pub const CallingConvention = union(enum(u8)) {
    pub const Tag = @typeInfo(CallingConvention).@"union".tag_type.?;

    /// This is an alias for the default C calling convention for this target.
    /// Functions marked as `extern` or `export` are given this calling convention by default.
    pub const c = builtin.target.cCallingConvention().?;

    pub const winapi: CallingConvention = switch (builtin.target.cpu.arch) {
        .x86_64 => .{ .x86_64_win = .{} },
        .x86 => .{ .x86_stdcall = .{} },
        .aarch64 => .{ .aarch64_aapcs_win = .{} },
        .thumb => .{ .arm_aapcs_vfp = .{} },
        else => unreachable,
    };

    pub const kernel: CallingConvention = switch (builtin.target.cpu.arch) {
        .amdgcn => .amdgcn_kernel,
        .nvptx, .nvptx64 => .nvptx_kernel,
        .spirv, .spirv32, .spirv64 => .spirv_kernel,
        else => unreachable,
    };

    /// Deprecated; use `.auto`.
    pub const Unspecified: CallingConvention = .auto;
    /// Deprecated; use `.c`.
    pub const C: CallingConvention = .c;
    /// Deprecated; use `.naked`.
    pub const Naked: CallingConvention = .naked;
    /// Deprecated; use `.@"async"`.
    pub const Async: CallingConvention = .@"async";
    /// Deprecated; use `.@"inline"`.
    pub const Inline: CallingConvention = .@"inline";
    /// Deprecated; use `.x86_64_interrupt`, `.x86_interrupt`, or `.avr_interrupt`.
    pub const Interrupt: CallingConvention = switch (builtin.target.cpu.arch) {
        .x86_64 => .{ .x86_64_interrupt = .{} },
        .x86 => .{ .x86_interrupt = .{} },
        .avr => .avr_interrupt,
        else => unreachable,
    };
    /// Deprecated; use `.avr_signal`.
    pub const Signal: CallingConvention = .avr_signal;
    /// Deprecated; use `.x86_stdcall`.
    pub const Stdcall: CallingConvention = .{ .x86_stdcall = .{} };
    /// Deprecated; use `.x86_fastcall`.
    pub const Fastcall: CallingConvention = .{ .x86_fastcall = .{} };
    /// Deprecated; use `.x86_64_vectorcall`, `.x86_vectorcall`, or `aarch64_vfabi`.
    pub const Vectorcall: CallingConvention = switch (builtin.target.cpu.arch) {
        .x86_64 => .{ .x86_64_vectorcall = .{} },
        .x86 => .{ .x86_vectorcall = .{} },
        .aarch64, .aarch64_be => .{ .aarch64_vfabi = .{} },
        else => unreachable,
    };
    /// Deprecated; use `.x86_thiscall`.
    pub const Thiscall: CallingConvention = .{ .x86_thiscall = .{} };
    /// Deprecated; use `.arm_aapcs`.
    pub const AAPCS: CallingConvention = .{ .arm_aapcs = .{} };
    /// Deprecated; use `.arm_aapcs_vfp`.
    pub const AAPCSVFP: CallingConvention = .{ .arm_aapcs_vfp = .{} };
    /// Deprecated; use `.x86_64_sysv`.
    pub const SysV: CallingConvention = .{ .x86_64_sysv = .{} };
    /// Deprecated; use `.x86_64_win`.
    pub const Win64: CallingConvention = .{ .x86_64_win = .{} };
    /// Deprecated; use `.kernel`.
    pub const Kernel: CallingConvention = .kernel;
    /// Deprecated; use `.spirv_fragment`.
    pub const Fragment: CallingConvention = .spirv_fragment;
    /// Deprecated; use `.spirv_vertex`.
    pub const Vertex: CallingConvention = .spirv_vertex;

    /// The default Zig calling convention when neither `export` nor `inline` is specified.
    /// This calling convention makes no guarantees about stack alignment, registers, etc.
    /// It can only be used within this Zig compilation unit.
    auto,

    /// The calling convention of a function that can be called with `async` syntax. An `async` call
    /// of a runtime-known function must target a function with this calling convention.
    /// Comptime-known functions with other calling conventions may be coerced to this one.
    @"async",

    /// Functions with this calling convention have no prologue or epilogue, making the function
    /// uncallable in regular Zig code. This can be useful when integrating with assembly.
    naked,

    /// This calling convention is exactly equivalent to using the `inline` keyword on a function
    /// definition. This function will be semantically inlined by the Zig compiler at call sites.
    /// Pointers to inline functions are comptime-only.
    @"inline",

    // Calling conventions for the `x86_64` architecture.
    x86_64_sysv: CommonOptions,
    x86_64_win: CommonOptions,
    x86_64_regcall_v3_sysv: CommonOptions,
    x86_64_regcall_v4_win: CommonOptions,
    x86_64_vectorcall: CommonOptions,
    x86_64_interrupt: CommonOptions,

    // Calling conventions for the `x86` architecture.
    x86_sysv: X86RegparmOptions,
    x86_win: X86RegparmOptions,
    x86_stdcall: X86RegparmOptions,
    x86_fastcall: CommonOptions,
    x86_thiscall: CommonOptions,
    x86_thiscall_mingw: CommonOptions,
    x86_regcall_v3: CommonOptions,
    x86_regcall_v4_win: CommonOptions,
    x86_vectorcall: CommonOptions,
    x86_interrupt: CommonOptions,

    // Calling conventions for the `aarch64` and `aarch64_be` architectures.
    aarch64_aapcs: CommonOptions,
    aarch64_aapcs_darwin: CommonOptions,
    aarch64_aapcs_win: CommonOptions,
    aarch64_vfabi: CommonOptions,
    aarch64_vfabi_sve: CommonOptions,

    // Calling convetions for the `arm`, `armeb`, `thumb`, and `thumbeb` architectures.
    /// ARM Architecture Procedure Call Standard
    arm_aapcs: CommonOptions,
    /// ARM Architecture Procedure Call Standard Vector Floating-Point
    arm_aapcs_vfp: CommonOptions,
    arm_interrupt: ArmInterruptOptions,

    // Calling conventions for the `mips64` and `mips64el` architectures.
    mips64_n64: CommonOptions,
    mips64_n32: CommonOptions,
    mips64_interrupt: MipsInterruptOptions,

    // Calling conventions for the `mips` and `mipsel` architectures.
    mips_o32: CommonOptions,
    mips_interrupt: MipsInterruptOptions,

    // Calling conventions for the `riscv64` architecture.
    riscv64_lp64: CommonOptions,
    riscv64_lp64_v: CommonOptions,
    riscv64_interrupt: RiscvInterruptOptions,

    // Calling conventions for the `riscv32` architecture.
    riscv32_ilp32: CommonOptions,
    riscv32_ilp32_v: CommonOptions,
    riscv32_interrupt: RiscvInterruptOptions,

    // Calling conventions for the `sparc64` architecture.
    sparc64_sysv: CommonOptions,

    // Calling conventions for the `sparc` architecture.
    sparc_sysv: CommonOptions,

    // Calling conventions for the `powerpc64` and `powerpc64le` architectures.
    powerpc64_elf: CommonOptions,
    powerpc64_elf_altivec: CommonOptions,
    powerpc64_elf_v2: CommonOptions,

    // Calling conventions for the `powerpc` and `powerpcle` architectures.
    powerpc_sysv: CommonOptions,
    powerpc_sysv_altivec: CommonOptions,
    powerpc_aix: CommonOptions,
    powerpc_aix_altivec: CommonOptions,

    /// The standard `wasm32` and `wasm64` calling convention, as specified in the WebAssembly Tool Conventions.
    wasm_mvp: CommonOptions,

    /// The standard `arc` calling convention.
    arc_sysv: CommonOptions,

    // Calling conventions for the `avr` architecture.
    avr_gnu,
    avr_builtin,
    avr_signal,
    avr_interrupt,

    /// The standard `bpfel`/`bpfeb` calling convention.
    bpf_std: CommonOptions,

    // Calling conventions for the `csky` architecture.
    csky_sysv: CommonOptions,
    csky_interrupt: CommonOptions,

    // Calling conventions for the `hexagon` architecture.
    hexagon_sysv: CommonOptions,
    hexagon_sysv_hvx: CommonOptions,

    /// The standard `lanai` calling convention.
    lanai_sysv: CommonOptions,

    /// The standard `loongarch64` calling convention.
    loongarch64_lp64: CommonOptions,

    /// The standard `loongarch32` calling convention.
    loongarch32_ilp32: CommonOptions,

    // Calling conventions for the `m68k` architecture.
    m68k_sysv: CommonOptions,
    m68k_gnu: CommonOptions,
    m68k_rtd: CommonOptions,
    m68k_interrupt: CommonOptions,

    /// The standard `msp430` calling convention.
    msp430_eabi: CommonOptions,

    /// The standard `propeller` calling convention.
    propeller_sysv: CommonOptions,

    // Calling conventions for the `s390x` architecture.
    s390x_sysv: CommonOptions,
    s390x_sysv_vx: CommonOptions,

    /// The standard `ve` calling convention.
    ve_sysv: CommonOptions,

    // Calling conventions for the `xcore` architecture.
    xcore_xs1: CommonOptions,
    xcore_xs2: CommonOptions,

    // Calling conventions for the `xtensa` architecture.
    xtensa_call0: CommonOptions,
    xtensa_windowed: CommonOptions,

    // Calling conventions for the `amdgcn` architecture.
    amdgcn_device: CommonOptions,
    amdgcn_kernel,
    amdgcn_cs: CommonOptions,

    // Calling conventions for the `nvptx` and `nvptx64` architectures.
    nvptx_device,
    nvptx_kernel,

    // Calling conventions for kernels and shaders on the `spirv`, `spirv32`, and `spirv64` architectures.
    spirv_device,
    spirv_kernel,
    spirv_fragment,
    spirv_vertex,

    /// Options shared across most calling conventions.
    pub const CommonOptions = struct {
        /// The boundary the stack is aligned to when the function is called.
        /// `null` means the default for this calling convention.
        incoming_stack_alignment: ?u64 = null,
    };

    /// Options for x86 calling conventions which support the regparm attribute to pass some
    /// arguments in registers.
    pub const X86RegparmOptions = struct {
        /// The boundary the stack is aligned to when the function is called.
        /// `null` means the default for this calling convention.
        incoming_stack_alignment: ?u64 = null,
        /// The number of arguments to pass in registers before passing the remaining arguments
        /// according to the calling convention.
        /// Equivalent to `__attribute__((regparm(x)))` in Clang and GCC.
        register_params: u2 = 0,
    };

    /// Options for the `arm_interrupt` calling convention.
    pub const ArmInterruptOptions = struct {
        /// The boundary the stack is aligned to when the function is called.
        /// `null` means the default for this calling convention.
        incoming_stack_alignment: ?u64 = null,
        /// The kind of interrupt being received.
        type: InterruptType = .generic,

        pub const InterruptType = enum(u3) {
            generic,
            irq,
            fiq,
            swi,
            abort,
            undef,
        };
    };

    /// Options for the `mips_interrupt` and `mips64_interrupt` calling conventions.
    pub const MipsInterruptOptions = struct {
        /// The boundary the stack is aligned to when the function is called.
        /// `null` means the default for this calling convention.
        incoming_stack_alignment: ?u64 = null,
        /// The interrupt mode.
        mode: InterruptMode = .eic,

        pub const InterruptMode = enum(u4) {
            eic,
            sw0,
            sw1,
            hw0,
            hw1,
            hw2,
            hw3,
            hw4,
            hw5,
        };
    };

    /// Options for the `riscv32_interrupt` and `riscv64_interrupt` calling conventions.
    pub const RiscvInterruptOptions = struct {
        /// The boundary the stack is aligned to when the function is called.
        /// `null` means the default for this calling convention.
        incoming_stack_alignment: ?u64 = null,
        /// The privilege mode.
        mode: PrivilegeMode,

        pub const PrivilegeMode = enum(u2) {
            supervisor,
            machine,
        };
    };

    /// Returns the array of `std.Target.Cpu.Arch` to which this `CallingConvention` applies.
    /// Asserts that `cc` is not `.auto`, `.@"async"`, `.naked`, or `.@"inline"`.
    pub fn archs(cc: CallingConvention) []const std.Target.Cpu.Arch {
        return std.Target.Cpu.Arch.fromCallingConvention(cc);
    }

    pub fn eql(a: CallingConvention, b: CallingConvention) bool {
        return std.meta.eql(a, b);
    }

    pub fn withStackAlign(cc: CallingConvention, incoming_stack_alignment: u64) CallingConvention {
        const tag: CallingConvention.Tag = cc;
        var result = cc;
        @field(result, @tagName(tag)).incoming_stack_alignment = incoming_stack_alignment;
        return result;
    }
}