; only
            // one return value may be in a register.
            ArgsOrRets::Rets => {
                if is_baldrdash || call_conv.extends_wasmtime() {
                    (1, 1) // x0 or v0, but not both
                } else {
                    (8, 16) // x0-x7 and v0-v7
                }
            }
        };

        for i in 0..params.len() {
            // Process returns backward, according to the SpiderMonkey ABI (which we
            // adopt internally if `is_baldrdash` is set).
            let param = match (args_or_rets, is_baldrdash) {
                (ArgsOrRets::Args, _) => &params[i],
                (ArgsOrRets::Rets, false) => &params[i],
                (ArgsOrRets::Rets, true) => &params[params.len() - 1 - i],
            };

            // Validate "purpose".
            match &param.purpose {
                &ir::ArgumentPurpose::VMContext
                | &ir::ArgumentPurpose::Normal
                | &ir::ArgumentPurpose::StackLimit
                | &ir::ArgumentPurpose::SignatureId
                | &ir::ArgumentPurpose::CallerTLS
                | &ir::ArgumentPurpose::CalleeTLS
                | &ir::ArgumentPurpose::StructReturn
                | &ir::ArgumentPurpose::StructArgument(_) => {}
                _ => panic!(
                    "Unsupported argument purpose {:?} in signature: {:?}",
                    param.purpose, params
                ),
            }

            assert!(
                legal_type_for_machine(param.value_type),
                "Invalid type for AArch64: {:?}",
                param.value_type
            );

            let (rcs, reg_types) = Inst::rc_for_type(param.value_type)?;

            if let Some(param) = try_fill_baldrdash_reg(call_conv, param) {
                assert!(rcs[0] == RegClass::I64);
                ret.push(param);
                continue;
            }

            if let ir::ArgumentPurpose::StructArgument(size) = param.purpose {
                let offset = next_stack as i64;
                let size = size as u64;
                assert!(size % 8 == 0, "StructArgument size is not properly aligned");
                next_stack += size;
                ret.push(ABIArg::StructArg {
                    offset,
                    size,
                    purpose: param.purpose,
                });
                continue;
            }

            // Handle multi register params
            //
            // See AArch64 ABI (https://github.com/ARM-software/abi-aa/blob/2021Q1/aapcs64/aapcs64.rst#642parameter-passing-rules), (Section 6.4.2 Stage C).
            //
            // For arguments with alignment of 16 we round up the the register number
            // to the next even value. So we can never allocate for example an i128
            // to X1 and X2, we have to skip one register and do X2, X3
            // (Stage C.8)
            // Note: The Apple ABI deviates a bit here. They don't respect Stage C.8
            // and will happily allocate a i128 to X1 and X2
            //
            // For integer types with alignment of 16 we also have the additional
            // restriction of passing the lower half in Xn and the upper half in Xn+1
            // (Stage C.9)
            //
            // For examples of how LLVM handles this: https://godbolt.org/z/bhd3vvEfh
            //
            // On the Apple ABI it is unspecified if we can spill half the value into the stack
            // i.e load the lower half into x7 and the upper half into the stack
            // LLVM does not seem to do this, so we are going to replicate that behaviour
            let is_multi_reg = rcs.len() >= 2;
            if is_multi_reg {
                assert!(
                    rcs.len() == 2,
                    "Unable to handle multi reg params with more than 2 regs"
                );
                assert!(
                    rcs == &[RegClass::I64, RegClass::I64],
                    "Unable to handle non i64 regs"
                );

                let reg_class_space = max_per_class_reg_vals - next_xreg;
                let reg_space = remaining_reg_vals;

                if reg_space >= 2 && reg_class_space >= 2 {
                    // The aarch64 ABI does not allow us to start a split argument
                    // at an odd numbered register. So we need to skip one register
                    //
                    // TODO: The Fast ABI should probably not skip the register
                    if !is_apple_cc && next_xreg % 2 != 0 {
                        next_xreg += 1;
                    }

                    let lower_reg = xreg(next_xreg);
                    let upper_reg = xreg(next_xreg + 1);

                    ret.push(ABIArg::Slots {
                        slots: vec![
                            ABIArgSlot::Reg {
                                reg: lower_reg.to_real_reg(),
                                ty: param.value_type,
                                extension: param.extension,
                            },
                            ABIArgSlot::Reg {
                                reg: upper_reg.to_real_reg(),
                                ty: param.value_type,
                                extension: param.extension,
                            },
                        ],
                        purpose: param.purpose,
                    });

                    next_xreg += 2;
                    remaining_reg_vals -= 2;
                    continue;
                }
            } else {
                // Single Register parameters
                let rc = rcs[0];
                let next_reg = match rc {
                    RegClass::I64 => &mut next_xreg,
                    RegClass::V128 => &mut next_vreg,
                    _ => panic!("Invalid register class: {:?}", rc),
                };

                if *next_reg < max_per_class_reg_vals && remaining_reg_vals > 0 {
                    let reg = match rc {
                        RegClass::I64 => xreg(*next_reg),
                        RegClass::V128 => vreg(*next_reg),
                        _ => unreachable!(),
                    };
                    ret.push(ABIArg::reg(
                        reg.to_real_reg(),
                        param.value_type,
                        param.extension,
                        param.purpose,
                    ));
                    *next_reg += 1;
                    remaining_reg_vals -= 1;
                    continue;
                }
            }

            // Spill to the stack

            // Compute the stack slot's size.
            let size = (ty_bits(param.value_type) / 8) as u64;

            let size = if is_apple_cc
                || (call_conv.extends_wasmtime() && args_or_rets == ArgsOrRets::Rets)
            {
                // MacOS aarch64 and Wasmtime allow stack slots with
                // sizes less than 8 bytes. They still need to be
                // properly aligned on their natural data alignment,
                // though.
                size
            } else {
                // Every arg takes a minimum slot of 8 bytes. (16-byte stack
                // alignment happens separately after all args.)
                std::cmp::max(size, 8)
            };

            // Align the stack slot.
            debug_assert!(size.is_power_of_two());
            next_stack = align_to(next_stack, size);

            let slots = reg_types
                .iter()
                .copied()
                // Build the stack locations from each slot
                .scan(next_stack, |next_stack, ty| {
                    let slot_offset = *next_stack as i64;
                    *next_stack += (ty_bits(ty) / 8) as u64;

                    Some((ty, slot_offset))
                })
                .map(|(ty, offset)| ABIArgSlot::Stack {
                    offset,
                    ty,
                    extension: param.extension,
                })
                .collect();

            ret.push(ABIArg::Slots {
                slots,
                purpose: param.purpose,
            });

            next_stack += size;
        }

        if args_or_rets == ArgsOrRets::Rets && is_baldrdash {
            ret.reverse();
        }

        let extra_arg = if add_ret_area_ptr {
            debug_assert!(args_or_rets == ArgsOrRets::Args);
            if next_xreg < max_per_class_reg_vals && remaining_reg_vals > 0 {
                ret.push(ABIArg::reg(
                    xreg(next_xreg).to_real_reg(),
                    I64,
                    ir::ArgumentExtension::None,
                    ir::ArgumentPurpose::Normal,
                ));
            } else {
                ret.push(ABIArg::stack(
                    next_stack as i64,
                    I64,
                    ir::ArgumentExtension::None,
                    ir::ArgumentPurpose::Normal,
                ));
                next_stack += 8;
            }
            Some(ret.len() - 1)
        } else {
            None
        };

        next_stack = align_to(next_stack, 16);

        // To avoid overflow issues, limit the arg/return size to something
        // reasonable -- here, 128 MB.
        if next_stack > STACK_ARG_RET_SIZE_LIMIT {
            return Err(CodegenError::ImplLimitExceeded);
        }

        Ok((ret, next_stack as i64, extra_arg))
    }

    fn fp_to_arg_offset(call_conv: isa::CallConv, flags: &settings::Flags) -> i64 {
        if call_conv.extends_baldrdash() {
            let num_words = flags.baldrdash_prologue_words() as i64;
            debug_assert!(num_words > 0, "baldrdash must set baldrdash_prologue_words");
            debug_assert_eq!(num_words % 2, 0, "stack must be 16-aligned");
            num_words * 8
        } else {
            16 // frame pointer + return address.
        }
    }

    fn gen_load_stack(mem: StackAMode, into_reg: Writable<Reg>, ty: Type) -> Inst {
        Inst::gen_load(into_reg, mem.into(), ty, MemFlags::trusted())
    }

    fn gen_store_stack(mem: StackAMode, from_reg: Reg, ty: Type) -> Inst {
        Inst::gen_store(mem.into(), from_reg, ty, MemFlags::trusted())
    }

    fn gen_move(to_reg: Writable<Reg>, from_reg: Reg, ty: Type) -> Inst {
        Inst::gen_move(to_reg, from_reg, ty)
    }

    fn gen_extend(
        to_reg: Writable<Reg>,
        from_reg: Reg,
        signed: bool,
        from_bits: u8,
        to_bits: u8,
    ) -> Inst {
        assert!(from_bits < to_bits);
        Inst::Extend {
            rd: to_reg,
            rn: from_reg,
            signed,
            from_bits,
            to_bits,
        }
    }

    fn gen_ret() -> Inst {
        Inst::Ret
    }

    fn gen_add_imm(into_reg: Writable<Reg>, from_reg: Reg, imm: u32) -> SmallInstVec<Inst> {
        let imm = imm as u64;
        let mut insts = SmallVec::new();
        if let Some(imm12) = Imm12::maybe_from_u64(imm) {
            insts.push(Inst::AluRRImm12 {
                alu_op: ALUOp::Add64,
                rd: into_reg,
                rn: from_reg,
                imm12,
            });
        } else {
            let scratch2 = writable_tmp2_reg();
            assert_ne!(scratch2.to_reg(), from_reg);
            insts.extend(Inst::load_constant(scratch2, imm.into()));
            insts.push(Inst::AluRRRExtend {
                alu_op: ALUOp::Add64,
                rd: into_reg,
                rn: from_reg,
                rm: scratch2.to_reg(),
                extendop: ExtendOp::UXTX,
            });
        }
        insts
    }

    fn gen_stack_lower_bound_trap(limit_reg: Reg) -> SmallInstVec<Inst> {
        let mut insts = SmallVec::new();
        insts.push(Inst::AluRRRExtend {
            alu_op: ALUOp::SubS64,
            rd: writable_zero_reg(),
            rn: stack_reg(),
            rm: limit_reg,
            extendop: ExtendOp::UXTX,
        });
        insts.push(Inst::TrapIf {
            trap_code: ir::TrapCode::StackOverflow,
            // Here `Lo` == "less than" when interpreting the two
            // operands as unsigned integers.
            kind: CondBrKind::Cond(Cond::Lo),
        });
        insts
    }

    fn gen_epilogue_placeholder() -> Inst {
        Inst::EpiloguePlaceholder
    }

    fn gen_get_stack_addr(mem: StackAMode, into_reg: Writable<Reg>, _ty: Type) -> Inst {
        let mem = mem.into();
        Inst::LoadAddr { rd: into_reg, mem }
    }

    fn get_stacklimit_reg() -> Reg {
        spilltmp_reg()
    }

    fn gen_load_base_offset(into_reg: Writable<Reg>, base: Reg, offset: i32, ty: Type) -> Inst {
        let mem = AMode::RegOffset(base, offset as i64, ty);
        Inst::gen_load(into_reg, mem, ty, MemFlags::trusted())
    }

    fn gen_store_base_offset(base: Reg, offset: i32, from_reg: Reg, ty: Type) -> Inst {
        let mem = AMode::RegOffset(base, offset as i64, ty);
        Inst::gen_store(mem, from_reg, ty, MemFlags::trusted())
    }

    fn gen_sp_reg_adjust(amount: i32) -> SmallInstVec<Inst> {
        if amount == 0 {
            return SmallVec::new();
        }

        let (amount, is_sub) = if amount > 0 {
            (amount as u64, false)
        } else {
            (-amount as u64, true)
        };

        let alu_op = if is_sub { ALUOp::Sub64 } else { ALUOp::Add64 };

        let mut ret = SmallVec::new();
        if let Some(imm12) = Imm12::maybe_from_u64(amount) {
            let adj_inst = Inst::AluRRImm12 {
                alu_op,
                rd: writable_stack_reg(),
                rn: stack_reg(),
                imm12,
            };
            ret.push(adj_inst);
        } else {
            let tmp = writable_spilltmp_reg();
            let const_inst = Inst::load_constant(tmp, amount);
            let adj_inst = Inst::AluRRRExtend {
                alu_op,
                rd: writable_stack_reg(),
                rn: stack_reg(),
                rm: tmp.to_reg(),
                extendop: ExtendOp::UXTX,
            };
            ret.extend(const_inst);
            ret.push(adj_inst);
        }
        ret
    }

    fn gen_nominal_sp_adj(offset: i32) -> Inst {
        Inst::VirtualSPOffsetAdj {
            offset: offset as i64,
        }
    }

    fn gen_prologue_frame_setup(flags: &settings::Flags) -> SmallInstVec<Inst> {
        let mut insts = SmallVec::new();

        if flags.unwind_info() {
            insts.push(Inst::Unwind {
                inst: UnwindInst::Aarch64SetPointerAuth {
                    return_addresses: false,
                },
            });
        }

        // stp fp (x29), lr (x30), [sp, #-16]!
        insts.push(Inst::StoreP64 {
            rt: fp_reg(),
            rt2: link_reg(),
            mem: PairAMode::PreIndexed(
                writable_stack_reg(),
                SImm7Scaled::maybe_from_i64(-16, types::I64).unwrap(),
            ),
            flags: MemFlags::trusted(),
        });

        if flags.unwind_info() {
            insts.push(Inst::Unwind {
                inst: UnwindInst::PushFrameRegs {
                    offset_upward_to_caller_sp: 16, // FP, LR
                },
            });
        }

        // mov fp (x29), sp. This uses the ADDI rd, rs, 0 form of `MOV` because
        // the usual encoding (`ORR`) does not work with SP.
        insts.push(Inst::AluRRImm12 {
            alu_op: ALUOp::Add64,
            rd: writable_fp_reg(),
            rn: stack_reg(),
            imm12: Imm12 {
                bits: 0,
                shift12: false,
            },
        });
        insts
    }

    fn gen_epilogue_frame_restore(_: &settings::Flags) -> SmallInstVec<Inst> {
        let mut insts = SmallVec::new();

        // N.B.: sp is already adjusted to the appropriate place by the
        // clobber-restore code (which also frees the fixed frame). Hence, there
        // is no need for the usual `mov sp, fp` here.

        // `ldp fp, lr, [sp], #16`
        insts.push(Inst::LoadP64 {
            rt: writable_fp_reg(),
            rt2: writable_link_reg(),
            mem: PairAMode::PostIndexed(
                writable_stack_reg(),
                SImm7Scaled::maybe_from_i64(16, types::I64).unwrap(),
            ),
            flags: MemFlags::trusted(),
        });
        insts
    }

    fn gen_probestack(_: u32) -> SmallInstVec<Self::I> {
        // TODO: implement if we ever require stack probes on an AArch64 host
        // (unlikely unless Lucet is ported)
        smallvec![]
    }

    // Returns stack bytes used as well as instructions. Does not adjust
    // nominal SP offset; abi_impl generic code will do that.
    fn gen_clobber_save(
        call_conv: isa::CallConv,
        flags: &settings::Flags,
        clobbers: &Set<Writable<RealReg>>,
        fixed_frame_storage_size: u32,
        _outgoing_args_size: u32,
    ) -> (u64, SmallVec<[Inst; 16]>) {
        let mut insts = SmallVec::new();
        let (clobbered_int, clobbered_vec) = get_regs_saved_in_prologue(call_conv, clobbers);

        let (int_save_bytes, vec_save_bytes) =
            saved_reg_stack_size(call_conv, &clobbered_int, &clobbered_vec);
        let total_save_bytes = int_save_bytes + vec_save_bytes;
        let clobber_size = total_save_bytes as i32;

        if flags.unwind_info() {
            // The *unwind* frame (but not the actual frame) starts at the
            // clobbers, just below the saved FP/LR pair.
            insts.push(Inst::Unwind {
                inst: UnwindInst::DefineNewFrame {
                    offset_downward_to_clobbers: clobber_size as u32,
                    offset_upward_to_caller_sp: 16, // FP, LR
                },
            });
        }

        // We use pre-indexed addressing modes here, rather than the possibly
        // more efficient "subtract sp once then used fixed offsets" scheme,
        // because (i) we cannot necessarily guarantee that the offset of a
        // clobber-save slot will be within a SImm7Scaled (+504-byte) offset
        // range of the whole frame including other slots, it is more complex to
        // conditionally generate a two-stage SP adjustment (clobbers then fixed
        // frame) otherwise, and generally we just want to maintain simplicity
        // here for maintainability.  Because clobbers are at the top of the
        // frame, just below FP, all that is necessary is to use the pre-indexed
        // "push" `[sp, #-16]!` addressing mode.
        //
        // `frame_offset` tracks offset above start-of-clobbers for unwind-info
        // purposes.
        let mut clobber_offset = clobber_size as u32;
        let clobber_offset_change = 16;
        let iter = clobbered_int.chunks_exact(2);

        if let [rd] = iter.remainder() {
            let rd = rd.to_reg().to_reg();

            debug_assert_eq!(rd.get_class(), RegClass::I64);
            // str rd, [sp, #-16]!
            insts.push(Inst::Store64 {
                rd,
                mem: AMode::PreIndexed(
                    writable_stack_reg(),
                    SImm9::maybe_from_i64(-clobber_offset_change).unwrap(),
                ),
                flags: MemFlags::trusted(),
            });

            if flags.unwind_info() {
                clobber_offset -= clobber_offset_change as u32;
                insts.push(Inst::Unwind {
                    inst: UnwindInst::SaveReg {
                        clobber_offset,
                        reg: rd.to_real_reg(),
                    },
                });
            }
        }

        let mut iter = iter.rev();

        while let Some([rt, rt2]) = iter.next() {
            // .to_reg().to_reg(): Writable<RealReg> --> RealReg --> Reg
            let rt = rt.to_reg().to_reg();
            let rt2 = rt2.to_reg().to_reg();

            debug_assert!(rt.get_class() == RegClass::I64);
            debug_assert!(rt2.get_class() == RegClass::I64);

            // stp rt, rt2, [sp, #-16]!
            insts.push(Inst::StoreP64 {
                rt,
                rt2,
                mem: PairAMode::PreIndexed(
                    writable_stack_reg(),
                    SImm7Scaled::maybe_from_i64(-clobber_offset_change, types::I64).unwrap(),
                ),
                flags: MemFlags::trusted(),
            });

            if flags.unwind_info() {
                clobber_offset -= clobber_offset_change as u32;
                insts.push(Inst::Unwind {
                    inst: UnwindInst::SaveReg {
                        clobber_offset,
                        reg: rt.to_real_reg(),
                    },
                });
                insts.push(Inst::Unwind {
                    inst: UnwindInst::SaveReg {
                        clobber_offset: clobber_offset + (clobber_offset_change / 2) as u32,
                        reg: rt2.to_real_reg(),
                    },
                });
            }
        }

        let store_vec_reg = |rd| {
            if call_conv.extends_baldrdash() {
                Inst::FpuStore128 {
                    rd,
                    mem: AMode::PreIndexed(
                        writable_stack_reg(),
                        SImm9::maybe_from_i64(-clobber_offset_change).unwrap(),
                    ),
                    flags: MemFlags::trusted(),
                }
            } else {
                Inst::FpuStore64 {
                    rd,
                    mem: AMode::PreIndexed(
                        writable_stack_reg(),
                        SImm9::maybe_from_i64(-clobber_offset_change).unwrap(),
                    ),
                    flags: MemFlags::trusted(),
                }
            }
        };
        let iter = clobbered_vec.chunks_exact(2);

        if let [rd] = iter.remainder() {
            let rd = rd.to_reg().to_reg();

            debug_assert_eq!(rd.get_class(), RegClass::V128);
            insts.push(store_vec_reg(rd));

            if flags.unwind_info() {
                clobber_offset -= clobber_offset_change as u32;
                insts.push(Inst::Unwind {
                    inst: UnwindInst::SaveReg {
                        clobber_offset,
                        reg: rd.to_real_reg(),
                    },
                });
            }
        }

        let store_vec_reg_pair = |rt, rt2| {
            if call_conv.extends_baldrdash() {
                let clobber_offset_change = 32;

                (
                    Inst::FpuStoreP128 {
                        rt,
                        rt2,
                        mem: PairAMode::PreIndexed(
                            writable_stack_reg(),
                            SImm7Scaled::maybe_from_i64(-clobber_offset_change, I8X16).unwrap(),
                        ),
                        flags: MemFlags::trusted(),
                    },
                    clobber_offset_change as u32,
                )
            } else {
                let clobber_offset_change = 16;

                (
                    Inst::FpuStoreP64 {
                        rt,
                        rt2,
                        mem: PairAMode::PreIndexed(
                            writable_stack_reg(),
                            SImm7Scaled::maybe_from_i64(-clobber_offset_change, F64).unwrap(),
                        ),
                        flags: MemFlags::trusted(),
                    },
                    clobber_offset_change as u32,
                )
            }
        };
        let mut iter = iter.rev();

        while let Some([rt, rt2]) = iter.next() {
            let rt = rt.to_reg().to_reg();
            let rt2 = rt2.to_reg().to_reg();

            debug_assert_eq!(rt.get_class(), RegClass::V128);
            debug_assert_eq!(rt2.get_class(), RegClass::V128);

            let (inst, clobber_offset_change) = store_vec_reg_pair(rt, rt2);

            insts.push(inst);

            if flags.unwind_info() {
                clobber_offset -= clobber_offset_change;
                insts.push(Inst::Unwind {
                    inst: UnwindInst::SaveReg {
                        clobber_offset,
                        reg: rt.to_real_reg(),
                    },
                });
                insts.push(Inst::Unwind {
                    inst: UnwindInst::SaveReg {
                        clobber_offset: clobber_offset + clobber_offset_change / 2,
                        reg: rt2.to_real_reg(),
                    },
                });
            }
        }

        // Allocate the fixed frame below the clobbers if necessary.
        if fixed_frame_storage_size > 0 {
            insts.extend(Self::gen_sp_reg_adjust(-(fixed_frame_storage_size as i32)));
        }

        (total_save_bytes as u64, insts)
    }

    fn gen_clobber_restore(
        call_conv: isa::CallConv,
        flags: &settings::Flags,
        clobbers: &Set<Writable<RealReg>>,
        fixed_frame_storage_size: u32,
        _outgoing_args_size: u32,
    ) -> SmallVec<[Inst; 16]> {
        let mut insts = SmallVec::new();
        let (clobbered_int, clobbered_vec) = get_regs_saved_in_prologue(call_conv, clobbers);

        // Free the fixed frame if necessary.
        if fixed_frame_storage_size > 0 {
            insts.extend(Self::gen_sp_reg_adjust(fixed_frame_storage_size as i32));
        }

        let load_vec_reg = |rd| {
            if call_conv.extends_baldrdash() {
                Inst::FpuLoad128 {
                    rd,
                    mem: AMode::PostIndexed(
                        writable_stack_reg(),
                        SImm9::maybe_from_i64(16).unwrap(),
                    ),
                    flags: MemFlags::trusted(),
                }
            } else {
                Inst::FpuLoad64 {
                    rd,
                    mem: AMode::PostIndexed(
                        writable_stack_reg(),
                        SImm9::maybe_from_i64(16).unwrap(),
                    ),
                    flags: MemFlags::trusted(),
                }
            }
        };
        let load_vec_reg_pair = |rt, rt2| {
            if call_conv.extends_baldrdash() {
                Inst::FpuLoadP128 {
                    rt,
                    rt2,
                    mem: PairAMode::PostIndexed(
                        writable_stack_reg(),
                        SImm7Scaled::maybe_from_i64(32, I8X16).unwrap(),
                    ),
                    flags: MemFlags::trusted(),
                }
            } else {
                Inst::FpuLoadP64 {
                    rt,
                    rt2,
                    mem: PairAMode::PostIndexed(
                        writable_stack_reg(),
                        SImm7Scaled::maybe_from_i64(16, F64).unwrap(),
                    ),
                    flags: MemFlags::trusted(),
                }
            }
        };

        let mut iter = clobbered_vec.chunks_exact(2);

        while let Some([rt, rt2]) = iter.next() {
            let rt = rt.map(|r| r.to_reg());
            let rt2 = rt2.map(|r| r.to_reg());

            debug_assert_eq!(rt.to_reg().get_class(), RegClass::V128);
            debug_assert_eq!(rt2.to_reg().get_class(), RegClass::V128);
            insts.push(load_vec_reg_pair(rt, rt2));
        }

        debug_assert!(iter.remainder().len() <= 1);

        if let [rd] = iter.remainder() {
            let rd = rd.map(|r| r.to_reg());

            debug_assert_eq!(rd.to_reg().get_class(), RegClass::V128);
            insts.push(load_vec_reg(rd));
        }

        let mut iter = clobbered_int.chunks_exact(2);

        while let Some([rt, rt2]) = iter.next() {
            let rt = rt.map(|r| r.to_reg());
            let rt2 = rt2.map(|r| r.to_reg());

            debug_assert_eq!(rt.to_reg().get_class(), RegClass::I64);
            debug_assert_eq!(rt2.to_reg().get_class(), RegClass::I64);
            // ldp rt, rt2, [sp], #16
            insts.push(Inst::LoadP64 {
                rt,
                rt2,
                mem: PairAMode::PostIndexed(
                    writable_stack_reg(),
                    SImm7Scaled::maybe_from_i64(16, I64).unwrap(),
                ),
                flags: MemFlags::trusted(),
            });
        }

        debug_assert!(iter.remainder().len() <= 1);

        if let [rd] = iter.remainder() {
            let rd = rd.map(|r| r.to_reg());

            debug_assert_eq!(rd.to_reg().get_class(), RegClass::I64);
            // ldr rd, [sp], #16
            insts.push(Inst::ULoad64 {
                rd,
                mem: AMode::PostIndexed(writable_stack_reg(), SImm9::maybe_from_i64(16).unwrap()),
                flags: MemFlags::trusted(),
            });
        }

        // If this is Baldrdash-2020, restore the callee (i.e., our) TLS
        // register. We may have allocated it for something else and clobbered
        // it, but the ABI expects us to leave the TLS register unchanged.
        if call_conv == isa::CallConv::Baldrdash2020 {
            let off = BALDRDASH_CALLEE_TLS_OFFSET + Self::fp_to_arg_offset(call_conv, flags);
            insts.push(Inst::gen_load(
                writable_xreg(BALDRDASH_TLS_REG),
                AMode::UnsignedOffset(fp_reg(), UImm12Scaled::maybe_from_i64(off, I64).unwrap()),
                I64,
                MemFlags::trusted(),
            ));
        }

        insts
    }

    fn gen_call(
        dest: &CallDest,
        uses: Vec<Reg>,
        defs: Vec<Writable<Reg>>,
        opcode: ir::Opcode,
        tmp: Writable<Reg>,
        callee_conv: isa::CallConv,
        caller_conv: isa::CallConv,
    ) -> SmallVec<[(InstIsSafepoint, Inst); 2]> {
        let mut insts = SmallVec::new();
        match &dest {
            &CallDest::ExtName(ref name, RelocDistance::Near) => insts.push((
                InstIsSafepoint::Yes,
                Inst::Call {
                    info: Box::new(CallInfo {
                        dest: name.clone(),
                        uses,
                        defs,
                        opcode,
                        caller_callconv: caller_conv,
                        callee_callconv: callee_conv,
                    }),
                },
            )),
            &CallDest::ExtName(ref name, RelocDistance::Far) => {
                insts.push((
                    InstIsSafepoint::No,
                    Inst::LoadExtName {
                        rd: tmp,
                        name: Box::new(name.clone()),
                        offset: 0,
                    },
                ));
                insts.push((
                    InstIsSafepoint::Yes,
                    Inst::CallInd {
                        info: Box::new(CallIndInfo {
                            rn: tmp.to_reg(),
                            uses,
                            defs,
                            opcode,
                            caller_callconv: caller_conv,
                            callee_callconv: callee_conv,
                        }),
                    },
                ));
            }
            &CallDest::Reg(reg) => insts.push((
                InstIsSafepoint::Yes,
                Inst::CallInd {
                    info: Box::new(CallIndInfo {
                        rn: *reg,
                        uses,
                        defs,
                        opcode,
                        caller_callconv: caller_conv,
                        callee_callconv: callee_conv,
                    }),
                },
            )),
        }

        insts
    }

    fn gen_memcpy(
        call_conv: isa::CallConv,
        dst: Reg,
        src: Reg,
        size: usize,
    ) -> SmallVec<[Self::I; 8]> {
        // Baldrdash should not use struct args.
        assert!(!call_conv.extends_baldrdash());
        let mut insts = SmallVec::new();
        let arg0 = writable_xreg(0);
        let arg1 = writable_xreg(1);
        let arg2 = writable_xreg(2);
        insts.push(Inst::gen_move(arg0, dst, I64));
        insts.push(Inst::gen_move(arg1, src, I64));
        insts.extend(Inst::load_constant(arg2, size as u64).into_iter());
        insts.push(Inst::Call {
            info: Box::new(CallInfo {
                dest: ExternalName::LibCall(LibCall::Memcpy),
                uses: vec![arg0.to_reg(), arg1.to_reg(), arg2.to_reg()],
                defs: Self::get_regs_clobbered_by_call(call_conv),
                opcode: Opcode::Call,
                caller_callconv: call_conv,
                callee_callconv: call_conv,
            }),
        });
        insts
    }

    fn get_number_of_spillslots_for_value(rc: RegClass, ty: Type) -> u32 {
        // We allocate in terms of 8-byte slots.
        match (rc, ty) {
            (RegClass::I64, _) => 1,
            (RegClass::V128, F32) | (RegClass::V128, F64) => 1,
            (RegClass::V128, _) => 2,
            _ => panic!("Unexpected register class!"),
        }
    }

    /// Get the current virtual-SP offset from an instruction-emission state.
    fn get_virtual_sp_offset_from_state(s: &EmitState) -> i64 {
        s.virtual_sp_offset
    }

    /// Get the nominal-SP-to-FP offset from an instruction-emission state.
    fn get_nominal_sp_to_fp(s: &EmitState) -> i64 {
        s.nominal_sp_to_fp
    }

    fn get_regs_clobbered_by_call(call_conv_of_callee: isa::CallConv) -> Vec<Writable<Reg>> {
        let mut caller_saved = Vec::new();
        for i in 0..29 {
            let x = writable_xreg(i);
            if is_reg_clobbered_by_call(call_conv_of_callee, x.to_reg().to_real_reg()) {
                caller_saved.push(x);
            }
        }
        for i in 0..32 {
            let v = writable_vreg(i);
            if is_reg_clobbered_by_call(call_conv_of_callee, v.to_reg().to_real_reg()) {
                caller_saved.push(v);
            }
        }
        caller_saved
    }

    fn get_ext_mode(
        call_conv: isa::CallConv,
        specified: ir::ArgumentExtension,
    ) -> ir::ArgumentExtension {
        if call_conv.extends_baldrdash() {
            // Baldrdash (SpiderMonkey) always extends args and return values to the full register.
            specified
        } else {
            // No other supported ABI on AArch64 does so.
            ir::ArgumentExtension::None
        }
    }
}

/// Is this type supposed to be seen on this machine? E.g. references of the
/// wrong width are invalid.
fn legal_type_for_machine(ty: Type) -> bool {
    match ty {
        R32 => false,
        _ => true,
    }
}

/// Is the given register saved in the prologue if clobbered, i.e., is it a
/// callee-save?
fn is_reg_saved_in_prologue(call_conv: isa::CallConv, r: RealReg) -> bool {
    if call_conv.extends_baldrdash() {
        match r.get_class() {
            RegClass::I64 => {
                let enc = r.get_hw_encoding();
                return BALDRDASH_JIT_CALLEE_SAVED_GPR[enc];
            }
            RegClass::V128 => {
                let enc = r.get_hw_encoding();
                return BALDRDASH_JIT_CALLEE_SAVED_FPU[enc];
            }
            _ => unimplemented!("baldrdash callee saved on non-i64 reg classes"),
        };
    }

    match r.get_class() {
        RegClass::I64 => {
            // x19 - x28 inclusive are callee-saves.
            r.get_hw_encoding() >= 19 && r.get_hw_encoding() <= 28
        }
        RegClass::V128 => {
            // v8 - v15 inclusive are callee-saves.
            r.get_hw_encoding() >= 8 && r.get_hw_encoding() <= 15
        }
        _ => panic!("Unexpected RegClass"),
    }
}

/// Return the set of all integer and vector registers that must be saved in the
/// prologue and restored in the epilogue, given the set of all registers
/// written by the function's body.
fn get_regs_saved_in_prologue(
    call_conv: isa::CallConv,
    regs: &Set<Writable<RealReg>>,
) -> (Vec<Writable<RealReg>>, Vec<Writable<RealReg>>) {
    let mut int_saves = vec![];
    let mut vec_saves = vec![];
    for &reg in regs.iter() {
        if is_reg_saved_in_prologue(call_conv, reg.to_reg()) {
            match reg.to_reg().get_class() {
                RegClass::I64 => int_saves.push(reg),
                RegClass::V128 => vec_saves.push(reg),
                _ => panic!("Unexpected RegClass"),
            }
        }
    }
    // Sort registers for deterministic code output. We can do an unstable sort because the
    // registers will be unique (there are no dups).
    int_saves.sort_unstable_by_key(|r| r.to_reg().get_index());
    vec_saves.sort_unstable_by_key(|r| r.to_reg().get_index());
    (int_saves, vec_saves)
}

fn is_reg_clobbered_by_call(call_conv_of_callee: isa::CallConv, r: RealReg) -> bool {
    if call_conv_of_callee.extends_baldrdash() {
        match r.get_class() {
            RegClass::I64 => {
                let enc = r.get_hw_encoding();
                if !BALDRDASH_JIT_CALLEE_SAVED_GPR[enc] {
                    return true;
                }
                // Otherwise, fall through to preserve native's ABI caller-saved.
            }
            RegClass::V128 => {
                let enc = r.get_hw_encoding();
                if !BALDRDASH_JIT_CALLEE_SAVED_FPU[enc] {
                    return true;
                }
                // Otherwise, fall through to preserve native's ABI caller-saved.
            }
            _ => unimplemented!("baldrdash callee saved on non-i64 reg classes"),
        };
    }

    match r.get_class() {
        RegClass::I64 => {
            // x0 - x17 inclusive are caller-saves.
            r.get_hw_encoding() <= 17
        }
        RegClass::V128 => {
            // v0 - v7 inclusive and v16 - v31 inclusive are caller-saves. The
            // upper 64 bits of v8 - v15 inclusive are also caller-saves.
            // However, because we cannot currently represent partial registers
            // to regalloc.rs, we indicate here that every vector register is
            // caller-save. Because this function is used at *callsites*,
            // approximating in this direction (save more than necessary) is
            // conservative and thus safe.
            //
            // Note that we set the 'not included in clobber set' flag in the
            // regalloc.rs API when a call instruction's callee has the same ABI
            // as the caller (the current function body); this is safe (anything
            // clobbered by callee can be clobbered by caller as well) and
            // avoids unnecessary saves of v8-v15 in the prologue even though we
            // include them as defs here.
            true
        }
        _ => panic!("Unexpected RegClass"),
    }
}