denoland-deno/ext/ffi/turbocall.rs

// Copyright 2018-2024 the Deno authors. All rights reserved. MIT license.

use std::cmp::max;
use std::ffi::c_void;
use std::iter::once;

use deno_core::v8::fast_api;
use dynasmrt::dynasm;
use dynasmrt::DynasmApi;
use dynasmrt::ExecutableBuffer;

use crate::NativeType;
use crate::Symbol;

pub(crate) fn is_compatible(sym: &Symbol) -> bool {
  // TODO: Support structs by value in fast call
  cfg!(any(
    all(target_arch = "x86_64", target_family = "unix"),
    all(target_arch = "x86_64", target_family = "windows"),
    all(target_arch = "aarch64", target_vendor = "apple")
  )) && !matches!(sym.result_type, NativeType::Struct(_))
    && !sym
      .parameter_types
      .iter()
      .any(|t| matches!(t, NativeType::Struct(_)))
}

// Unused on linux aarch64
#[allow(unused)]
pub(crate) fn compile_trampoline(sym: &Symbol) -> Trampoline {
  #[cfg(all(target_arch = "x86_64", target_family = "unix"))]
  return SysVAmd64::compile(sym);
  #[cfg(all(target_arch = "x86_64", target_family = "windows"))]
  return Win64::compile(sym);
  #[cfg(all(target_arch = "aarch64", target_vendor = "apple"))]
  return Aarch64Apple::compile(sym);
  #[allow(unreachable_code)]
  {
    unimplemented!("fast API is not implemented for the current target");
  }
}

pub(crate) fn make_template(
  sym: &Symbol,
  trampoline: &Trampoline,
) -> fast_api::FastFunction {
  let params = once(fast_api::Type::V8Value) // Receiver
    .chain(sym.parameter_types.iter().map(|t| t.into()))
    .collect::<Vec<_>>();

  let ret = if sym.result_type == NativeType::Buffer {
    // Buffer can be used as a return type and converts differently than in parameters.
    fast_api::CType::Pointer
  } else {
    fast_api::CType::from(&fast_api::Type::from(&sym.result_type))
  };

  fast_api::FastFunction::new_with_bigint(
    Box::leak(params.into_boxed_slice()),
    ret,
    trampoline.ptr(),
  )
}

/// Trampoline for fast-call FFI functions
///
/// Calls the FFI function without the first argument (the receiver)
pub(crate) struct Trampoline(ExecutableBuffer);

impl Trampoline {
  fn ptr(&self) -> *const c_void {
    &self.0[0] as *const u8 as *const c_void
  }
}

impl From<&NativeType> for fast_api::Type {
  fn from(native_type: &NativeType) -> Self {
    match native_type {
      NativeType::Bool => fast_api::Type::Bool,
      NativeType::U8 | NativeType::U16 | NativeType::U32 => {
        fast_api::Type::Uint32
      }
      NativeType::I8 | NativeType::I16 | NativeType::I32 => {
        fast_api::Type::Int32
      }
      NativeType::F32 => fast_api::Type::Float32,
      NativeType::F64 => fast_api::Type::Float64,
      NativeType::Void => fast_api::Type::Void,
      NativeType::I64 => fast_api::Type::Int64,
      NativeType::U64 => fast_api::Type::Uint64,
      NativeType::ISize => fast_api::Type::Int64,
      NativeType::USize => fast_api::Type::Uint64,
      NativeType::Pointer | NativeType::Function => fast_api::Type::Pointer,
      NativeType::Buffer => fast_api::Type::TypedArray(fast_api::CType::Uint8),
      NativeType::Struct(_) => {
        fast_api::Type::TypedArray(fast_api::CType::Uint8)
      }
    }
  }
}

macro_rules! x64 {
  ($assembler:expr; $($tokens:tt)+) => {
    dynasm!($assembler; .arch x64; $($tokens)+)
  }
}

macro_rules! aarch64 {
  ($assembler:expr; $($tokens:tt)+) => {
    dynasm!($assembler; .arch aarch64; $($tokens)+)
  }
}

struct SysVAmd64 {
  // Reference: https://refspecs.linuxfoundation.org/elf/x86_64-abi-0.99.pdf
  assmblr: dynasmrt::x64::Assembler,
  // Parameter counters
  integral_params: u32,
  float_params: u32,
  // Stack offset accumulators
  offset_trampoline: u32,
  offset_callee: u32,
  allocated_stack: u32,
  frame_pointer: u32,
}

#[cfg_attr(
  not(all(target_aarch = "x86_64", target_family = "unix")),
  allow(dead_code)
)]
impl SysVAmd64 {
  // Integral arguments go to the following GPR, in order: rdi, rsi, rdx, rcx, r8, r9
  const INTEGRAL_REGISTERS: u32 = 6;
  // SSE arguments go to the first 8 SSE registers: xmm0-xmm7
  const FLOAT_REGISTERS: u32 = 8;

  fn new() -> Self {
    Self {
      assmblr: dynasmrt::x64::Assembler::new().unwrap(),
      integral_params: 0,
      float_params: 0,
      // Start at 8 to account for trampoline caller's return address
      offset_trampoline: 8,
      // default to tail-call mode. If a new stack frame is allocated this becomes 0
      offset_callee: 8,
      allocated_stack: 0,
      frame_pointer: 0,
    }
  }

  fn compile(sym: &Symbol) -> Trampoline {
    let mut compiler = Self::new();

    let must_cast_return_value =
      compiler.must_cast_return_value(&sym.result_type);
    let cannot_tailcall = must_cast_return_value;

    if cannot_tailcall {
      compiler.allocate_stack(&sym.parameter_types);
    }

    for param in sym.parameter_types.iter().cloned() {
      compiler.move_left(param)
    }
    if !compiler.is_recv_arg_overridden() {
      // the receiver object should never be expected. Avoid its unexpected or deliberate leak
      compiler.zero_first_arg();
    }

    if cannot_tailcall {
      compiler.call(sym.ptr.as_ptr());
      if must_cast_return_value {
        compiler.cast_return_value(&sym.result_type);
      }
      compiler.deallocate_stack();
      compiler.ret();
    } else {
      compiler.tailcall(sym.ptr.as_ptr());
    }

    Trampoline(compiler.finalize())
  }

  fn move_left(&mut self, param: NativeType) {
    // Section 3.2.3 of the SysV ABI spec, on argument classification:
    // - INTEGER:
    //    > Arguments of types (signed and unsigned) _Bool, char, short, int,
    //    > long, long long, and pointers are in the INTEGER class.
    // - SSE:
    //    > Arguments of types float, double, _Decimal32, _Decimal64 and
    //    > __m64 are in class SSE.
    match param.into() {
      Int(integral) => self.move_integral(integral),
      Float(float) => self.move_float(float),
    }
  }

  fn move_float(&mut self, param: Floating) {
    // Section 3.2.3 of the SysV AMD64 ABI:
    // > If the class is SSE, the next available vector register is used, the registers
    // > are taken in the order from %xmm0 to %xmm7.
    // [...]
    // > Once registers are assigned, the arguments passed in memory are pushed on
    // > the stack in reversed (right-to-left) order
    let param_i = self.float_params;

    let is_in_stack = param_i >= Self::FLOAT_REGISTERS;
    // floats are only moved to accommodate integer movement in the stack
    let stack_has_moved = self.allocated_stack > 0
      || self.integral_params >= Self::INTEGRAL_REGISTERS;

    if is_in_stack && stack_has_moved {
      let s = &mut self.assmblr;
      let ot = self.offset_trampoline as i32;
      let oc = self.offset_callee as i32;
      match param {
        Single => x64!(s
          ; movss xmm8, [rsp + ot]
          ; movss [rsp + oc], xmm8
        ),
        Double => x64!(s
          ; movsd xmm8, [rsp + ot]
          ; movsd [rsp + oc], xmm8
        ),
      }

      // Section 3.2.3 of the SysV AMD64 ABI:
      // > The size of each argument gets rounded up to eightbytes. [...] Therefore the stack will always be eightbyte aligned.
      self.offset_trampoline += 8;
      self.offset_callee += 8;

      debug_assert!(
        self.allocated_stack == 0 || self.offset_callee <= self.allocated_stack
      );
    }
    self.float_params += 1;
  }

  fn move_integral(&mut self, arg: Integral) {
    // Section 3.2.3 of the SysV AMD64 ABI:
    // > If the class is INTEGER, the next available register of the sequence %rdi,
    // > %rsi, %rdx, %rcx, %r8 and %r9 is used
    // [...]
    // > Once registers are assigned, the arguments passed in memory are pushed on
    // > the stack in reversed (right-to-left) order
    let s = &mut self.assmblr;
    let param_i = self.integral_params;

    // move each argument one position to the left. The first argument in the stack moves to the last integer register (r9).
    // If the FFI function is called with a new stack frame, the arguments remaining in the stack are copied to the new stack frame.
    // Otherwise, they are copied 8 bytes lower in the same frame
    match (param_i, arg) {
      // u8 and u16 parameters are defined as u32 parameters in the V8's fast API function. The trampoline takes care of the cast.
      // Conventionally, many compilers expect 8 and 16 bit arguments to be sign/zero extended by the caller
      // See https://stackoverflow.com/a/36760539/2623340
      (0, U(B)) => x64!(s; movzx edi, sil),
      (0, I(B)) => x64!(s; movsx edi, sil),
      (0, U(W)) => x64!(s; movzx edi, si),
      (0, I(W)) => x64!(s; movsx edi, si),
      (0, U(DW) | I(DW)) => x64!(s; mov edi, esi),
      (0, U(QW) | I(QW)) => x64!(s; mov rdi, rsi),
      // The fast API expects buffer arguments passed as a pointer to a FastApiTypedArray<Uint8> struct
      // Here we blindly follow the layout of https://github.com/denoland/rusty_v8/blob/main/src/fast_api.rs#L190-L200
      // although that might be problematic: https://discord.com/channels/684898665143206084/956626010248478720/1009450940866252823
      (0, Buffer) => x64!(s; mov rdi, [rsi + 8]),

      (1, U(B)) => x64!(s; movzx esi, dl),
      (1, I(B)) => x64!(s; movsx esi, dl),
      (1, U(W)) => x64!(s; movzx esi, dx),
      (1, I(W)) => x64!(s; movsx esi, dx),
      (1, U(DW) | I(DW)) => x64!(s; mov esi, edx),
      (1, U(QW) | I(QW)) => x64!(s; mov rsi, rdx),
      (1, Buffer) => x64!(s; mov rsi, [rdx + 8]),

      (2, U(B)) => x64!(s; movzx edx, cl),
      (2, I(B)) => x64!(s; movsx edx, cl),
      (2, U(W)) => x64!(s; movzx edx, cx),
      (2, I(W)) => x64!(s; movsx edx, cx),
      (2, U(DW) | I(DW)) => x64!(s; mov edx, ecx),
      (2, U(QW) | I(QW)) => x64!(s; mov rdx, rcx),
      (2, Buffer) => x64!(s; mov rdx, [rcx + 8]),

      (3, U(B)) => x64!(s; movzx ecx, r8b),
      (3, I(B)) => x64!(s; movsx ecx, r8b),
      (3, U(W)) => x64!(s; movzx ecx, r8w),
      (3, I(W)) => x64!(s; movsx ecx, r8w),
      (3, U(DW) | I(DW)) => x64!(s; mov ecx, r8d),
      (3, U(QW) | I(QW)) => x64!(s; mov rcx, r8),
      (3, Buffer) => x64!(s; mov rcx, [r8 + 8]),

      (4, U(B)) => x64!(s; movzx r8d, r9b),
      (4, I(B)) => x64!(s; movsx r8d, r9b),
      (4, U(W)) => x64!(s; movzx r8d, r9w),
      (4, I(W)) => x64!(s; movsx r8d, r9w),
      (4, U(DW) | I(DW)) => x64!(s; mov r8d, r9d),
      (4, U(QW) | I(QW)) => x64!(s; mov r8, r9),
      (4, Buffer) => x64!(s; mov r8, [r9 + 8]),

      (5, param) => {
        let ot = self.offset_trampoline as i32;
        // First argument in stack goes to last register (r9)
        match param {
          U(B) => x64!(s; movzx r9d, BYTE [rsp + ot]),
          I(B) => x64!(s; movsx r9d, BYTE [rsp + ot]),
          U(W) => x64!(s; movzx r9d, WORD [rsp + ot]),
          I(W) => x64!(s; movsx r9d, WORD [rsp + ot]),
          U(DW) | I(DW) => x64!(s; mov r9d, [rsp + ot]),
          U(QW) | I(QW) => x64!(s; mov r9, [rsp + ot]),
          Buffer => x64!(s
            ; mov r9, [rsp + ot]
            ; mov r9, [r9 + 8]
          ),
        }
        // Section 3.2.3 of the SysV AMD64 ABI:
        // > The size of each argument gets rounded up to eightbytes. [...] Therefore the stack will always be eightbyte aligned.
        self.offset_trampoline += 8;
      }

      (6.., param) => {
        let ot = self.offset_trampoline as i32;
        let oc = self.offset_callee as i32;
        match param {
          U(B) => x64!(s
            // TODO: optimize to [rsp] (without immediate) when offset is 0
            ; movzx eax, BYTE [rsp + ot]
            ; mov [rsp + oc], eax
          ),
          I(B) => x64!(s
            ; movsx eax, BYTE [rsp + ot]
            ; mov [rsp + oc], eax
          ),
          U(W) => x64!(s
            ; movzx eax, WORD [rsp + ot]
            ; mov [rsp + oc], eax
          ),
          I(W) => x64!(s
            ; movsx eax, WORD [rsp + ot]
            ; mov [rsp + oc], eax
          ),
          U(DW) | I(DW) => x64!(s
            ; mov eax, [rsp + ot]
            ; mov [rsp + oc], eax
          ),
          U(QW) | I(QW) => x64!(s
            ; mov rax, [rsp + ot]
            ; mov [rsp + oc], rax
          ),
          Buffer => x64!(s
            ; mov rax, [rsp + ot]
            ; mov rax, [rax + 8]
            ; mov [rsp + oc], rax
          ),
        }
        // Section 3.2.3 of the SysV AMD64 ABI:
        // > The size of each argument gets rounded up to eightbytes. [...] Therefore the stack will always be eightbyte aligned.
        self.offset_trampoline += 8;
        self.offset_callee += 8;

        debug_assert!(
          self.allocated_stack == 0
            || self.offset_callee <= self.allocated_stack
        );
      }
    }
    self.integral_params += 1;
  }

  fn zero_first_arg(&mut self) {
    debug_assert!(
      self.integral_params == 0,
      "the trampoline would zero the first argument after having overridden it with the second one"
    );
    dynasm!(self.assmblr
      ; .arch x64
      ; xor edi, edi
    );
  }

  fn cast_return_value(&mut self, rv: &NativeType) {
    let s = &mut self.assmblr;
    // V8 only supports 32bit integers. We support 8 and 16 bit integers casting them to 32bits.
    // In SysV-AMD64 the convention dictates that the unused bits of the return value contain garbage, so we
    // need to zero/sign extend the return value explicitly
    match rv {
      NativeType::U8 => x64!(s; movzx eax, al),
      NativeType::I8 => x64!(s; movsx eax, al),
      NativeType::U16 => x64!(s; movzx eax, ax),
      NativeType::I16 => x64!(s; movsx eax, ax),
      _ => (),
    }
  }

  fn save_out_array_to_preserved_register(&mut self) {
    let s = &mut self.assmblr;
    // functions returning 64 bit integers have the out array appended as their last parameter,
    // and it is a *FastApiTypedArray<Int32>
    match self.integral_params {
      // Trampoline's signature is (receiver, [param0, param1, ...], *FastApiTypedArray)
      // self.integral_params account only for the original params [param0, param1, ...]
      // and the out array has not been moved left
      0 => x64!(s; mov rbx, [rsi + 8]),
      1 => x64!(s; mov rbx, [rdx + 8]),
      2 => x64!(s; mov rbx, [rcx + 8]),
      3 => x64!(s; mov rbx, [r8 + 8]),
      4 => x64!(s; mov rbx, [r9 + 8]),
      5.. => {
        x64!(s
          ; mov rax, [rsp + self.offset_trampoline as i32]
          ; mov rbx, [rax + 8]
        )
      }
    }
  }

  fn wrap_return_value_in_out_array(&mut self) {
    x64!(self.assmblr; mov [rbx], rax);
  }

  fn save_preserved_register_to_stack(&mut self) {
    x64!(self.assmblr; push rbx);
    self.offset_trampoline += 8;
    // stack pointer has been modified, and the callee stack parameters are expected at the top of the stack
    self.offset_callee = 0;
    self.frame_pointer += 8;
  }

  fn recover_preserved_register(&mut self) {
    debug_assert!(
      self.frame_pointer >= 8,
      "the trampoline would try to pop from the stack beyond its frame pointer"
    );
    x64!(self.assmblr; pop rbx);
    self.frame_pointer -= 8;
    // parameter offsets are invalid once this method is called
  }

  fn allocate_stack(&mut self, params: &[NativeType]) {
    let mut int_params = 0u32;
    let mut float_params = 0u32;
    for param in params {
      match param {
        NativeType::F32 | NativeType::F64 => float_params += 1,
        _ => int_params += 1,
      }
    }
    let mut stack_size = (int_params.saturating_sub(Self::INTEGRAL_REGISTERS)
      + float_params.saturating_sub(Self::FLOAT_REGISTERS))
      * 8;

    // Align new stack frame (accounting for the 8 byte of the trampoline caller's return address
    // and any other potential addition to the stack prior to this allocation)
    // Section 3.2.2 of the SysV AMD64 ABI:
    // > The end of the input argument area shall be aligned on a 16 (32 or 64, if
    // > __m256 or __m512 is passed on stack) byte boundary. In other words, the value
    // > (%rsp + 8) is always a multiple of 16 (32 or 64) when control is transferred to
    // > the function entry point. The stack pointer, %rsp, always points to the end of the
    // > latest allocated stack frame.
    stack_size += padding_to_align(16, self.frame_pointer + stack_size + 8);

    if stack_size > 0 {
      x64!(self.assmblr; sub rsp, stack_size as i32);
      self.offset_trampoline += stack_size;
      // stack pointer has been modified, and the callee stack parameters are expected at the top of the stack
      self.offset_callee = 0;
      self.allocated_stack += stack_size;
      self.frame_pointer += stack_size;
    }
  }

  fn deallocate_stack(&mut self) {
    debug_assert!(
      self.frame_pointer >= self.allocated_stack,
      "the trampoline would try to deallocate stack beyond its frame pointer"
    );
    if self.allocated_stack > 0 {
      x64!(self.assmblr; add rsp, self.allocated_stack as i32);

      self.frame_pointer -= self.allocated_stack;
      self.allocated_stack = 0;
    }
  }

  fn call(&mut self, ptr: *const c_void) {
    // the stack has been aligned during stack allocation and/or pushing of preserved registers
    debug_assert!(
      (8 + self.frame_pointer) % 16 == 0,
      "the trampoline would call the FFI function with an unaligned stack"
    );
    x64!(self.assmblr
      ; mov rax, QWORD ptr as _
      ; call rax
    );
  }

  fn tailcall(&mut self, ptr: *const c_void) {
    // stack pointer is never modified and remains aligned
    // return address remains the one provided by the trampoline's caller (V8)
    debug_assert!(
      self.allocated_stack == 0,
      "the trampoline would tail call the FFI function with an outstanding stack allocation"
    );
    debug_assert!(
      self.frame_pointer == 0,
      "the trampoline would tail call the FFI function with outstanding locals in the frame"
    );
    x64!(self.assmblr
      ; mov rax, QWORD ptr as _
      ; jmp rax
    );
  }

  fn ret(&mut self) {
    debug_assert!(
      self.allocated_stack == 0,
      "the trampoline would return with an outstanding stack allocation"
    );
    debug_assert!(
      self.frame_pointer == 0,
      "the trampoline would return with outstanding locals in the frame"
    );
    x64!(self.assmblr; ret);
  }

  fn is_recv_arg_overridden(&self) -> bool {
    // V8 receiver is the first parameter of the trampoline function and is a pointer
    self.integral_params > 0
  }

  fn must_cast_return_value(&self, rv: &NativeType) -> bool {
    // V8 only supports i32 and u32 return types for integers
    // We support 8 and 16 bit integers by extending them to 32 bits in the trampoline before returning
    matches!(
      rv,
      NativeType::U8 | NativeType::I8 | NativeType::U16 | NativeType::I16
    )
  }

  fn finalize(self) -> ExecutableBuffer {
    self.assmblr.finalize().unwrap()
  }
}

struct Aarch64Apple {
  // Reference https://github.com/ARM-software/abi-aa/blob/main/aapcs64/aapcs64.rst
  assmblr: dynasmrt::aarch64::Assembler,
  // Parameter counters
  integral_params: u32,
  float_params: u32,
  // Stack offset accumulators
  offset_trampoline: u32,
  offset_callee: u32,
  allocated_stack: u32,
}

#[cfg_attr(
  not(all(target_aarch = "aarch64", target_vendor = "apple")),
  allow(dead_code)
)]
impl Aarch64Apple {
  // Integral arguments go to the first 8 GPR: x0-x7
  const INTEGRAL_REGISTERS: u32 = 8;
  // Floating-point arguments go to the first 8 SIMD & Floating-Point registers: v0-v1
  const FLOAT_REGISTERS: u32 = 8;

  fn new() -> Self {
    Self {
      assmblr: dynasmrt::aarch64::Assembler::new().unwrap(),
      integral_params: 0,
      float_params: 0,
      offset_trampoline: 0,
      offset_callee: 0,
      allocated_stack: 0,
    }
  }

  fn compile(sym: &Symbol) -> Trampoline {
    let mut compiler = Self::new();

    for param in sym.parameter_types.iter().cloned() {
      compiler.move_left(param)
    }
    if !compiler.is_recv_arg_overridden() {
      // the receiver object should never be expected. Avoid its unexpected or deliberate leak
      compiler.zero_first_arg();
    }

    compiler.tailcall(sym.ptr.as_ptr());

    Trampoline(compiler.finalize())
  }

  fn move_left(&mut self, param: NativeType) {
    // Section 6.4.2 of the Aarch64 Procedure Call Standard (PCS), on argument classification:
    // - INTEGRAL or POINTER:
    //    > If the argument is an Integral or Pointer Type, the size of the argument is less than or equal to 8 bytes
    //    > and the NGRN is less than 8, the argument is copied to the least significant bits in x[NGRN].
    //
    // - Floating-Point or Vector:
    //    > If the argument is a Half-, Single-, Double- or Quad- precision Floating-point or short vector type
    //    > and the NSRN is less than 8, then the argument is allocated to the least significant bits of register v[NSRN]
    match param.into() {
      Int(integral) => self.move_integral(integral),
      Float(float) => self.move_float(float),
    }
  }

  fn move_float(&mut self, param: Floating) {
    // Section 6.4.2 of the Aarch64 PCS:
    // > If the argument is a Half-, Single-, Double- or Quad- precision Floating-point or short vector type and the NSRN is less than 8, then the
    // > argument is allocated to the least significant bits of register v[NSRN]. The NSRN is incremented by one. The argument has now been allocated.
    // > [if NSRN is equal or more than 8]
    // > The argument is copied to memory at the adjusted NSAA. The NSAA is incremented by the size of the argument. The argument has now been allocated.
    let param_i = self.float_params;

    let is_in_stack = param_i >= Self::FLOAT_REGISTERS;
    if is_in_stack {
      // https://developer.apple.com/documentation/xcode/writing-arm64-code-for-apple-platforms:
      // > Function arguments may consume slots on the stack that are not multiples of 8 bytes.
      // (i.e. natural alignment instead of eightbyte alignment)
      let padding_trampl =
        (param.size() - self.offset_trampoline % param.size()) % param.size();
      let padding_callee =
        (param.size() - self.offset_callee % param.size()) % param.size();

      // floats are only moved to accommodate integer movement in the stack
      let stack_has_moved = self.integral_params >= Self::INTEGRAL_REGISTERS;
      if stack_has_moved {
        let s = &mut self.assmblr;
        let ot = self.offset_trampoline;
        let oc = self.offset_callee;
        match param {
          Single => aarch64!(s
            // 6.1.2 Aarch64 PCS:
            // > Registers v8-v15 must be preserved by a callee across subroutine calls;
            // > the remaining registers (v0-v7, v16-v31) do not need to be preserved (or should be preserved by the caller).
            ; ldr s16, [sp, ot + padding_trampl]
            ; str s16, [sp, oc + padding_callee]
          ),
          Double => aarch64!(s
            ; ldr d16, [sp, ot + padding_trampl]
            ; str d16, [sp, oc + padding_callee]
          ),
        }
      }
      self.offset_trampoline += padding_trampl + param.size();
      self.offset_callee += padding_callee + param.size();

      debug_assert!(
        self.allocated_stack == 0 || self.offset_callee <= self.allocated_stack
      );
    }
    self.float_params += 1;
  }

  fn move_integral(&mut self, param: Integral) {
    let s = &mut self.assmblr;
    // Section 6.4.2 of the Aarch64 PCS:
    // If the argument is an Integral or Pointer Type, the size of the argument is less than or
    // equal to 8 bytes and the NGRN is less than 8, the argument is copied to the least
    // significant bits in x[NGRN]. The NGRN is incremented by one. The argument has now been
    // allocated.
    // [if NGRN is equal or more than 8]
    // The argument is copied to memory at the adjusted NSAA. The NSAA is incremented by the size
    // of the argument. The argument has now been allocated.
    let param_i = self.integral_params;

    // move each argument one position to the left. The first argument in the stack moves to the last integer register (x7).
    match (param_i, param) {
      // From https://developer.apple.com/documentation/xcode/writing-arm64-code-for-apple-platforms:
      // > The caller of a function is responsible for signing or zero-extending any argument with fewer than 32 bits.
      // > The standard ABI expects the callee to sign or zero-extend those arguments.
      // (this applies to register parameters, as stack parameters are not eightbyte aligned in Apple)
      (0, I(B)) => aarch64!(s; sxtb w0, w1),
      (0, U(B)) => aarch64!(s; and w0, w1, 0xFF),
      (0, I(W)) => aarch64!(s; sxth w0, w1),
      (0, U(W)) => aarch64!(s; and w0, w1, 0xFFFF),
      (0, I(DW) | U(DW)) => aarch64!(s; mov w0, w1),
      (0, I(QW) | U(QW)) => aarch64!(s; mov x0, x1),
      // The fast API expects buffer arguments passed as a pointer to a FastApiTypedArray<Uint8> struct
      // Here we blindly follow the layout of https://github.com/denoland/rusty_v8/blob/main/src/fast_api.rs#L190-L200
      // although that might be problematic: https://discord.com/channels/684898665143206084/956626010248478720/1009450940866252823
      (0, Buffer) => aarch64!(s; ldr x0, [x1, 8]),

      (1, I(B)) => aarch64!(s; sxtb w1, w2),
      (1, U(B)) => aarch64!(s; and w1, w2, 0xFF),
      (1, I(W)) => aarch64!(s; sxth w1, w2),
      (1, U(W)) => aarch64!(s; and w1, w2, 0xFFFF),
      (1, I(DW) | U(DW)) => aarch64!(s; mov w1, w2),
      (1, I(QW) | U(QW)) => aarch64!(s; mov x1, x2),
      (1, Buffer) => aarch64!(s; ldr x1, [x2, 8]),

      (2, I(B)) => aarch64!(s; sxtb w2, w3),
      (2, U(B)) => aarch64!(s; and w2, w3, 0xFF),
      (2, I(W)) => aarch64!(s; sxth w2, w3),
      (2, U(W)) => aarch64!(s; and w2, w3, 0xFFFF),
      (2, I(DW) | U(DW)) => aarch64!(s; mov w2, w3),
      (2, I(QW) | U(QW)) => aarch64!(s; mov x2, x3),
      (2, Buffer) => aarch64!(s; ldr x2, [x3, 8]),

      (3, I(B)) => aarch64!(s; sxtb w3, w4),
      (3, U(B)) => aarch64!(s; and w3, w4, 0xFF),
      (3, I(W)) => aarch64!(s; sxth w3, w4),
      (3, U(W)) => aarch64!(s; and w3, w4, 0xFFFF),
      (3, I(DW) | U(DW)) => aarch64!(s; mov w3, w4),
      (3, I(QW) | U(QW)) => aarch64!(s; mov x3, x4),
      (3, Buffer) => aarch64!(s; ldr x3, [x4, 8]),

      (4, I(B)) => aarch64!(s; sxtb w4, w5),
      (4, U(B)) => aarch64!(s; and w4, w5, 0xFF),
      (4, I(W)) => aarch64!(s; sxth w4, w5),
      (4, U(W)) => aarch64!(s; and w4, w5, 0xFFFF),
      (4, I(DW) | U(DW)) => aarch64!(s; mov w4, w5),
      (4, I(QW) | U(QW)) => aarch64!(s; mov x4, x5),
      (4, Buffer) => aarch64!(s; ldr x4, [x5, 8]),

      (5, I(B)) => aarch64!(s; sxtb w5, w6),
      (5, U(B)) => aarch64!(s; and w5, w6, 0xFF),
      (5, I(W)) => aarch64!(s; sxth w5, w6),
      (5, U(W)) => aarch64!(s; and w5, w6, 0xFFFF),
      (5, I(DW) | U(DW)) => aarch64!(s; mov w5, w6),
      (5, I(QW) | U(QW)) => aarch64!(s; mov x5, x6),
      (5, Buffer) => aarch64!(s; ldr x5, [x6, 8]),

      (6, I(B)) => aarch64!(s; sxtb w6, w7),
      (6, U(B)) => aarch64!(s; and w6, w7, 0xFF),
      (6, I(W)) => aarch64!(s; sxth w6, w7),
      (6, U(W)) => aarch64!(s; and w6, w7, 0xFFFF),
      (6, I(DW) | U(DW)) => aarch64!(s; mov w6, w7),
      (6, I(QW) | U(QW)) => aarch64!(s; mov x6, x7),
      (6, Buffer) => aarch64!(s; ldr x6, [x7, 8]),

      (7, param) => {
        let ot = self.offset_trampoline;
        match param {
          I(B) => {
            aarch64!(s; ldrsb w7, [sp, ot])
          }
          U(B) => {
            // ldrb zero-extends the byte to fill the 32bits of the register
            aarch64!(s; ldrb w7, [sp, ot])
          }
          I(W) => {
            aarch64!(s; ldrsh w7, [sp, ot])
          }
          U(W) => {
            // ldrh zero-extends the half-word to fill the 32bits of the register
            aarch64!(s; ldrh w7, [sp, ot])
          }
          I(DW) | U(DW) => {
            aarch64!(s; ldr w7, [sp, ot])
          }
          I(QW) | U(QW) => {
            aarch64!(s; ldr x7, [sp, ot])
          }
          Buffer => {
            aarch64!(s
              ; ldr x7, [sp, ot]
              ; ldr x7, [x7, 8]
            )
          }
        }
        // 16 and 8 bit integers are 32 bit integers in v8
        self.offset_trampoline += max(param.size(), 4);
      }

      (8.., param) => {
        // https://developer.apple.com/documentation/xcode/writing-arm64-code-for-apple-platforms:
        // > Function arguments may consume slots on the stack that are not multiples of 8 bytes.
        // (i.e. natural alignment instead of eightbyte alignment)
        //
        // N.B. V8 does not currently follow this Apple's policy, and instead aligns all arguments to 8 Byte boundaries.
        // The current implementation follows the V8 incorrect calling convention for the sake of a seamless experience
        // for the Deno users. Whenever upgrading V8 we should make sure that the bug has not been amended, and revert this
        // workaround once it has been. The bug is being tracked in https://bugs.chromium.org/p/v8/issues/detail?id=13171
        let size_original = param.size();
        // 16 and 8 bit integers are 32 bit integers in v8
        // let size_trampl = max(size_original, 4);  // <-- Apple alignment
        let size_trampl = 8; // <-- V8 incorrect alignment
        let padding_trampl =
          padding_to_align(size_trampl, self.offset_trampoline);
        let padding_callee =
          padding_to_align(size_original, self.offset_callee);
        let ot = self.offset_trampoline;
        let oc = self.offset_callee;
        match param {
          I(B) | U(B) => aarch64!(s
            ; ldr w8, [sp, ot + padding_trampl]
            ; strb w8, [sp, oc + padding_callee]
          ),
          I(W) | U(W) => aarch64!(s
            ; ldr w8, [sp, ot + padding_trampl]
            ; strh w8, [sp, oc + padding_callee]
          ),
          I(DW) | U(DW) => aarch64!(s
            ;  ldr w8, [sp, ot + padding_trampl]
            ; str w8, [sp, oc + padding_callee]
          ),
          I(QW) | U(QW) => aarch64!(s
            ; ldr x8, [sp, ot + padding_trampl]
            ; str x8, [sp, oc + padding_callee]
          ),
          Buffer => aarch64!(s
            ; ldr x8, [sp, ot + padding_trampl]
            ; ldr x8, [x8, 8]
            ; str x8, [sp, oc + padding_callee]
          ),
        }
        self.offset_trampoline += padding_trampl + size_trampl;
        self.offset_callee += padding_callee + size_original;

        debug_assert!(
          self.allocated_stack == 0
            || self.offset_callee <= self.allocated_stack
        );
      }
    };
    self.integral_params += 1;
  }

  fn zero_first_arg(&mut self) {
    debug_assert!(
      self.integral_params == 0,
      "the trampoline would zero the first argument after having overridden it with the second one"
    );
    aarch64!(self.assmblr; mov x0, xzr);
  }

  fn save_out_array_to_preserved_register(&mut self) {
    let s = &mut self.assmblr;
    // functions returning 64 bit integers have the out array appended as their last parameter,
    // and it is a *FastApiTypedArray<Int32>
    match self.integral_params {
      // x0 is always V8's receiver
      0 => aarch64!(s; ldr x19, [x1, 8]),
      1 => aarch64!(s; ldr x19, [x2, 8]),
      2 => aarch64!(s; ldr x19, [x3, 8]),
      3 => aarch64!(s; ldr x19, [x4, 8]),
      4 => aarch64!(s; ldr x19, [x5, 8]),
      5 => aarch64!(s; ldr x19, [x6, 8]),
      6 => aarch64!(s; ldr x19, [x7, 8]),
      7.. => {
        aarch64!(s
          ; ldr x19, [sp, self.offset_trampoline]
          ; ldr x19, [x19, 8]
        )
      }
    }
  }

  fn wrap_return_value_in_out_array(&mut self) {
    aarch64!(self.assmblr; str x0, [x19]);
  }

  #[allow(clippy::unnecessary_cast)]
  fn save_frame_record(&mut self) {
    debug_assert!(
      self.allocated_stack >= 16,
      "the trampoline would try to save the frame record to the stack without having allocated enough space for it"
    );
    aarch64!(self.assmblr
      // Frame record is stored at the bottom of the stack frame
      ; stp x29, x30, [sp, self.allocated_stack - 16]
      ; add x29, sp, self.allocated_stack - 16
    )
  }

  #[allow(clippy::unnecessary_cast)]
  fn recover_frame_record(&mut self) {
    // The stack cannot have been deallocated before the frame record is restored
    debug_assert!(
      self.allocated_stack >= 16,
      "the trampoline would try to load the frame record from the stack, but it couldn't possibly contain it"
    );
    // Frame record is stored at the bottom of the stack frame
    aarch64!(self.assmblr; ldp x29, x30, [sp, self.allocated_stack - 16])
  }

  fn save_preserved_register_to_stack(&mut self) {
    // If a preserved register needs to be used, we must have allocated at least 32 bytes in the stack
    // 16 for the frame record, 8 for the preserved register, and 8 for 16-byte alignment.
    debug_assert!(
      self.allocated_stack >= 32,
      "the trampoline would try to save a register to the stack without having allocated enough space for it"
    );
    // preserved register is stored after frame record
    aarch64!(self.assmblr; str x19, [sp, self.allocated_stack - 24]);
  }

  fn recover_preserved_register(&mut self) {
    // The stack cannot have been deallocated before the preserved register is restored
    // 16 for the frame record, 8 for the preserved register, and 8 for 16-byte alignment.
    debug_assert!(
      self.allocated_stack >= 32,
      "the trampoline would try to recover the value of a register from the stack, but it couldn't possibly contain it"
    );
    // preserved register is stored after frame record
    aarch64!(self.assmblr; ldr x19, [sp, self.allocated_stack - 24]);
  }

  fn allocate_stack(&mut self, symbol: &Symbol) {
    // https://developer.apple.com/documentation/xcode/writing-arm64-code-for-apple-platforms:
    // > Function arguments may consume slots on the stack that are not multiples of 8 bytes.
    // (i.e. natural alignment instead of eightbyte alignment)
    let mut int_params = 0u32;
    let mut float_params = 0u32;
    let mut stack_size = 0u32;
    for param in symbol.parameter_types.iter().cloned() {
      match param.into() {
        Float(float_param) => {
          float_params += 1;
          if float_params > Self::FLOAT_REGISTERS {
            stack_size += float_param.size();
          }
        }
        Int(integral_param) => {
          int_params += 1;
          if int_params > Self::INTEGRAL_REGISTERS {
            stack_size += integral_param.size();
          }
        }
      }
    }

    // Section 6.2.3 of the Aarch64 PCS:
    // > Each frame shall link to the frame of its caller by means of a frame record of two 64-bit values on the stack
    stack_size += 16;

    // Section 6.2.2 of Aarch64 PCS:
    // > At any point at which memory is accessed via SP, the hardware requires that
    // > - SP mod 16 = 0. The stack must be quad-word aligned.
    // > The stack must also conform to the following constraint at a public interface:
    // > - SP mod 16 = 0. The stack must be quad-word aligned.
    stack_size += padding_to_align(16, stack_size);

    if stack_size > 0 {
      aarch64!(self.assmblr; sub sp, sp, stack_size);
      self.offset_trampoline += stack_size;
      // stack pointer has been modified, and the callee stack parameters are expected at the top of the stack
      self.offset_callee = 0;
      self.allocated_stack += stack_size;
    }
  }

  fn deallocate_stack(&mut self) {
    if self.allocated_stack > 0 {
      aarch64!(self.assmblr; add sp, sp, self.allocated_stack);
      self.allocated_stack = 0;
    }
  }

  fn call(&mut self, ptr: *const c_void) {
    // the stack has been aligned during stack allocation
    // Frame record has been stored in stack and frame pointer points to it
    debug_assert!(
      self.allocated_stack % 16 == 0,
      "the trampoline would call the FFI function with an unaligned stack"
    );
    debug_assert!(
      self.allocated_stack >= 16,
      "the trampoline would call the FFI function without allocating enough stack for the frame record"
    );
    self.load_callee_address(ptr);
    aarch64!(self.assmblr; blr x8);
  }

  fn tailcall(&mut self, ptr: *const c_void) {
    // stack pointer is never modified and remains aligned
    // frame pointer and link register remain the one provided by the trampoline's caller (V8)
    debug_assert!(
      self.allocated_stack == 0,
      "the trampoline would tail call the FFI function with an outstanding stack allocation"
    );
    self.load_callee_address(ptr);
    aarch64!(self.assmblr; br x8);
  }

  fn ret(&mut self) {
    debug_assert!(
      self.allocated_stack == 0,
      "the trampoline would return with an outstanding stack allocation"
    );
    aarch64!(self.assmblr; ret);
  }

  fn load_callee_address(&mut self, ptr: *const c_void) {
    // Like all ARM instructions, move instructions are 32bit long and can fit at most 16bit immediates.
    // bigger immediates are loaded in multiple steps applying a left-shift modifier
    let mut address = ptr as u64;
    let mut imm16 = address & 0xFFFF;
    aarch64!(self.assmblr; movz x8, imm16 as u32);
    address >>= 16;
    let mut shift = 16;
    while address > 0 {
      imm16 = address & 0xFFFF;
      if imm16 != 0 {
        aarch64!(self.assmblr; movk x8, imm16 as u32, lsl shift);
      }
      address >>= 16;
      shift += 16;
    }
  }

  fn is_recv_arg_overridden(&self) -> bool {
    // V8 receiver is the first parameter of the trampoline function and is a pointer
    self.integral_params > 0
  }

  fn finalize(self) -> ExecutableBuffer {
    self.assmblr.finalize().unwrap()
  }
}

struct Win64 {
  // Reference: https://github.com/MicrosoftDocs/cpp-docs/blob/main/docs/build/x64-calling-convention.md
  assmblr: dynasmrt::x64::Assembler,
  // Params counter (Windows does not distinguish by type with regards to parameter position)
  params: u32,
  // Stack offset accumulators
  offset_trampoline: u32,
  offset_callee: u32,
  allocated_stack: u32,
  frame_pointer: u32,
}

#[cfg_attr(
  not(all(target_aarch = "x86_64", target_family = "windows")),
  allow(dead_code)
)]
impl Win64 {
  // Section "Parameter Passing" of the Windows x64 calling convention:
  // > By default, the x64 calling convention passes the first four arguments to a function in registers.
  const REGISTERS: u32 = 4;

  fn new() -> Self {
    Self {
      assmblr: dynasmrt::x64::Assembler::new().unwrap(),
      params: 0,
      // trampoline caller's return address + trampoline's shadow space
      offset_trampoline: 8 + 32,
      offset_callee: 8 + 32,
      allocated_stack: 0,
      frame_pointer: 0,
    }
  }

  fn compile(sym: &Symbol) -> Trampoline {
    let mut compiler = Self::new();

    let must_cast_return_value =
      compiler.must_cast_return_value(&sym.result_type);
    let cannot_tailcall = must_cast_return_value;

    if cannot_tailcall {
      compiler.allocate_stack(&sym.parameter_types);
    }

    for param in sym.parameter_types.iter().cloned() {
      compiler.move_left(param)
    }
    if !compiler.is_recv_arg_overridden() {
      // the receiver object should never be expected. Avoid its unexpected or deliberate leak
      compiler.zero_first_arg();
    }

    if cannot_tailcall {
      compiler.call(sym.ptr.as_ptr());
      if must_cast_return_value {
        compiler.cast_return_value(&sym.result_type);
      }
      compiler.deallocate_stack();
      compiler.ret();
    } else {
      compiler.tailcall(sym.ptr.as_ptr());
    }

    Trampoline(compiler.finalize())
  }

  fn move_left(&mut self, param: NativeType) {
    // Section "Parameter Passing" of the Windows x64 calling convention:
    // > By default, the x64 calling convention passes the first four arguments to a function in registers.
    // > The registers used for these arguments depend on the position and type of the argument.
    // > Remaining arguments get pushed on the stack in right-to-left order.
    // > [...]
    // > Integer valued arguments in the leftmost four positions are passed in left-to-right order in RCX, RDX, R8, and R9
    // > [...]
    // > Any floating-point and double-precision arguments in the first four parameters are passed in XMM0 - XMM3, depending on position
    let s = &mut self.assmblr;
    let param_i = self.params;

    // move each argument one position to the left. The first argument in the stack moves to the last register (r9 or xmm3).
    // If the FFI function is called with a new stack frame, the arguments remaining in the stack are copied to the new stack frame.
    // Otherwise, they are copied 8 bytes lower in the same frame
    match (param_i, param.into()) {
      // Section "Parameter Passing" of the Windows x64 calling convention:
      // > All integer arguments in registers are right-justified, so the callee can ignore the upper bits of the register
      // > and access only the portion of the register necessary.
      // (i.e. unlike in SysV or Aarch64-Apple, 8/16 bit integers are not expected to be zero/sign extended)
      (0, Int(U(B | W | DW) | I(B | W | DW))) => x64!(s; mov ecx, edx),
      (0, Int(U(QW) | I(QW))) => x64!(s; mov rcx, rdx),
      // The fast API expects buffer arguments passed as a pointer to a FastApiTypedArray<Uint8> struct
      // Here we blindly follow the layout of https://github.com/denoland/rusty_v8/blob/main/src/fast_api.rs#L190-L200
      // although that might be problematic: https://discord.com/channels/684898665143206084/956626010248478720/1009450940866252823
      (0, Int(Buffer)) => x64!(s; mov rcx, [rdx + 8]),
      // Use movaps for singles and doubles, benefits of smaller encoding outweigh those of using the correct instruction for the type,
      // which for doubles should technically be movapd
      (0, Float(_)) => {
        x64!(s; movaps xmm0, xmm1);
        self.zero_first_arg();
      }

      (1, Int(U(B | W | DW) | I(B | W | DW))) => x64!(s; mov edx, r8d),
      (1, Int(U(QW) | I(QW))) => x64!(s; mov rdx, r8),
      (1, Int(Buffer)) => x64!(s; mov rdx, [r8 + 8]),
      (1, Float(_)) => x64!(s; movaps xmm1, xmm2),

      (2, Int(U(B | W | DW) | I(B | W | DW))) => x64!(s; mov r8d, r9d),
      (2, Int(U(QW) | I(QW))) => x64!(s; mov r8, r9),
      (2, Int(Buffer)) => x64!(s; mov r8, [r9 + 8]),
      (2, Float(_)) => x64!(s; movaps xmm2, xmm3),

      (3, param) => {
        let ot = self.offset_trampoline as i32;
        match param {
          Int(U(B | W | DW) | I(B | W | DW)) => {
            x64!(s; mov r9d, [rsp + ot])
          }
          Int(U(QW) | I(QW)) => {
            x64!(s; mov r9, [rsp + ot])
          }
          Int(Buffer) => {
            x64!(s
              ; mov r9, [rsp + ot]
              ; mov r9, [r9 + 8])
          }
          Float(_) => {
            // parameter 4 is always 16-byte aligned, so we can use movaps instead of movups
            x64!(s; movaps xmm3, [rsp + ot])
          }
        }
        // Section "x64 Aggregate and Union layout" of the windows x64 software conventions doc:
        // > The alignment of the beginning of a structure or a union is the maximum alignment of any individual member
        // Ref: https://github.com/MicrosoftDocs/cpp-docs/blob/main/docs/build/x64-software-conventions.md#x64-aggregate-and-union-layout
        self.offset_trampoline += 8;
      }
      (4.., param) => {
        let ot = self.offset_trampoline as i32;
        let oc = self.offset_callee as i32;
        match param {
          Int(U(B | W | DW) | I(B | W | DW)) => {
            x64!(s
              ; mov eax, [rsp + ot]
              ; mov [rsp + oc], eax
            )
          }
          Int(U(QW) | I(QW)) => {
            x64!(s
              ; mov rax, [rsp + ot]
              ; mov [rsp + oc], rax
            )
          }
          Int(Buffer) => {
            x64!(s
              ; mov rax, [rsp + ot]
              ; mov rax, [rax + 8]
              ; mov [rsp + oc], rax
            )
          }
          Float(_) => {
            x64!(s
              ; movups xmm4, [rsp + ot]
              ; movups [rsp + oc], xmm4
            )
          }
        }
        // Section "x64 Aggregate and Union layout" of the windows x64 software conventions doc:
        // > The alignment of the beginning of a structure or a union is the maximum alignment of any individual member
        // Ref: https://github.com/MicrosoftDocs/cpp-docs/blob/main/docs/build/x64-software-conventions.md#x64-aggregate-and-union-layout
        self.offset_trampoline += 8;
        self.offset_callee += 8;

        debug_assert!(
          self.allocated_stack == 0
            || self.offset_callee <= self.allocated_stack
        );
      }
    }
    self.params += 1;
  }

  fn zero_first_arg(&mut self) {
    debug_assert!(
      self.params == 0,
      "the trampoline would zero the first argument after having overridden it with the second one"
    );
    x64!(self.assmblr; xor ecx, ecx);
  }

  fn cast_return_value(&mut self, rv: &NativeType) {
    let s = &mut self.assmblr;
    // V8 only supports 32bit integers. We support 8 and 16 bit integers casting them to 32bits.
    // Section "Return Values" of the Windows x64 Calling Convention doc:
    // > The state of unused bits in the value returned in RAX or XMM0 is undefined.
    match rv {
      NativeType::U8 => x64!(s; movzx eax, al),
      NativeType::I8 => x64!(s; movsx eax, al),
      NativeType::U16 => x64!(s; movzx eax, ax),
      NativeType::I16 => x64!(s; movsx eax, ax),
      _ => (),
    }
  }

  fn save_out_array_to_preserved_register(&mut self) {
    let s = &mut self.assmblr;
    // functions returning 64 bit integers have the out array appended as their last parameter,
    // and it is a *FastApiTypedArray<Int32>
    match self.params {
      // rcx is always V8 receiver
      0 => x64!(s; mov rbx, [rdx + 8]),
      1 => x64!(s; mov rbx, [r8 + 8]),
      2 => x64!(s; mov rbx, [r9 + 8]),
      3.. => {
        x64!(s
          ; mov rax, [rsp + self.offset_trampoline as i32]
          ; mov rbx, [rax + 8]
        )
      }
    }
  }

  fn wrap_return_value_in_out_array(&mut self) {
    x64!(self.assmblr; mov [rbx], rax)
  }

  fn save_preserved_register_to_stack(&mut self) {
    x64!(self.assmblr; push rbx);
    self.offset_trampoline += 8;
    // stack pointer has been modified, and the callee stack parameters are expected at the top of the stack
    self.offset_callee = 0;
    self.frame_pointer += 8;
  }

  fn recover_preserved_register(&mut self) {
    debug_assert!(
      self.frame_pointer >= 8,
      "the trampoline would try to pop from the stack beyond its frame pointer"
    );
    x64!(self.assmblr; pop rbx);
    self.frame_pointer -= 8;
    // parameter offsets are invalid once this method is called
  }

  fn allocate_stack(&mut self, params: &[NativeType]) {
    let mut stack_size = 0;
    // Section "Calling Convention Defaults" of the x64-calling-convention and Section "Stack Allocation" of the stack-usage docs:
    // > The x64 Application Binary Interface (ABI) uses a four-register fast-call calling convention by default.
    // > Space is allocated on the call stack as a shadow store for callees to save those registers.
    // > [...]
    // > Any parameters beyond the first four must be stored on the stack after the shadow store before the call
    // > [...]
    // > Even if the called function has fewer than 4 parameters, these 4 stack locations are effectively owned by the called function,
    // > and may be used by the called function for other purposes besides saving parameter register values
    stack_size += max(params.len() as u32, 4) * 8;

    // Align new stack frame (accounting for the 8 byte of the trampoline caller's return address
    // and any other potential addition to the stack prior to this allocation)
    // Section "Stack Allocation" of stack-usage docs:
    // > The stack will always be maintained 16-byte aligned, except within the prolog (for example, after the return address is pushed)
    stack_size += padding_to_align(16, self.frame_pointer + stack_size + 8);

    x64!(self.assmblr; sub rsp, stack_size as i32);
    self.offset_trampoline += stack_size;
    // stack pointer has been modified, and the callee stack parameters are expected at the top of the stack right after the shadow space
    self.offset_callee = 32;
    self.allocated_stack += stack_size;
    self.frame_pointer += stack_size;
  }

  fn deallocate_stack(&mut self) {
    debug_assert!(
      self.frame_pointer >= self.allocated_stack,
      "the trampoline would try to deallocate stack beyond its frame pointer"
    );
    x64!(self.assmblr; add rsp, self.allocated_stack as i32);
    self.frame_pointer -= self.allocated_stack;
    self.allocated_stack = 0;
  }

  fn call(&mut self, ptr: *const c_void) {
    // the stack has been aligned during stack allocation and/or pushing of preserved registers
    debug_assert!(
      (8 + self.frame_pointer) % 16 == 0,
      "the trampoline would call the FFI function with an unaligned stack"
    );
    x64!(self.assmblr
      ; mov rax, QWORD ptr as _
      ; call rax
    );
  }

  fn tailcall(&mut self, ptr: *const c_void) {
    // stack pointer is never modified and remains aligned
    // return address remains the one provided by the trampoline's caller (V8)
    debug_assert!(
      self.allocated_stack == 0,
      "the trampoline would tail call the FFI function with an outstanding stack allocation"
    );
    debug_assert!(
      self.frame_pointer == 0,
      "the trampoline would tail call the FFI function with outstanding locals in the frame"
    );
    x64!(self.assmblr
      ; mov rax, QWORD ptr as _
      ; jmp rax
    );
  }

  fn ret(&mut self) {
    debug_assert!(
      self.allocated_stack == 0,
      "the trampoline would return with an outstanding stack allocation"
    );
    debug_assert!(
      self.frame_pointer == 0,
      "the trampoline would return with outstanding locals in the frame"
    );
    x64!(self.assmblr; ret);
  }

  fn is_recv_arg_overridden(&self) -> bool {
    self.params > 0
  }

  fn must_cast_return_value(&self, rv: &NativeType) -> bool {
    // V8 only supports i32 and u32 return types for integers
    // We support 8 and 16 bit integers by extending them to 32 bits in the trampoline before returning
    matches!(
      rv,
      NativeType::U8 | NativeType::I8 | NativeType::U16 | NativeType::I16
    )
  }

  fn finalize(self) -> ExecutableBuffer {
    self.assmblr.finalize().unwrap()
  }
}

fn padding_to_align(alignment: u32, size: u32) -> u32 {
  (alignment - size % alignment) % alignment
}

#[derive(Clone, Copy, Debug)]
enum Floating {
  Single = 4,
  Double = 8,
}

impl Floating {
  fn size(self) -> u32 {
    self as u32
  }
}

use Floating::*;

#[derive(Clone, Copy, Debug)]
enum Integral {
  I(Size),
  U(Size),
  Buffer,
}

impl Integral {
  fn size(self) -> u32 {
    match self {
      I(size) | U(size) => size as u32,
      Buffer => 8,
    }
  }
}

use Integral::*;

#[derive(Clone, Copy, Debug)]
enum Size {
  B = 1,
  W = 2,
  DW = 4,
  QW = 8,
}
use Size::*;

#[allow(clippy::enum_variant_names)]
#[derive(Clone, Copy, Debug)]
enum Param {
  Int(Integral),
  Float(Floating),
}

use Param::*;

impl From<NativeType> for Param {
  fn from(native: NativeType) -> Self {
    match native {
      NativeType::F32 => Float(Single),
      NativeType::F64 => Float(Double),
      NativeType::Bool | NativeType::U8 => Int(U(B)),
      NativeType::U16 => Int(U(W)),
      NativeType::U32 | NativeType::Void => Int(U(DW)),
      NativeType::U64
      | NativeType::USize
      | NativeType::Pointer
      | NativeType::Function => Int(U(QW)),
      NativeType::I8 => Int(I(B)),
      NativeType::I16 => Int(I(W)),
      NativeType::I32 => Int(I(DW)),
      NativeType::I64 | NativeType::ISize => Int(I(QW)),
      NativeType::Buffer => Int(Buffer),
      NativeType::Struct(_) => unimplemented!(),
    }
  }
}

#[cfg(test)]
mod tests {
  use std::ptr::null_mut;

  use libffi::middle::Type;

  use crate::NativeType;
  use crate::Symbol;

  fn symbol(parameters: Vec<NativeType>, ret: NativeType) -> Symbol {
    Symbol {
      cif: libffi::middle::Cif::new(vec![], Type::void()),
      ptr: libffi::middle::CodePtr(null_mut()),
      parameter_types: parameters,
      result_type: ret,
    }
  }

  mod sysv_amd64 {
    use std::ops::Deref;

    use dynasmrt::dynasm;
    use dynasmrt::DynasmApi;

    use super::super::SysVAmd64;
    use super::symbol;
    use crate::NativeType::*;

    #[test]
    fn tailcall() {
      let trampoline = SysVAmd64::compile(&symbol(
        vec![
          U8, U16, I16, I8, U32, U64, Buffer, Function, I64, I32, I16, I8, F32,
          F32, F32, F32, F64, F64, F64, F64, F32, F64,
        ],
        Void,
      ));

      let mut assembler = dynasmrt::x64::Assembler::new().unwrap();
      // See https://godbolt.org/z/KE9x1h9xq
      dynasm!(assembler
        ; .arch x64
        ; movzx edi, sil                   // u8
        ; movzx esi, dx                    // u16
        ; movsx edx, cx                    // i16
        ; movsx ecx, r8b                   // i8
        ; mov r8d, r9d                     // u32
        ; mov r9, [DWORD rsp + 8]          // u64
        ; mov rax, [DWORD rsp + 16]        // Buffer
        ; mov rax, [rax + 8]               // ..
        ; mov [DWORD rsp + 8], rax         // ..
        ; mov rax, [DWORD rsp + 24]        // Function
        ; mov [DWORD rsp + 16], rax        // ..
        ; mov rax, [DWORD rsp + 32]        // i64
        ; mov [DWORD rsp + 24], rax        // ..
        ; mov eax, [DWORD rsp + 40]        // i32
        ; mov [DWORD rsp + 32], eax        // ..
        ; movsx eax, WORD [DWORD rsp + 48] // i16
        ; mov [DWORD rsp + 40], eax        // ..
        ; movsx eax, BYTE [DWORD rsp + 56] // i8
        ; mov [DWORD rsp + 48], eax        // ..
        ; movss xmm8, [DWORD rsp + 64]     // f32
        ; movss [DWORD rsp + 56], xmm8     // ..
        ; movsd xmm8, [DWORD rsp + 72]     // f64
        ; movsd [DWORD rsp + 64], xmm8     // ..
        ; mov rax, QWORD 0
        ; jmp rax
      );
      let expected = assembler.finalize().unwrap();
      assert_eq!(trampoline.0.deref(), expected.deref());
    }

    #[test]
    fn integer_casting() {
      let trampoline = SysVAmd64::compile(&symbol(
        vec![U8, U16, I8, I16, U8, U16, I8, I16, U8, U16, I8, I16],
        I8,
      ));

      let mut assembler = dynasmrt::x64::Assembler::new().unwrap();
      // See https://godbolt.org/z/qo59bPsfv
      dynasm!(assembler
        ; .arch x64
        ; sub rsp, DWORD 56                 // stack allocation
        ; movzx edi, sil                    // u8
        ; movzx esi, dx                     // u16
        ; movsx edx, cl                     // i8
        ; movsx ecx, r8w                    // i16
        ; movzx r8d, r9b                    // u8
        ; movzx r9d, WORD [DWORD rsp + 64]  // u16
        ; movsx eax, BYTE [DWORD rsp + 72]  // i8
        ; mov [DWORD rsp + 0], eax          // ..
        ; movsx eax, WORD [DWORD rsp + 80]  // i16
        ; mov [DWORD rsp + 8], eax          // ..
        ; movzx eax, BYTE [DWORD rsp + 88]  // u8
        ; mov [DWORD rsp + 16], eax         // ..
        ; movzx eax, WORD [DWORD rsp + 96]  // u16
        ; mov [DWORD rsp + 24], eax         // ..
        ; movsx eax, BYTE [DWORD rsp + 104] // i8
        ; mov [DWORD rsp + 32], eax         // ..
        ; movsx eax, WORD [DWORD rsp + 112] // i16
        ; mov [DWORD rsp + 40], eax         // ..
        ; mov rax, QWORD 0
        ; call rax
        ; movsx eax, al      // return value cast
        ; add rsp, DWORD 56  // stack deallocation
        ; ret
      );
      let expected = assembler.finalize().unwrap();
      assert_eq!(trampoline.0.deref(), expected.deref());
    }

    #[test]
    fn buffer_parameters() {
      let trampoline = SysVAmd64::compile(&symbol(
        vec![
          Buffer, Buffer, Buffer, Buffer, Buffer, Buffer, Buffer, Buffer,
        ],
        Void,
      ));

      let mut assembler = dynasmrt::x64::Assembler::new().unwrap();
      // See https://godbolt.org/z/hqv63M3Ko
      dynasm!(assembler
        ; .arch x64
        ; mov rdi, [rsi + 8]               // Buffer
        ; mov rsi, [rdx + 8]               // Buffer
        ; mov rdx, [rcx + 8]               // Buffer
        ; mov rcx, [r8 + 8]                // Buffer
        ; mov r8, [r9 + 8]                 // Buffer
        ; mov r9, [DWORD rsp + 8]          // Buffer
        ; mov r9, [r9 + 8]                 // ..
        ; mov rax, [DWORD rsp + 16]        // Buffer
        ; mov rax, [rax + 8]               // ..
        ; mov [DWORD rsp + 8], rax         // ..
        ; mov rax, [DWORD rsp + 24]        // Buffer
        ; mov rax, [rax + 8]               // ..
        ; mov [DWORD rsp + 16], rax        // ..
        ; mov rax, QWORD 0
        ; jmp rax
      );
      let expected = assembler.finalize().unwrap();
      assert_eq!(trampoline.0.deref(), expected.deref());
    }
  }

  mod aarch64_apple {
    use std::ops::Deref;

    use dynasmrt::dynasm;

    use super::super::Aarch64Apple;
    use super::symbol;
    use crate::NativeType::*;

    #[test]
    fn tailcall() {
      let trampoline = Aarch64Apple::compile(&symbol(
        vec![
          U8, U16, I16, I8, U32, U64, Buffer, Function, I64, I32, I16, I8, F32,
          F32, F32, F32, F64, F64, F64, F64, F32, F64,
        ],
        Void,
      ));

      let mut assembler = dynasmrt::aarch64::Assembler::new().unwrap();
      // See https://godbolt.org/z/oefqYWT13
      dynasm!(assembler
        ; .arch aarch64
        ; and w0, w1, 0xFF   // u8
        ; and w1, w2, 0xFFFF // u16
        ; sxth w2, w3        // i16
        ; sxtb w3, w4        // i8
        ; mov w4, w5         // u32
        ; mov x5, x6         // u64
        ; ldr x6, [x7, 8]    // Buffer
        ; ldr x7, [sp]       // Function
        ; ldr x8, [sp, 8]    // i64
        ; str x8, [sp]       // ..
        ; ldr w8, [sp, 16]   // i32
        ; str w8, [sp, 8]    // ..
        ; ldr w8, [sp, 24]   // i16
        ; strh w8, [sp, 12]  // ..
        ; ldr w8, [sp, 32]   // i8
        ; strb w8, [sp, 14]  // ..
        ; ldr s16, [sp, 40]  // f32
        ; str s16, [sp, 16]  // ..
        ; ldr d16, [sp, 48]  // f64
        ; str d16, [sp, 24]  // ..
        ; movz x8, 0
        ; br x8
      );
      let expected = assembler.finalize().unwrap();
      assert_eq!(trampoline.0.deref(), expected.deref());
    }

    #[test]
    fn integer_casting() {
      let trampoline = Aarch64Apple::compile(&symbol(
        vec![U8, U16, I8, I16, U8, U16, I8, I16, U8, U16, I8, I16],
        I8,
      ));

      let mut assembler = dynasmrt::aarch64::Assembler::new().unwrap();
      // See https://godbolt.org/z/7qfzbzobM
      dynasm!(assembler
        ; .arch aarch64
        ; and w0, w1, 0xFF   // u8
        ; and w1, w2, 0xFFFF // u16
        ; sxtb w2, w3        // i8
        ; sxth w3, w4        // i16
        ; and w4, w5, 0xFF   // u8
        ; and w5, w6, 0xFFFF // u16
        ; sxtb w6, w7        // i8
        ; ldrsh w7, [sp]     // i16
        ; ldr w8, [sp, 8]    // u8
        ; strb w8, [sp]      // ..
        ; ldr w8, [sp, 16]    // u16
        ; strh w8, [sp, 2]   // ..
        ; ldr w8, [sp, 24]   // i8
        ; strb w8, [sp, 4]   // ..
        ; ldr w8, [sp, 32]   // i16
        ; strh w8, [sp, 6]   // ..
        ; movz x8, 0
        ; br x8
      );
      let expected = assembler.finalize().unwrap();
      assert_eq!(trampoline.0.deref(), expected.deref());
    }

    #[test]
    fn buffer_parameters() {
      let trampoline = Aarch64Apple::compile(&symbol(
        vec![
          Buffer, Buffer, Buffer, Buffer, Buffer, Buffer, Buffer, Buffer,
          Buffer, Buffer,
        ],
        Void,
      ));

      let mut assembler = dynasmrt::aarch64::Assembler::new().unwrap();
      // See https://godbolt.org/z/obd6z6vsf
      dynasm!(assembler
        ; .arch aarch64
        ; ldr x0, [x1, 8]               // Buffer
        ; ldr x1, [x2, 8]               // Buffer
        ; ldr x2, [x3, 8]               // Buffer
        ; ldr x3, [x4, 8]               // Buffer
        ; ldr x4, [x5, 8]               // Buffer
        ; ldr x5, [x6, 8]               // Buffer
        ; ldr x6, [x7, 8]               // Buffer
        ; ldr x7, [sp]                  // Buffer
        ; ldr x7, [x7, 8]               // ..
        ; ldr x8, [sp, 8]               // Buffer
        ; ldr x8, [x8, 8]               // ..
        ; str x8, [sp]                  // ..
        ; ldr x8, [sp, 16]              // Buffer
        ; ldr x8, [x8, 8]               // ..
        ; str x8, [sp, 8]               // ..
        ; movz x8, 0
        ; br x8
      );
      let expected = assembler.finalize().unwrap();
      assert_eq!(trampoline.0.deref(), expected.deref());
    }
  }

  mod x64_windows {
    use std::ops::Deref;

    use dynasmrt::dynasm;
    use dynasmrt::DynasmApi;

    use super::super::Win64;
    use super::symbol;
    use crate::NativeType::*;

    #[test]
    fn tailcall() {
      let trampoline =
        Win64::compile(&symbol(vec![U8, I16, F64, F32, U32, I8, Buffer], Void));

      let mut assembler = dynasmrt::x64::Assembler::new().unwrap();
      // See https://godbolt.org/z/TYzqrf9aj
      dynasm!(assembler
        ; .arch x64
        ; mov ecx, edx                  // u8
        ; mov edx, r8d                  // i16
        ; movaps xmm2, xmm3             // f64
        ; movaps xmm3, [DWORD rsp + 40] // f32
        ; mov eax, [DWORD rsp + 48]     // u32
        ; mov [DWORD rsp + 40], eax     // ..
        ; mov eax, [DWORD rsp + 56]     // i8
        ; mov [DWORD rsp + 48], eax     // ..
        ; mov rax, [DWORD rsp + 64]     // Buffer
        ; mov rax, [rax + 8]            // ..
        ; mov [DWORD rsp + 56], rax     // ..
        ; mov rax, QWORD 0
        ; jmp rax
      );
      let expected = assembler.finalize().unwrap();
      assert_eq!(trampoline.0.deref(), expected.deref());
    }

    #[test]
    fn integer_casting() {
      let trampoline = Win64::compile(&symbol(
        vec![U8, U16, I8, I16, U8, U16, I8, I16, U8, U16, I8, I16],
        I8,
      ));

      let mut assembler = dynasmrt::x64::Assembler::new().unwrap();
      // See https://godbolt.org/z/KMx56KGTq
      dynasm!(assembler
        ; .arch x64
        ; sub rsp, DWORD 104          // stack allocation
        ; mov ecx, edx                // u8
        ; mov edx, r8d                // u16
        ; mov r8d, r9d                // i8
        ; mov r9d, [DWORD rsp + 144]  // i16
        ; mov eax, [DWORD rsp + 152]  // u8
        ; mov [DWORD rsp + 32], eax   // ..
        ; mov eax, [DWORD rsp + 160]  // u16
        ; mov [DWORD rsp + 40], eax   // u16
        ; mov eax, [DWORD rsp + 168]  // i8
        ; mov [DWORD rsp + 48], eax   // ..
        ; mov eax, [DWORD rsp + 176]  // i16
        ; mov [DWORD rsp + 56], eax   // ..
        ; mov eax, [DWORD rsp + 184]  // u8
        ; mov [DWORD rsp + 64], eax   // ..
        ; mov eax, [DWORD rsp + 192]  // u16
        ; mov [DWORD rsp + 72], eax   // ..
        ; mov eax, [DWORD rsp + 200]  // i8
        ; mov [DWORD rsp + 80], eax   // ..
        ; mov eax, [DWORD rsp + 208]  // i16
        ; mov [DWORD rsp + 88], eax   // ..
        ; mov rax, QWORD 0
        ; call rax
        ; movsx eax, al       // return value cast
        ; add rsp, DWORD 104  // stack deallocation
        ; ret
      );
      let expected = assembler.finalize().unwrap();
      assert_eq!(trampoline.0.deref(), expected.deref());
    }

    #[test]
    fn buffer_parameters() {
      let trampoline = Win64::compile(&symbol(
        vec![Buffer, Buffer, Buffer, Buffer, Buffer, Buffer],
        Void,
      ));

      let mut assembler = dynasmrt::x64::Assembler::new().unwrap();
      // See https://godbolt.org/z/TYzqrf9aj
      dynasm!(assembler
        ; .arch x64
        ; mov rcx, [rdx + 8]               // Buffer
        ; mov rdx, [r8 + 8]                // Buffer
        ; mov r8, [r9 + 8]                 // Buffer
        ; mov r9, [DWORD rsp + 40]         // Buffer
        ; mov r9, [r9 + 8]                 // ..
        ; mov rax, [DWORD rsp + 48]        // Buffer
        ; mov rax, [rax + 8]               // ..
        ; mov [DWORD rsp + 40], rax        // ..
        ; mov rax, [DWORD rsp + 56]        // Buffer
        ; mov rax, [rax + 8]               // ..
        ; mov [DWORD rsp + 48], rax        // ..
        ; mov rax, QWORD 0
        ; jmp rax
      );
      let expected = assembler.finalize().unwrap();
      assert_eq!(trampoline.0.deref(), expected.deref());
    }
  }
}