const std = @import("std");
const Type = @import("type.zig").Type;
const log2 = std.math.log2;
const assert = std.debug.assert;
const BigIntConst = std.math.big.int.Const;
const BigIntMutable = std.math.big.int.Mutable;
const Target = std.Target;
const Allocator = std.mem.Allocator;
const Module = @import("Module.zig");
const Air = @import("Air.zig");

/// This is the raw data, with no bookkeeping, no memory awareness,
/// no de-duplication, and no type system awareness.
/// It's important for this type to be small.
/// This union takes advantage of the fact that the first page of memory
/// is unmapped, giving us 4096 possible enum tags that have no payload.
pub const Value = extern union {
    /// If the tag value is less than Tag.no_payload_count, then no pointer
    /// dereference is needed.
    tag_if_small_enough: usize,
    ptr_otherwise: *Payload,

    pub const Tag = enum {
        // The first section of this enum are tags that require no payload.
        u1_type,
        u8_type,
        i8_type,
        u16_type,
        i16_type,
        u32_type,
        i32_type,
        u64_type,
        i64_type,
        u128_type,
        i128_type,
        usize_type,
        isize_type,
        c_short_type,
        c_ushort_type,
        c_int_type,
        c_uint_type,
        c_long_type,
        c_ulong_type,
        c_longlong_type,
        c_ulonglong_type,
        c_longdouble_type,
        f16_type,
        f32_type,
        f64_type,
        f128_type,
        c_void_type,
        bool_type,
        void_type,
        type_type,
        anyerror_type,
        comptime_int_type,
        comptime_float_type,
        noreturn_type,
        anyframe_type,
        null_type,
        undefined_type,
        enum_literal_type,
        atomic_ordering_type,
        atomic_rmw_op_type,
        calling_convention_type,
        float_mode_type,
        reduce_op_type,
        call_options_type,
        export_options_type,
        extern_options_type,
        type_info_type,
        manyptr_u8_type,
        manyptr_const_u8_type,
        fn_noreturn_no_args_type,
        fn_void_no_args_type,
        fn_naked_noreturn_no_args_type,
        fn_ccc_void_no_args_type,
        single_const_pointer_to_comptime_int_type,
        const_slice_u8_type,
        anyerror_void_error_union_type,
        generic_poison_type,

        undef,
        zero,
        one,
        void_value,
        unreachable_value,
        null_value,
        bool_true,
        bool_false,
        generic_poison,

        abi_align_default,
        empty_struct_value,
        empty_array, // See last_no_payload_tag below.
        // After this, the tag requires a payload.

        ty,
        int_type,
        int_u64,
        int_i64,
        int_big_positive,
        int_big_negative,
        function,
        extern_fn,
        variable,
        /// Represents a pointer to a Decl.
        /// When machine codegen backend sees this, it must set the Decl's `alive` field to true.
        decl_ref,
        /// Pointer to a Decl, but allows comptime code to mutate the Decl's Value.
        /// This Tag will never be seen by machine codegen backends. It is changed into a
        /// `decl_ref` when a comptime variable goes out of scope.
        decl_ref_mut,
        elem_ptr,
        field_ptr,
        /// A slice of u8 whose memory is managed externally.
        bytes,
        /// This value is repeated some number of times. The amount of times to repeat
        /// is stored externally.
        repeated,
        /// Each element stored as a `Value`.
        array,
        /// Pointer and length as sub `Value` objects.
        slice,
        float_16,
        float_32,
        float_64,
        float_128,
        enum_literal,
        /// A specific enum tag, indicated by the field index (declaration order).
        enum_field_index,
        @"error",
        /// When the type is error union:
        /// * If the tag is `.@"error"`, the error union is an error.
        /// * If the tag is `.eu_payload`, the error union is a payload.
        /// * A nested error such as `anyerror!(anyerror!T)` in which the the outer error union
        ///   is non-error, but the inner error union is an error, is represented as
        ///   a tag of `.eu_payload`, with a sub-tag of `.@"error"`.
        eu_payload,
        /// A pointer to the payload of an error union, based on a pointer to an error union.
        eu_payload_ptr,
        /// When the type is optional:
        /// * If the tag is `.null_value`, the optional is null.
        /// * If the tag is `.opt_payload`, the optional is a payload.
        /// * A nested optional such as `??T` in which the the outer optional
        ///   is non-null, but the inner optional is null, is represented as
        ///   a tag of `.opt_payload`, with a sub-tag of `.null_value`.
        opt_payload,
        /// A pointer to the payload of an optional, based on a pointer to an optional.
        opt_payload_ptr,
        /// An instance of a struct.
        @"struct",
        /// An instance of a union.
        @"union",
        /// This is a special value that tracks a set of types that have been stored
        /// to an inferred allocation. It does not support any of the normal value queries.
        inferred_alloc,
        /// Used to coordinate alloc_inferred, store_to_inferred_ptr, and resolve_inferred_alloc
        /// instructions for comptime code.
        inferred_alloc_comptime,

        pub const last_no_payload_tag = Tag.empty_array;
        pub const no_payload_count = @enumToInt(last_no_payload_tag) + 1;

        pub fn Type(comptime t: Tag) type {
            return switch (t) {
                .u1_type,
                .u8_type,
                .i8_type,
                .u16_type,
                .i16_type,
                .u32_type,
                .i32_type,
                .u64_type,
                .i64_type,
                .u128_type,
                .i128_type,
                .usize_type,
                .isize_type,
                .c_short_type,
                .c_ushort_type,
                .c_int_type,
                .c_uint_type,
                .c_long_type,
                .c_ulong_type,
                .c_longlong_type,
                .c_ulonglong_type,
                .c_longdouble_type,
                .f16_type,
                .f32_type,
                .f64_type,
                .f128_type,
                .c_void_type,
                .bool_type,
                .void_type,
                .type_type,
                .anyerror_type,
                .comptime_int_type,
                .comptime_float_type,
                .noreturn_type,
                .null_type,
                .undefined_type,
                .fn_noreturn_no_args_type,
                .fn_void_no_args_type,
                .fn_naked_noreturn_no_args_type,
                .fn_ccc_void_no_args_type,
                .single_const_pointer_to_comptime_int_type,
                .anyframe_type,
                .const_slice_u8_type,
                .anyerror_void_error_union_type,
                .generic_poison_type,
                .enum_literal_type,
                .undef,
                .zero,
                .one,
                .void_value,
                .unreachable_value,
                .empty_struct_value,
                .empty_array,
                .null_value,
                .bool_true,
                .bool_false,
                .abi_align_default,
                .manyptr_u8_type,
                .manyptr_const_u8_type,
                .atomic_ordering_type,
                .atomic_rmw_op_type,
                .calling_convention_type,
                .float_mode_type,
                .reduce_op_type,
                .call_options_type,
                .export_options_type,
                .extern_options_type,
                .type_info_type,
                .generic_poison,
                => @compileError("Value Tag " ++ @tagName(t) ++ " has no payload"),

                .int_big_positive,
                .int_big_negative,
                => Payload.BigInt,

                .extern_fn,
                .decl_ref,
                .inferred_alloc_comptime,
                => Payload.Decl,

                .repeated,
                .eu_payload,
                .eu_payload_ptr,
                .opt_payload,
                .opt_payload_ptr,
                => Payload.SubValue,

                .bytes,
                .enum_literal,
                => Payload.Bytes,

                .array => Payload.Array,
                .slice => Payload.Slice,

                .enum_field_index => Payload.U32,

                .ty => Payload.Ty,
                .int_type => Payload.IntType,
                .int_u64 => Payload.U64,
                .int_i64 => Payload.I64,
                .function => Payload.Function,
                .variable => Payload.Variable,
                .decl_ref_mut => Payload.DeclRefMut,
                .elem_ptr => Payload.ElemPtr,
                .field_ptr => Payload.FieldPtr,
                .float_16 => Payload.Float_16,
                .float_32 => Payload.Float_32,
                .float_64 => Payload.Float_64,
                .float_128 => Payload.Float_128,
                .@"error" => Payload.Error,
                .inferred_alloc => Payload.InferredAlloc,
                .@"struct" => Payload.Struct,
                .@"union" => Payload.Union,
            };
        }

        pub fn create(comptime t: Tag, ally: *Allocator, data: Data(t)) error{OutOfMemory}!Value {
            const ptr = try ally.create(t.Type());
            ptr.* = .{
                .base = .{ .tag = t },
                .data = data,
            };
            return Value{ .ptr_otherwise = &ptr.base };
        }

        pub fn Data(comptime t: Tag) type {
            return std.meta.fieldInfo(t.Type(), .data).field_type;
        }
    };

    pub fn initTag(small_tag: Tag) Value {
        assert(@enumToInt(small_tag) < Tag.no_payload_count);
        return .{ .tag_if_small_enough = @enumToInt(small_tag) };
    }

    pub fn initPayload(payload: *Payload) Value {
        assert(@enumToInt(payload.tag) >= Tag.no_payload_count);
        return .{ .ptr_otherwise = payload };
    }

    pub fn tag(self: Value) Tag {
        if (self.tag_if_small_enough < Tag.no_payload_count) {
            return @intToEnum(Tag, @intCast(std.meta.Tag(Tag), self.tag_if_small_enough));
        } else {
            return self.ptr_otherwise.tag;
        }
    }

    /// Prefer `castTag` to this.
    pub fn cast(self: Value, comptime T: type) ?*T {
        if (@hasField(T, "base_tag")) {
            return base.castTag(T.base_tag);
        }
        if (self.tag_if_small_enough < Tag.no_payload_count) {
            return null;
        }
        inline for (@typeInfo(Tag).Enum.fields) |field| {
            if (field.value < Tag.no_payload_count)
                continue;
            const t = @intToEnum(Tag, field.value);
            if (self.ptr_otherwise.tag == t) {
                if (T == t.Type()) {
                    return @fieldParentPtr(T, "base", self.ptr_otherwise);
                }
                return null;
            }
        }
        unreachable;
    }

    pub fn castTag(self: Value, comptime t: Tag) ?*t.Type() {
        if (self.tag_if_small_enough < Tag.no_payload_count)
            return null;

        if (self.ptr_otherwise.tag == t)
            return @fieldParentPtr(t.Type(), "base", self.ptr_otherwise);

        return null;
    }

    pub fn copy(self: Value, allocator: *Allocator) error{OutOfMemory}!Value {
        if (self.tag_if_small_enough < Tag.no_payload_count) {
            return Value{ .tag_if_small_enough = self.tag_if_small_enough };
        } else switch (self.ptr_otherwise.tag) {
            .u1_type,
            .u8_type,
            .i8_type,
            .u16_type,
            .i16_type,
            .u32_type,
            .i32_type,
            .u64_type,
            .i64_type,
            .u128_type,
            .i128_type,
            .usize_type,
            .isize_type,
            .c_short_type,
            .c_ushort_type,
            .c_int_type,
            .c_uint_type,
            .c_long_type,
            .c_ulong_type,
            .c_longlong_type,
            .c_ulonglong_type,
            .c_longdouble_type,
            .f16_type,
            .f32_type,
            .f64_type,
            .f128_type,
            .c_void_type,
            .bool_type,
            .void_type,
            .type_type,
            .anyerror_type,
            .comptime_int_type,
            .comptime_float_type,
            .noreturn_type,
            .null_type,
            .undefined_type,
            .fn_noreturn_no_args_type,
            .fn_void_no_args_type,
            .fn_naked_noreturn_no_args_type,
            .fn_ccc_void_no_args_type,
            .single_const_pointer_to_comptime_int_type,
            .anyframe_type,
            .const_slice_u8_type,
            .anyerror_void_error_union_type,
            .generic_poison_type,
            .enum_literal_type,
            .undef,
            .zero,
            .one,
            .void_value,
            .unreachable_value,
            .empty_array,
            .null_value,
            .bool_true,
            .bool_false,
            .empty_struct_value,
            .abi_align_default,
            .manyptr_u8_type,
            .manyptr_const_u8_type,
            .atomic_ordering_type,
            .atomic_rmw_op_type,
            .calling_convention_type,
            .float_mode_type,
            .reduce_op_type,
            .call_options_type,
            .export_options_type,
            .extern_options_type,
            .type_info_type,
            .generic_poison,
            => unreachable,

            .ty => {
                const payload = self.castTag(.ty).?;
                const new_payload = try allocator.create(Payload.Ty);
                new_payload.* = .{
                    .base = payload.base,
                    .data = try payload.data.copy(allocator),
                };
                return Value{ .ptr_otherwise = &new_payload.base };
            },
            .int_type => return self.copyPayloadShallow(allocator, Payload.IntType),
            .int_u64 => return self.copyPayloadShallow(allocator, Payload.U64),
            .int_i64 => return self.copyPayloadShallow(allocator, Payload.I64),
            .int_big_positive, .int_big_negative => {
                const old_payload = self.cast(Payload.BigInt).?;
                const new_payload = try allocator.create(Payload.BigInt);
                new_payload.* = .{
                    .base = .{ .tag = self.ptr_otherwise.tag },
                    .data = try allocator.dupe(std.math.big.Limb, old_payload.data),
                };
                return Value{ .ptr_otherwise = &new_payload.base };
            },
            .function => return self.copyPayloadShallow(allocator, Payload.Function),
            .extern_fn => return self.copyPayloadShallow(allocator, Payload.Decl),
            .variable => return self.copyPayloadShallow(allocator, Payload.Variable),
            .decl_ref => return self.copyPayloadShallow(allocator, Payload.Decl),
            .decl_ref_mut => return self.copyPayloadShallow(allocator, Payload.DeclRefMut),
            .elem_ptr => {
                const payload = self.castTag(.elem_ptr).?;
                const new_payload = try allocator.create(Payload.ElemPtr);
                new_payload.* = .{
                    .base = payload.base,
                    .data = .{
                        .array_ptr = try payload.data.array_ptr.copy(allocator),
                        .index = payload.data.index,
                    },
                };
                return Value{ .ptr_otherwise = &new_payload.base };
            },
            .field_ptr => {
                const payload = self.castTag(.field_ptr).?;
                const new_payload = try allocator.create(Payload.FieldPtr);
                new_payload.* = .{
                    .base = payload.base,
                    .data = .{
                        .container_ptr = try payload.data.container_ptr.copy(allocator),
                        .field_index = payload.data.field_index,
                    },
                };
                return Value{ .ptr_otherwise = &new_payload.base };
            },
            .bytes => return self.copyPayloadShallow(allocator, Payload.Bytes),
            .repeated,
            .eu_payload,
            .eu_payload_ptr,
            .opt_payload,
            .opt_payload_ptr,
            => {
                const payload = self.cast(Payload.SubValue).?;
                const new_payload = try allocator.create(Payload.SubValue);
                new_payload.* = .{
                    .base = payload.base,
                    .data = try payload.data.copy(allocator),
                };
                return Value{ .ptr_otherwise = &new_payload.base };
            },
            .array => {
                const payload = self.castTag(.array).?;
                const new_payload = try allocator.create(Payload.Array);
                new_payload.* = .{
                    .base = payload.base,
                    .data = try allocator.alloc(Value, payload.data.len),
                };
                std.mem.copy(Value, new_payload.data, payload.data);
                return Value{ .ptr_otherwise = &new_payload.base };
            },
            .slice => {
                const payload = self.castTag(.slice).?;
                const new_payload = try allocator.create(Payload.Slice);
                new_payload.* = .{
                    .base = payload.base,
                    .data = .{
                        .ptr = try payload.data.ptr.copy(allocator),
                        .len = try payload.data.len.copy(allocator),
                    },
                };
                return Value{ .ptr_otherwise = &new_payload.base };
            },
            .float_16 => return self.copyPayloadShallow(allocator, Payload.Float_16),
            .float_32 => return self.copyPayloadShallow(allocator, Payload.Float_32),
            .float_64 => return self.copyPayloadShallow(allocator, Payload.Float_64),
            .float_128 => return self.copyPayloadShallow(allocator, Payload.Float_128),
            .enum_literal => {
                const payload = self.castTag(.enum_literal).?;
                const new_payload = try allocator.create(Payload.Bytes);
                new_payload.* = .{
                    .base = payload.base,
                    .data = try allocator.dupe(u8, payload.data),
                };
                return Value{ .ptr_otherwise = &new_payload.base };
            },
            .enum_field_index => return self.copyPayloadShallow(allocator, Payload.U32),
            .@"error" => return self.copyPayloadShallow(allocator, Payload.Error),
            .@"struct" => @panic("TODO can't copy struct value without knowing the type"),
            .@"union" => @panic("TODO can't copy union value without knowing the type"),

            .inferred_alloc => unreachable,
            .inferred_alloc_comptime => unreachable,
        }
    }

    fn copyPayloadShallow(self: Value, allocator: *Allocator, comptime T: type) error{OutOfMemory}!Value {
        const payload = self.cast(T).?;
        const new_payload = try allocator.create(T);
        new_payload.* = payload.*;
        return Value{ .ptr_otherwise = &new_payload.base };
    }

    /// TODO this should become a debug dump() function. In order to print values in a meaningful way
    /// we also need access to the type.
    pub fn format(
        start_val: Value,
        comptime fmt: []const u8,
        options: std.fmt.FormatOptions,
        out_stream: anytype,
    ) !void {
        comptime assert(fmt.len == 0);
        var val = start_val;
        while (true) switch (val.tag()) {
            .u1_type => return out_stream.writeAll("u1"),
            .u8_type => return out_stream.writeAll("u8"),
            .i8_type => return out_stream.writeAll("i8"),
            .u16_type => return out_stream.writeAll("u16"),
            .i16_type => return out_stream.writeAll("i16"),
            .u32_type => return out_stream.writeAll("u32"),
            .i32_type => return out_stream.writeAll("i32"),
            .u64_type => return out_stream.writeAll("u64"),
            .i64_type => return out_stream.writeAll("i64"),
            .u128_type => return out_stream.writeAll("u128"),
            .i128_type => return out_stream.writeAll("i128"),
            .isize_type => return out_stream.writeAll("isize"),
            .usize_type => return out_stream.writeAll("usize"),
            .c_short_type => return out_stream.writeAll("c_short"),
            .c_ushort_type => return out_stream.writeAll("c_ushort"),
            .c_int_type => return out_stream.writeAll("c_int"),
            .c_uint_type => return out_stream.writeAll("c_uint"),
            .c_long_type => return out_stream.writeAll("c_long"),
            .c_ulong_type => return out_stream.writeAll("c_ulong"),
            .c_longlong_type => return out_stream.writeAll("c_longlong"),
            .c_ulonglong_type => return out_stream.writeAll("c_ulonglong"),
            .c_longdouble_type => return out_stream.writeAll("c_longdouble"),
            .f16_type => return out_stream.writeAll("f16"),
            .f32_type => return out_stream.writeAll("f32"),
            .f64_type => return out_stream.writeAll("f64"),
            .f128_type => return out_stream.writeAll("f128"),
            .c_void_type => return out_stream.writeAll("c_void"),
            .bool_type => return out_stream.writeAll("bool"),
            .void_type => return out_stream.writeAll("void"),
            .type_type => return out_stream.writeAll("type"),
            .anyerror_type => return out_stream.writeAll("anyerror"),
            .comptime_int_type => return out_stream.writeAll("comptime_int"),
            .comptime_float_type => return out_stream.writeAll("comptime_float"),
            .noreturn_type => return out_stream.writeAll("noreturn"),
            .null_type => return out_stream.writeAll("@Type(.Null)"),
            .undefined_type => return out_stream.writeAll("@Type(.Undefined)"),
            .fn_noreturn_no_args_type => return out_stream.writeAll("fn() noreturn"),
            .fn_void_no_args_type => return out_stream.writeAll("fn() void"),
            .fn_naked_noreturn_no_args_type => return out_stream.writeAll("fn() callconv(.Naked) noreturn"),
            .fn_ccc_void_no_args_type => return out_stream.writeAll("fn() callconv(.C) void"),
            .single_const_pointer_to_comptime_int_type => return out_stream.writeAll("*const comptime_int"),
            .anyframe_type => return out_stream.writeAll("anyframe"),
            .const_slice_u8_type => return out_stream.writeAll("[]const u8"),
            .anyerror_void_error_union_type => return out_stream.writeAll("anyerror!void"),
            .generic_poison_type => return out_stream.writeAll("(generic poison type)"),
            .generic_poison => return out_stream.writeAll("(generic poison)"),
            .enum_literal_type => return out_stream.writeAll("@Type(.EnumLiteral)"),
            .manyptr_u8_type => return out_stream.writeAll("[*]u8"),
            .manyptr_const_u8_type => return out_stream.writeAll("[*]const u8"),
            .atomic_ordering_type => return out_stream.writeAll("std.builtin.AtomicOrdering"),
            .atomic_rmw_op_type => return out_stream.writeAll("std.builtin.AtomicRmwOp"),
            .calling_convention_type => return out_stream.writeAll("std.builtin.CallingConvention"),
            .float_mode_type => return out_stream.writeAll("std.builtin.FloatMode"),
            .reduce_op_type => return out_stream.writeAll("std.builtin.ReduceOp"),
            .call_options_type => return out_stream.writeAll("std.builtin.CallOptions"),
            .export_options_type => return out_stream.writeAll("std.builtin.ExportOptions"),
            .extern_options_type => return out_stream.writeAll("std.builtin.ExternOptions"),
            .type_info_type => return out_stream.writeAll("std.builtin.TypeInfo"),
            .abi_align_default => return out_stream.writeAll("(default ABI alignment)"),

            .empty_struct_value => return out_stream.writeAll("struct {}{}"),
            .@"struct" => {
                return out_stream.writeAll("(struct value)");
            },
            .@"union" => {
                return out_stream.writeAll("(union value)");
            },
            .null_value => return out_stream.writeAll("null"),
            .undef => return out_stream.writeAll("undefined"),
            .zero => return out_stream.writeAll("0"),
            .one => return out_stream.writeAll("1"),
            .void_value => return out_stream.writeAll("{}"),
            .unreachable_value => return out_stream.writeAll("unreachable"),
            .bool_true => return out_stream.writeAll("true"),
            .bool_false => return out_stream.writeAll("false"),
            .ty => return val.castTag(.ty).?.data.format("", options, out_stream),
            .int_type => {
                const int_type = val.castTag(.int_type).?.data;
                return out_stream.print("{s}{d}", .{
                    if (int_type.signed) "s" else "u",
                    int_type.bits,
                });
            },
            .int_u64 => return std.fmt.formatIntValue(val.castTag(.int_u64).?.data, "", options, out_stream),
            .int_i64 => return std.fmt.formatIntValue(val.castTag(.int_i64).?.data, "", options, out_stream),
            .int_big_positive => return out_stream.print("{}", .{val.castTag(.int_big_positive).?.asBigInt()}),
            .int_big_negative => return out_stream.print("{}", .{val.castTag(.int_big_negative).?.asBigInt()}),
            .function => return out_stream.print("(function '{s}')", .{val.castTag(.function).?.data.owner_decl.name}),
            .extern_fn => return out_stream.writeAll("(extern function)"),
            .variable => return out_stream.writeAll("(variable)"),
            .decl_ref_mut => {
                const decl = val.castTag(.decl_ref_mut).?.data.decl;
                return out_stream.print("(decl_ref_mut '{s}')", .{decl.name});
            },
            .decl_ref => return out_stream.writeAll("(decl ref)"),
            .elem_ptr => {
                const elem_ptr = val.castTag(.elem_ptr).?.data;
                try out_stream.print("&[{}] ", .{elem_ptr.index});
                val = elem_ptr.array_ptr;
            },
            .field_ptr => {
                const field_ptr = val.castTag(.field_ptr).?.data;
                try out_stream.print("fieldptr({d}) ", .{field_ptr.field_index});
                val = field_ptr.container_ptr;
            },
            .empty_array => return out_stream.writeAll(".{}"),
            .enum_literal => return out_stream.print(".{}", .{std.zig.fmtId(val.castTag(.enum_literal).?.data)}),
            .enum_field_index => return out_stream.print("(enum field {d})", .{val.castTag(.enum_field_index).?.data}),
            .bytes => return out_stream.print("\"{}\"", .{std.zig.fmtEscapes(val.castTag(.bytes).?.data)}),
            .repeated => {
                try out_stream.writeAll("(repeated) ");
                val = val.castTag(.repeated).?.data;
            },
            .array => return out_stream.writeAll("(array)"),
            .slice => return out_stream.writeAll("(slice)"),
            .float_16 => return out_stream.print("{}", .{val.castTag(.float_16).?.data}),
            .float_32 => return out_stream.print("{}", .{val.castTag(.float_32).?.data}),
            .float_64 => return out_stream.print("{}", .{val.castTag(.float_64).?.data}),
            .float_128 => return out_stream.print("{}", .{val.castTag(.float_128).?.data}),
            .@"error" => return out_stream.print("error.{s}", .{val.castTag(.@"error").?.data.name}),
            // TODO to print this it should be error{ Set, Items }!T(val), but we need the type for that
            .eu_payload => {
                try out_stream.writeAll("(eu_payload) ");
                val = val.castTag(.eu_payload).?.data;
            },
            .opt_payload => {
                try out_stream.writeAll("(opt_payload) ");
                val = val.castTag(.opt_payload).?.data;
            },
            .inferred_alloc => return out_stream.writeAll("(inferred allocation value)"),
            .inferred_alloc_comptime => return out_stream.writeAll("(inferred comptime allocation value)"),
            .eu_payload_ptr => {
                try out_stream.writeAll("(eu_payload_ptr)");
                val = val.castTag(.eu_payload_ptr).?.data;
            },
            .opt_payload_ptr => {
                try out_stream.writeAll("(opt_payload_ptr)");
                val = val.castTag(.opt_payload_ptr).?.data;
            },
        };
    }

    /// Asserts that the value is representable as an array of bytes.
    /// Copies the value into a freshly allocated slice of memory, which is owned by the caller.
    pub fn toAllocatedBytes(self: Value, allocator: *Allocator) ![]u8 {
        if (self.castTag(.bytes)) |payload| {
            return std.mem.dupe(allocator, u8, payload.data);
        }
        if (self.castTag(.enum_literal)) |payload| {
            return std.mem.dupe(allocator, u8, payload.data);
        }
        if (self.castTag(.repeated)) |payload| {
            _ = payload;
            @panic("TODO implement toAllocatedBytes for this Value tag");
        }
        if (self.castTag(.decl_ref)) |payload| {
            const val = try payload.data.value();
            return val.toAllocatedBytes(allocator);
        }
        unreachable;
    }

    pub const ToTypeBuffer = Type.Payload.Bits;

    /// Asserts that the value is representable as a type.
    pub fn toType(self: Value, buffer: *ToTypeBuffer) Type {
        return switch (self.tag()) {
            .ty => self.castTag(.ty).?.data,
            .u1_type => Type.initTag(.u1),
            .u8_type => Type.initTag(.u8),
            .i8_type => Type.initTag(.i8),
            .u16_type => Type.initTag(.u16),
            .i16_type => Type.initTag(.i16),
            .u32_type => Type.initTag(.u32),
            .i32_type => Type.initTag(.i32),
            .u64_type => Type.initTag(.u64),
            .i64_type => Type.initTag(.i64),
            .u128_type => Type.initTag(.u128),
            .i128_type => Type.initTag(.i128),
            .usize_type => Type.initTag(.usize),
            .isize_type => Type.initTag(.isize),
            .c_short_type => Type.initTag(.c_short),
            .c_ushort_type => Type.initTag(.c_ushort),
            .c_int_type => Type.initTag(.c_int),
            .c_uint_type => Type.initTag(.c_uint),
            .c_long_type => Type.initTag(.c_long),
            .c_ulong_type => Type.initTag(.c_ulong),
            .c_longlong_type => Type.initTag(.c_longlong),
            .c_ulonglong_type => Type.initTag(.c_ulonglong),
            .c_longdouble_type => Type.initTag(.c_longdouble),
            .f16_type => Type.initTag(.f16),
            .f32_type => Type.initTag(.f32),
            .f64_type => Type.initTag(.f64),
            .f128_type => Type.initTag(.f128),
            .c_void_type => Type.initTag(.c_void),
            .bool_type => Type.initTag(.bool),
            .void_type => Type.initTag(.void),
            .type_type => Type.initTag(.type),
            .anyerror_type => Type.initTag(.anyerror),
            .comptime_int_type => Type.initTag(.comptime_int),
            .comptime_float_type => Type.initTag(.comptime_float),
            .noreturn_type => Type.initTag(.noreturn),
            .null_type => Type.initTag(.@"null"),
            .undefined_type => Type.initTag(.@"undefined"),
            .fn_noreturn_no_args_type => Type.initTag(.fn_noreturn_no_args),
            .fn_void_no_args_type => Type.initTag(.fn_void_no_args),
            .fn_naked_noreturn_no_args_type => Type.initTag(.fn_naked_noreturn_no_args),
            .fn_ccc_void_no_args_type => Type.initTag(.fn_ccc_void_no_args),
            .single_const_pointer_to_comptime_int_type => Type.initTag(.single_const_pointer_to_comptime_int),
            .anyframe_type => Type.initTag(.@"anyframe"),
            .const_slice_u8_type => Type.initTag(.const_slice_u8),
            .anyerror_void_error_union_type => Type.initTag(.anyerror_void_error_union),
            .generic_poison_type => Type.initTag(.generic_poison),
            .enum_literal_type => Type.initTag(.enum_literal),
            .manyptr_u8_type => Type.initTag(.manyptr_u8),
            .manyptr_const_u8_type => Type.initTag(.manyptr_const_u8),
            .atomic_ordering_type => Type.initTag(.atomic_ordering),
            .atomic_rmw_op_type => Type.initTag(.atomic_rmw_op),
            .calling_convention_type => Type.initTag(.calling_convention),
            .float_mode_type => Type.initTag(.float_mode),
            .reduce_op_type => Type.initTag(.reduce_op),
            .call_options_type => Type.initTag(.call_options),
            .export_options_type => Type.initTag(.export_options),
            .extern_options_type => Type.initTag(.extern_options),
            .type_info_type => Type.initTag(.type_info),

            .int_type => {
                const payload = self.castTag(.int_type).?.data;
                buffer.* = .{
                    .base = .{
                        .tag = if (payload.signed) .int_signed else .int_unsigned,
                    },
                    .data = payload.bits,
                };
                return Type.initPayload(&buffer.base);
            },

            else => unreachable,
        };
    }

    /// Asserts the type is an enum type.
    pub fn toEnum(val: Value, comptime E: type) E {
        switch (val.tag()) {
            .enum_field_index => {
                const field_index = val.castTag(.enum_field_index).?.data;
                // TODO should `@intToEnum` do this `@intCast` for you?
                return @intToEnum(E, @intCast(@typeInfo(E).Enum.tag_type, field_index));
            },
            else => unreachable,
        }
    }

    pub fn enumToInt(val: Value, ty: Type, buffer: *Payload.U64) Value {
        if (val.castTag(.enum_field_index)) |enum_field_payload| {
            const field_index = enum_field_payload.data;
            switch (ty.tag()) {
                .enum_full, .enum_nonexhaustive => {
                    const enum_full = ty.cast(Type.Payload.EnumFull).?.data;
                    if (enum_full.values.count() != 0) {
                        return enum_full.values.keys()[field_index];
                    } else {
                        // Field index and integer values are the same.
                        buffer.* = .{
                            .base = .{ .tag = .int_u64 },
                            .data = field_index,
                        };
                        return Value.initPayload(&buffer.base);
                    }
                },
                .enum_simple => {
                    // Field index and integer values are the same.
                    buffer.* = .{
                        .base = .{ .tag = .int_u64 },
                        .data = field_index,
                    };
                    return Value.initPayload(&buffer.base);
                },
                else => unreachable,
            }
        }
        // Assume it is already an integer and return it directly.
        return val;
    }

    /// Asserts the value is an integer.
    pub fn toBigInt(self: Value, space: *BigIntSpace) BigIntConst {
        switch (self.tag()) {
            .zero,
            .bool_false,
            => return BigIntMutable.init(&space.limbs, 0).toConst(),

            .one,
            .bool_true,
            => return BigIntMutable.init(&space.limbs, 1).toConst(),

            .int_u64 => return BigIntMutable.init(&space.limbs, self.castTag(.int_u64).?.data).toConst(),
            .int_i64 => return BigIntMutable.init(&space.limbs, self.castTag(.int_i64).?.data).toConst(),
            .int_big_positive => return self.castTag(.int_big_positive).?.asBigInt(),
            .int_big_negative => return self.castTag(.int_big_negative).?.asBigInt(),

            .undef => unreachable,
            else => unreachable,
        }
    }

    /// Asserts the value is an integer and it fits in a u64
    pub fn toUnsignedInt(self: Value) u64 {
        switch (self.tag()) {
            .zero,
            .bool_false,
            => return 0,

            .one,
            .bool_true,
            => return 1,

            .int_u64 => return self.castTag(.int_u64).?.data,
            .int_i64 => return @intCast(u64, self.castTag(.int_i64).?.data),
            .int_big_positive => return self.castTag(.int_big_positive).?.asBigInt().to(u64) catch unreachable,
            .int_big_negative => return self.castTag(.int_big_negative).?.asBigInt().to(u64) catch unreachable,

            .undef => unreachable,
            else => unreachable,
        }
    }

    /// Asserts the value is an integer and it fits in a i64
    pub fn toSignedInt(self: Value) i64 {
        switch (self.tag()) {
            .zero,
            .bool_false,
            => return 0,

            .one,
            .bool_true,
            => return 1,

            .int_u64 => return @intCast(i64, self.castTag(.int_u64).?.data),
            .int_i64 => return self.castTag(.int_i64).?.data,
            .int_big_positive => return self.castTag(.int_big_positive).?.asBigInt().to(i64) catch unreachable,
            .int_big_negative => return self.castTag(.int_big_negative).?.asBigInt().to(i64) catch unreachable,

            .undef => unreachable,
            else => unreachable,
        }
    }

    pub fn toBool(self: Value) bool {
        return switch (self.tag()) {
            .bool_true => true,
            .bool_false, .zero => false,
            else => unreachable,
        };
    }

    /// Asserts that the value is a float or an integer.
    pub fn toFloat(self: Value, comptime T: type) T {
        return switch (self.tag()) {
            .float_16 => @panic("TODO soft float"),
            .float_32 => @floatCast(T, self.castTag(.float_32).?.data),
            .float_64 => @floatCast(T, self.castTag(.float_64).?.data),
            .float_128 => @floatCast(T, self.castTag(.float_128).?.data),

            .zero => 0,
            .one => 1,
            .int_u64 => @intToFloat(T, self.castTag(.int_u64).?.data),
            .int_i64 => @intToFloat(T, self.castTag(.int_i64).?.data),

            .int_big_positive, .int_big_negative => @panic("big int to f128"),
            else => unreachable,
        };
    }

    /// Asserts the value is an integer and not undefined.
    /// Returns the number of bits the value requires to represent stored in twos complement form.
    pub fn intBitCountTwosComp(self: Value) usize {
        switch (self.tag()) {
            .zero,
            .bool_false,
            => return 0,

            .one,
            .bool_true,
            => return 1,

            .int_u64 => {
                const x = self.castTag(.int_u64).?.data;
                if (x == 0) return 0;
                return @intCast(usize, std.math.log2(x) + 1);
            },
            .int_i64 => {
                @panic("TODO implement i64 intBitCountTwosComp");
            },
            .int_big_positive => return self.castTag(.int_big_positive).?.asBigInt().bitCountTwosComp(),
            .int_big_negative => return self.castTag(.int_big_negative).?.asBigInt().bitCountTwosComp(),

            else => unreachable,
        }
    }

    /// Asserts the value is an integer, and the destination type is ComptimeInt or Int.
    pub fn intFitsInType(self: Value, ty: Type, target: Target) bool {
        switch (self.tag()) {
            .zero,
            .undef,
            .bool_false,
            => return true,

            .one,
            .bool_true,
            => {
                const info = ty.intInfo(target);
                return switch (info.signedness) {
                    .signed => info.bits >= 2,
                    .unsigned => info.bits >= 1,
                };
            },

            .int_u64 => switch (ty.zigTypeTag()) {
                .Int => {
                    const x = self.castTag(.int_u64).?.data;
                    if (x == 0) return true;
                    const info = ty.intInfo(target);
                    const needed_bits = std.math.log2(x) + 1 + @boolToInt(info.signedness == .signed);
                    return info.bits >= needed_bits;
                },
                .ComptimeInt => return true,
                else => unreachable,
            },
            .int_i64 => switch (ty.zigTypeTag()) {
                .Int => {
                    const x = self.castTag(.int_i64).?.data;
                    if (x == 0) return true;
                    const info = ty.intInfo(target);
                    if (info.signedness == .unsigned and x < 0)
                        return false;
                    @panic("TODO implement i64 intFitsInType");
                },
                .ComptimeInt => return true,
                else => unreachable,
            },
            .int_big_positive => switch (ty.zigTypeTag()) {
                .Int => {
                    const info = ty.intInfo(target);
                    return self.castTag(.int_big_positive).?.asBigInt().fitsInTwosComp(info.signedness, info.bits);
                },
                .ComptimeInt => return true,
                else => unreachable,
            },
            .int_big_negative => switch (ty.zigTypeTag()) {
                .Int => {
                    const info = ty.intInfo(target);
                    return self.castTag(.int_big_negative).?.asBigInt().fitsInTwosComp(info.signedness, info.bits);
                },
                .ComptimeInt => return true,
                else => unreachable,
            },

            else => unreachable,
        }
    }

    /// Converts an integer or a float to a float.
    /// Returns `error.Overflow` if the value does not fit in the new type.
    pub fn floatCast(self: Value, allocator: *Allocator, dest_ty: Type) !Value {
        switch (dest_ty.tag()) {
            .f16 => {
                @panic("TODO add __trunctfhf2 to compiler-rt");
                //const res = try Value.Tag.float_16.create(allocator, self.toFloat(f16));
                //if (!self.eql(res))
                //    return error.Overflow;
                //return res;
            },
            .f32 => {
                const res = try Value.Tag.float_32.create(allocator, self.toFloat(f32));
                if (!self.eql(res, dest_ty))
                    return error.Overflow;
                return res;
            },
            .f64 => {
                const res = try Value.Tag.float_64.create(allocator, self.toFloat(f64));
                if (!self.eql(res, dest_ty))
                    return error.Overflow;
                return res;
            },
            .f128, .comptime_float, .c_longdouble => {
                return Value.Tag.float_128.create(allocator, self.toFloat(f128));
            },
            else => unreachable,
        }
    }

    /// Asserts the value is a float
    pub fn floatHasFraction(self: Value) bool {
        return switch (self.tag()) {
            .zero,
            .one,
            => false,

            .float_16 => @rem(self.castTag(.float_16).?.data, 1) != 0,
            .float_32 => @rem(self.castTag(.float_32).?.data, 1) != 0,
            .float_64 => @rem(self.castTag(.float_64).?.data, 1) != 0,
            // .float_128 => @rem(self.castTag(.float_128).?.data, 1) != 0,
            .float_128 => @panic("TODO lld: error: undefined symbol: fmodl"),

            else => unreachable,
        };
    }

    /// Asserts the value is numeric
    pub fn isZero(self: Value) bool {
        return switch (self.tag()) {
            .zero => true,
            .one => false,

            .int_u64 => self.castTag(.int_u64).?.data == 0,
            .int_i64 => self.castTag(.int_i64).?.data == 0,

            .float_16 => self.castTag(.float_16).?.data == 0,
            .float_32 => self.castTag(.float_32).?.data == 0,
            .float_64 => self.castTag(.float_64).?.data == 0,
            .float_128 => self.castTag(.float_128).?.data == 0,

            .int_big_positive => self.castTag(.int_big_positive).?.asBigInt().eqZero(),
            .int_big_negative => self.castTag(.int_big_negative).?.asBigInt().eqZero(),
            else => unreachable,
        };
    }

    pub fn orderAgainstZero(lhs: Value) std.math.Order {
        return switch (lhs.tag()) {
            .zero,
            .bool_false,
            => .eq,

            .one,
            .bool_true,
            => .gt,

            .int_u64 => std.math.order(lhs.castTag(.int_u64).?.data, 0),
            .int_i64 => std.math.order(lhs.castTag(.int_i64).?.data, 0),
            .int_big_positive => lhs.castTag(.int_big_positive).?.asBigInt().orderAgainstScalar(0),
            .int_big_negative => lhs.castTag(.int_big_negative).?.asBigInt().orderAgainstScalar(0),

            .float_16 => std.math.order(lhs.castTag(.float_16).?.data, 0),
            .float_32 => std.math.order(lhs.castTag(.float_32).?.data, 0),
            .float_64 => std.math.order(lhs.castTag(.float_64).?.data, 0),
            .float_128 => std.math.order(lhs.castTag(.float_128).?.data, 0),

            else => unreachable,
        };
    }

    /// Asserts the value is comparable.
    pub fn order(lhs: Value, rhs: Value) std.math.Order {
        const lhs_tag = lhs.tag();
        const rhs_tag = rhs.tag();
        const lhs_is_zero = lhs_tag == .zero;
        const rhs_is_zero = rhs_tag == .zero;
        if (lhs_is_zero) return rhs.orderAgainstZero().invert();
        if (rhs_is_zero) return lhs.orderAgainstZero();

        const lhs_float = lhs.isFloat();
        const rhs_float = rhs.isFloat();
        if (lhs_float and rhs_float) {
            if (lhs_tag == rhs_tag) {
                return switch (lhs.tag()) {
                    .float_16 => return std.math.order(lhs.castTag(.float_16).?.data, rhs.castTag(.float_16).?.data),
                    .float_32 => return std.math.order(lhs.castTag(.float_32).?.data, rhs.castTag(.float_32).?.data),
                    .float_64 => return std.math.order(lhs.castTag(.float_64).?.data, rhs.castTag(.float_64).?.data),
                    .float_128 => return std.math.order(lhs.castTag(.float_128).?.data, rhs.castTag(.float_128).?.data),
                    else => unreachable,
                };
            }
        }
        if (lhs_float or rhs_float) {
            const lhs_f128 = lhs.toFloat(f128);
            const rhs_f128 = rhs.toFloat(f128);
            return std.math.order(lhs_f128, rhs_f128);
        }

        var lhs_bigint_space: BigIntSpace = undefined;
        var rhs_bigint_space: BigIntSpace = undefined;
        const lhs_bigint = lhs.toBigInt(&lhs_bigint_space);
        const rhs_bigint = rhs.toBigInt(&rhs_bigint_space);
        return lhs_bigint.order(rhs_bigint);
    }

    /// Asserts the value is comparable. Does not take a type parameter because it supports
    /// comparisons between heterogeneous types.
    pub fn compareHetero(lhs: Value, op: std.math.CompareOperator, rhs: Value) bool {
        return order(lhs, rhs).compare(op);
    }

    /// Asserts the value is comparable. Both operands have type `ty`.
    pub fn compare(lhs: Value, op: std.math.CompareOperator, rhs: Value, ty: Type) bool {
        return switch (op) {
            .eq => lhs.eql(rhs, ty),
            .neq => !lhs.eql(rhs, ty),
            else => compareHetero(lhs, op, rhs),
        };
    }

    /// Asserts the value is comparable.
    pub fn compareWithZero(lhs: Value, op: std.math.CompareOperator) bool {
        return orderAgainstZero(lhs).compare(op);
    }

    pub fn eql(a: Value, b: Value, ty: Type) bool {
        const a_tag = a.tag();
        const b_tag = b.tag();
        assert(a_tag != .undef);
        assert(b_tag != .undef);
        if (a_tag == b_tag) {
            switch (a_tag) {
                .void_value, .null_value => return true,
                .enum_literal => {
                    const a_name = a.castTag(.enum_literal).?.data;
                    const b_name = b.castTag(.enum_literal).?.data;
                    return std.mem.eql(u8, a_name, b_name);
                },
                .enum_field_index => {
                    const a_field_index = a.castTag(.enum_field_index).?.data;
                    const b_field_index = b.castTag(.enum_field_index).?.data;
                    return a_field_index == b_field_index;
                },
                else => {},
            }
        }
        if (ty.zigTypeTag() == .Type) {
            var buf_a: ToTypeBuffer = undefined;
            var buf_b: ToTypeBuffer = undefined;
            const a_type = a.toType(&buf_a);
            const b_type = b.toType(&buf_b);
            return a_type.eql(b_type);
        }
        return order(a, b).compare(.eq);
    }

    pub fn hash(val: Value, ty: Type, hasher: *std.hash.Wyhash) void {
        const zig_ty_tag = ty.zigTypeTag();
        std.hash.autoHash(hasher, zig_ty_tag);

        switch (zig_ty_tag) {
            .BoundFn => unreachable, // TODO remove this from the language

            .Void,
            .NoReturn,
            .Undefined,
            .Null,
            => {},

            .Type => {
                var buf: ToTypeBuffer = undefined;
                return val.toType(&buf).hashWithHasher(hasher);
            },
            .Bool => {
                std.hash.autoHash(hasher, val.toBool());
            },
            .Int, .ComptimeInt => {
                var space: BigIntSpace = undefined;
                const big = val.toBigInt(&space);
                std.hash.autoHash(hasher, big.positive);
                for (big.limbs) |limb| {
                    std.hash.autoHash(hasher, limb);
                }
            },
            .Float, .ComptimeFloat => {
                // TODO double check the lang spec. should we to bitwise hashing here,
                // or a hash that normalizes the float value?
                const float = val.toFloat(f128);
                std.hash.autoHash(hasher, @bitCast(u128, float));
            },
            .Pointer => {
                @panic("TODO implement hashing pointer values");
            },
            .Array, .Vector => {
                @panic("TODO implement hashing array/vector values");
            },
            .Struct => {
                @panic("TODO implement hashing struct values");
            },
            .Optional => {
                if (val.castTag(.opt_payload)) |payload| {
                    std.hash.autoHash(hasher, true); // non-null
                    const sub_val = payload.data;
                    var buffer: Type.Payload.ElemType = undefined;
                    const sub_ty = ty.optionalChild(&buffer);
                    sub_val.hash(sub_ty, hasher);
                } else {
                    std.hash.autoHash(hasher, false); // non-null
                }
            },
            .ErrorUnion => {
                @panic("TODO implement hashing error union values");
            },
            .ErrorSet => {
                @panic("TODO implement hashing error set values");
            },
            .Enum => {
                var enum_space: Payload.U64 = undefined;
                const int_val = val.enumToInt(ty, &enum_space);

                var space: BigIntSpace = undefined;
                const big = int_val.toBigInt(&space);

                std.hash.autoHash(hasher, big.positive);
                for (big.limbs) |limb| {
                    std.hash.autoHash(hasher, limb);
                }
            },
            .Union => {
                @panic("TODO implement hashing union values");
            },
            .Fn => {
                @panic("TODO implement hashing function values");
            },
            .Opaque => {
                @panic("TODO implement hashing opaque values");
            },
            .Frame => {
                @panic("TODO implement hashing frame values");
            },
            .AnyFrame => {
                @panic("TODO implement hashing anyframe values");
            },
            .EnumLiteral => {
                @panic("TODO implement hashing enum literal values");
            },
        }
    }

    pub const ArrayHashContext = struct {
        ty: Type,

        pub fn hash(self: @This(), val: Value) u32 {
            const other_context: HashContext = .{ .ty = self.ty };
            return @truncate(u32, other_context.hash(val));
        }
        pub fn eql(self: @This(), a: Value, b: Value) bool {
            return a.eql(b, self.ty);
        }
    };

    pub const HashContext = struct {
        ty: Type,

        pub fn hash(self: @This(), val: Value) u64 {
            var hasher = std.hash.Wyhash.init(0);
            val.hash(self.ty, &hasher);
            return hasher.final();
        }

        pub fn eql(self: @This(), a: Value, b: Value) bool {
            return a.eql(b, self.ty);
        }
    };

    /// Asserts the value is a pointer and dereferences it.
    /// Returns error.AnalysisFail if the pointer points to a Decl that failed semantic analysis.
    pub fn pointerDeref(
        self: Value,
        allocator: *Allocator,
    ) error{ AnalysisFail, OutOfMemory }!?Value {
        const sub_val: Value = switch (self.tag()) {
            .decl_ref_mut => val: {
                // The decl whose value we are obtaining here may be overwritten with
                // a different value, which would invalidate this memory. So we must
                // copy here.
                const val = try self.castTag(.decl_ref_mut).?.data.decl.value();
                break :val try val.copy(allocator);
            },
            .decl_ref => try self.castTag(.decl_ref).?.data.value(),
            .elem_ptr => blk: {
                const elem_ptr = self.castTag(.elem_ptr).?.data;
                const array_val = (try elem_ptr.array_ptr.pointerDeref(allocator)) orelse return null;
                break :blk try array_val.elemValue(allocator, elem_ptr.index);
            },
            .field_ptr => blk: {
                const field_ptr = self.castTag(.field_ptr).?.data;
                const container_val = (try field_ptr.container_ptr.pointerDeref(allocator)) orelse return null;
                break :blk try container_val.fieldValue(allocator, field_ptr.field_index);
            },
            .eu_payload_ptr => blk: {
                const err_union_ptr = self.castTag(.eu_payload_ptr).?.data;
                const err_union_val = (try err_union_ptr.pointerDeref(allocator)) orelse return null;
                break :blk err_union_val.castTag(.eu_payload).?.data;
            },
            .opt_payload_ptr => blk: {
                const opt_ptr = self.castTag(.opt_payload_ptr).?.data;
                const opt_val = (try opt_ptr.pointerDeref(allocator)) orelse return null;
                break :blk opt_val.castTag(.opt_payload).?.data;
            },

            .zero,
            .one,
            .int_u64,
            .int_i64,
            .int_big_positive,
            .int_big_negative,
            .variable,
            .extern_fn,
            .function,
            => return null,

            else => unreachable,
        };
        if (sub_val.tag() == .variable) {
            // This would be loading a runtime value at compile-time so we return
            // the indicator that this pointer dereference requires being done at runtime.
            return null;
        }
        return sub_val;
    }

    pub fn sliceLen(val: Value) u64 {
        return switch (val.tag()) {
            .empty_array => 0,
            .bytes => val.castTag(.bytes).?.data.len,
            .array => val.castTag(.array).?.data.len,
            .slice => val.castTag(.slice).?.data.len.toUnsignedInt(),
            .decl_ref => {
                const decl = val.castTag(.decl_ref).?.data;
                if (decl.ty.zigTypeTag() == .Array) {
                    return decl.ty.arrayLen();
                } else {
                    return 1;
                }
            },
            else => unreachable,
        };
    }

    /// Asserts the value is a single-item pointer to an array, or an array,
    /// or an unknown-length pointer, and returns the element value at the index.
    pub fn elemValue(self: Value, allocator: *Allocator, index: usize) error{OutOfMemory}!Value {
        switch (self.tag()) {
            .empty_array => unreachable, // out of bounds array index

            .bytes => return Tag.int_u64.create(allocator, self.castTag(.bytes).?.data[index]),

            // No matter the index; all the elements are the same!
            .repeated => return self.castTag(.repeated).?.data,

            .array => return self.castTag(.array).?.data[index],
            .slice => return self.castTag(.slice).?.data.ptr.elemValue(allocator, index),

            else => unreachable,
        }
    }

    pub fn fieldValue(val: Value, allocator: *Allocator, index: usize) error{OutOfMemory}!Value {
        _ = allocator;
        switch (val.tag()) {
            .@"struct" => {
                const field_values = val.castTag(.@"struct").?.data;
                return field_values[index];
            },
            .@"union" => {
                const payload = val.castTag(.@"union").?.data;
                // TODO assert the tag is correct
                return payload.val;
            },

            else => unreachable,
        }
    }

    /// Returns a pointer to the element value at the index.
    pub fn elemPtr(self: Value, allocator: *Allocator, index: usize) !Value {
        if (self.castTag(.elem_ptr)) |elem_ptr| {
            return Tag.elem_ptr.create(allocator, .{
                .array_ptr = elem_ptr.data.array_ptr,
                .index = elem_ptr.data.index + index,
            });
        }

        return Tag.elem_ptr.create(allocator, .{
            .array_ptr = self,
            .index = index,
        });
    }

    pub fn isUndef(self: Value) bool {
        return self.tag() == .undef;
    }

    /// Valid for all types. Asserts the value is not undefined and not unreachable.
    pub fn isNull(self: Value) bool {
        return switch (self.tag()) {
            .null_value => true,
            .opt_payload => false,

            .undef => unreachable,
            .unreachable_value => unreachable,
            .inferred_alloc => unreachable,
            .inferred_alloc_comptime => unreachable,
            else => unreachable,
        };
    }

    /// Valid for all types. Asserts the value is not undefined and not unreachable.
    /// Prefer `errorUnionIsPayload` to find out whether something is an error or not
    /// because it works without having to figure out the string.
    pub fn getError(self: Value) ?[]const u8 {
        return switch (self.tag()) {
            .@"error" => self.castTag(.@"error").?.data.name,
            .int_u64 => @panic("TODO"),
            .int_i64 => @panic("TODO"),
            .int_big_positive => @panic("TODO"),
            .int_big_negative => @panic("TODO"),
            .one => @panic("TODO"),
            .undef => unreachable,
            .unreachable_value => unreachable,
            .inferred_alloc => unreachable,
            .inferred_alloc_comptime => unreachable,

            else => null,
        };
    }

    /// Assumes the type is an error union. Returns true if and only if the value is
    /// the error union payload, not an error.
    pub fn errorUnionIsPayload(val: Value) bool {
        return switch (val.tag()) {
            .eu_payload => true,
            else => false,

            .undef => unreachable,
            .inferred_alloc => unreachable,
            .inferred_alloc_comptime => unreachable,
        };
    }

    /// Valid for all types. Asserts the value is not undefined.
    pub fn isFloat(self: Value) bool {
        return switch (self.tag()) {
            .undef => unreachable,
            .inferred_alloc => unreachable,
            .inferred_alloc_comptime => unreachable,

            .float_16,
            .float_32,
            .float_64,
            .float_128,
            => true,
            else => false,
        };
    }

    pub fn intAdd(lhs: Value, rhs: Value, allocator: *Allocator) !Value {
        // TODO is this a performance issue? maybe we should try the operation without
        // resorting to BigInt first.
        var lhs_space: Value.BigIntSpace = undefined;
        var rhs_space: Value.BigIntSpace = undefined;
        const lhs_bigint = lhs.toBigInt(&lhs_space);
        const rhs_bigint = rhs.toBigInt(&rhs_space);
        const limbs = try allocator.alloc(
            std.math.big.Limb,
            std.math.max(lhs_bigint.limbs.len, rhs_bigint.limbs.len) + 1,
        );
        var result_bigint = BigIntMutable{ .limbs = limbs, .positive = undefined, .len = undefined };
        result_bigint.add(lhs_bigint, rhs_bigint);
        const result_limbs = result_bigint.limbs[0..result_bigint.len];

        if (result_bigint.positive) {
            return Value.Tag.int_big_positive.create(allocator, result_limbs);
        } else {
            return Value.Tag.int_big_negative.create(allocator, result_limbs);
        }
    }

    pub fn intSub(lhs: Value, rhs: Value, allocator: *Allocator) !Value {
        // TODO is this a performance issue? maybe we should try the operation without
        // resorting to BigInt first.
        var lhs_space: Value.BigIntSpace = undefined;
        var rhs_space: Value.BigIntSpace = undefined;
        const lhs_bigint = lhs.toBigInt(&lhs_space);
        const rhs_bigint = rhs.toBigInt(&rhs_space);
        const limbs = try allocator.alloc(
            std.math.big.Limb,
            std.math.max(lhs_bigint.limbs.len, rhs_bigint.limbs.len) + 1,
        );
        var result_bigint = BigIntMutable{ .limbs = limbs, .positive = undefined, .len = undefined };
        result_bigint.sub(lhs_bigint, rhs_bigint);
        const result_limbs = result_bigint.limbs[0..result_bigint.len];

        if (result_bigint.positive) {
            return Value.Tag.int_big_positive.create(allocator, result_limbs);
        } else {
            return Value.Tag.int_big_negative.create(allocator, result_limbs);
        }
    }

    pub fn intDiv(lhs: Value, rhs: Value, allocator: *Allocator) !Value {
        // TODO is this a performance issue? maybe we should try the operation without
        // resorting to BigInt first.
        var lhs_space: Value.BigIntSpace = undefined;
        var rhs_space: Value.BigIntSpace = undefined;
        const lhs_bigint = lhs.toBigInt(&lhs_space);
        const rhs_bigint = rhs.toBigInt(&rhs_space);
        const limbs_q = try allocator.alloc(
            std.math.big.Limb,
            lhs_bigint.limbs.len + rhs_bigint.limbs.len + 1,
        );
        const limbs_r = try allocator.alloc(
            std.math.big.Limb,
            lhs_bigint.limbs.len,
        );
        const limbs_buffer = try allocator.alloc(
            std.math.big.Limb,
            std.math.big.int.calcDivLimbsBufferLen(lhs_bigint.limbs.len, rhs_bigint.limbs.len),
        );
        var result_q = BigIntMutable{ .limbs = limbs_q, .positive = undefined, .len = undefined };
        var result_r = BigIntMutable{ .limbs = limbs_r, .positive = undefined, .len = undefined };
        result_q.divTrunc(&result_r, lhs_bigint, rhs_bigint, limbs_buffer, null);
        const result_limbs = result_q.limbs[0..result_q.len];

        if (result_q.positive) {
            return Value.Tag.int_big_positive.create(allocator, result_limbs);
        } else {
            return Value.Tag.int_big_negative.create(allocator, result_limbs);
        }
    }

    pub fn intMul(lhs: Value, rhs: Value, allocator: *Allocator) !Value {
        // TODO is this a performance issue? maybe we should try the operation without
        // resorting to BigInt first.
        var lhs_space: Value.BigIntSpace = undefined;
        var rhs_space: Value.BigIntSpace = undefined;
        const lhs_bigint = lhs.toBigInt(&lhs_space);
        const rhs_bigint = rhs.toBigInt(&rhs_space);
        const limbs = try allocator.alloc(
            std.math.big.Limb,
            lhs_bigint.limbs.len + rhs_bigint.limbs.len + 1,
        );
        var result_bigint = BigIntMutable{ .limbs = limbs, .positive = undefined, .len = undefined };
        var limbs_buffer = try allocator.alloc(
            std.math.big.Limb,
            std.math.big.int.calcMulLimbsBufferLen(lhs_bigint.limbs.len, rhs_bigint.limbs.len, 1),
        );
        defer allocator.free(limbs_buffer);
        result_bigint.mul(lhs_bigint, rhs_bigint, limbs_buffer, allocator);
        const result_limbs = result_bigint.limbs[0..result_bigint.len];

        if (result_bigint.positive) {
            return Value.Tag.int_big_positive.create(allocator, result_limbs);
        } else {
            return Value.Tag.int_big_negative.create(allocator, result_limbs);
        }
    }

    pub fn intTrunc(val: Value, arena: *Allocator, bits: u16) !Value {
        const x = val.toUnsignedInt(); // TODO: implement comptime truncate on big ints
        if (bits == 64) return val;
        const mask = (@as(u64, 1) << @intCast(u6, bits)) - 1;
        const truncated = x & mask;
        return Tag.int_u64.create(arena, truncated);
    }

    pub fn shr(lhs: Value, rhs: Value, allocator: *Allocator) !Value {
        // TODO is this a performance issue? maybe we should try the operation without
        // resorting to BigInt first.
        var lhs_space: Value.BigIntSpace = undefined;
        const lhs_bigint = lhs.toBigInt(&lhs_space);
        const shift = rhs.toUnsignedInt();
        const limbs = try allocator.alloc(
            std.math.big.Limb,
            lhs_bigint.limbs.len - (shift / (@sizeOf(std.math.big.Limb) * 8)),
        );
        var result_bigint = BigIntMutable{
            .limbs = limbs,
            .positive = undefined,
            .len = undefined,
        };
        result_bigint.shiftRight(lhs_bigint, shift);
        const result_limbs = result_bigint.limbs[0..result_bigint.len];

        if (result_bigint.positive) {
            return Value.Tag.int_big_positive.create(allocator, result_limbs);
        } else {
            return Value.Tag.int_big_negative.create(allocator, result_limbs);
        }
    }

    pub fn floatAdd(
        lhs: Value,
        rhs: Value,
        float_type: Type,
        arena: *Allocator,
    ) !Value {
        switch (float_type.tag()) {
            .f16 => {
                @panic("TODO add __trunctfhf2 to compiler-rt");
                //const lhs_val = lhs.toFloat(f16);
                //const rhs_val = rhs.toFloat(f16);
                //return Value.Tag.float_16.create(arena, lhs_val + rhs_val);
            },
            .f32 => {
                const lhs_val = lhs.toFloat(f32);
                const rhs_val = rhs.toFloat(f32);
                return Value.Tag.float_32.create(arena, lhs_val + rhs_val);
            },
            .f64 => {
                const lhs_val = lhs.toFloat(f64);
                const rhs_val = rhs.toFloat(f64);
                return Value.Tag.float_64.create(arena, lhs_val + rhs_val);
            },
            .f128, .comptime_float, .c_longdouble => {
                const lhs_val = lhs.toFloat(f128);
                const rhs_val = rhs.toFloat(f128);
                return Value.Tag.float_128.create(arena, lhs_val + rhs_val);
            },
            else => unreachable,
        }
    }

    pub fn floatSub(
        lhs: Value,
        rhs: Value,
        float_type: Type,
        arena: *Allocator,
    ) !Value {
        switch (float_type.tag()) {
            .f16 => {
                @panic("TODO add __trunctfhf2 to compiler-rt");
                //const lhs_val = lhs.toFloat(f16);
                //const rhs_val = rhs.toFloat(f16);
                //return Value.Tag.float_16.create(arena, lhs_val - rhs_val);
            },
            .f32 => {
                const lhs_val = lhs.toFloat(f32);
                const rhs_val = rhs.toFloat(f32);
                return Value.Tag.float_32.create(arena, lhs_val - rhs_val);
            },
            .f64 => {
                const lhs_val = lhs.toFloat(f64);
                const rhs_val = rhs.toFloat(f64);
                return Value.Tag.float_64.create(arena, lhs_val - rhs_val);
            },
            .f128, .comptime_float, .c_longdouble => {
                const lhs_val = lhs.toFloat(f128);
                const rhs_val = rhs.toFloat(f128);
                return Value.Tag.float_128.create(arena, lhs_val - rhs_val);
            },
            else => unreachable,
        }
    }

    pub fn floatDiv(
        lhs: Value,
        rhs: Value,
        float_type: Type,
        arena: *Allocator,
    ) !Value {
        switch (float_type.tag()) {
            .f16 => {
                @panic("TODO add __trunctfhf2 to compiler-rt");
                //const lhs_val = lhs.toFloat(f16);
                //const rhs_val = rhs.toFloat(f16);
                //return Value.Tag.float_16.create(arena, lhs_val / rhs_val);
            },
            .f32 => {
                const lhs_val = lhs.toFloat(f32);
                const rhs_val = rhs.toFloat(f32);
                return Value.Tag.float_32.create(arena, lhs_val / rhs_val);
            },
            .f64 => {
                const lhs_val = lhs.toFloat(f64);
                const rhs_val = rhs.toFloat(f64);
                return Value.Tag.float_64.create(arena, lhs_val / rhs_val);
            },
            .f128, .comptime_float, .c_longdouble => {
                const lhs_val = lhs.toFloat(f128);
                const rhs_val = rhs.toFloat(f128);
                return Value.Tag.float_128.create(arena, lhs_val / rhs_val);
            },
            else => unreachable,
        }
    }

    pub fn floatMul(
        lhs: Value,
        rhs: Value,
        float_type: Type,
        arena: *Allocator,
    ) !Value {
        switch (float_type.tag()) {
            .f16 => {
                @panic("TODO add __trunctfhf2 to compiler-rt");
                //const lhs_val = lhs.toFloat(f16);
                //const rhs_val = rhs.toFloat(f16);
                //return Value.Tag.float_16.create(arena, lhs_val * rhs_val);
            },
            .f32 => {
                const lhs_val = lhs.toFloat(f32);
                const rhs_val = rhs.toFloat(f32);
                return Value.Tag.float_32.create(arena, lhs_val * rhs_val);
            },
            .f64 => {
                const lhs_val = lhs.toFloat(f64);
                const rhs_val = rhs.toFloat(f64);
                return Value.Tag.float_64.create(arena, lhs_val * rhs_val);
            },
            .f128, .comptime_float, .c_longdouble => {
                const lhs_val = lhs.toFloat(f128);
                const rhs_val = rhs.toFloat(f128);
                return Value.Tag.float_128.create(arena, lhs_val * rhs_val);
            },
            else => unreachable,
        }
    }

    /// This type is not copyable since it may contain pointers to its inner data.
    pub const Payload = struct {
        tag: Tag,

        pub const U32 = struct {
            base: Payload,
            data: u32,
        };

        pub const U64 = struct {
            base: Payload,
            data: u64,
        };

        pub const I64 = struct {
            base: Payload,
            data: i64,
        };

        pub const BigInt = struct {
            base: Payload,
            data: []const std.math.big.Limb,

            pub fn asBigInt(self: BigInt) BigIntConst {
                const positive = switch (self.base.tag) {
                    .int_big_positive => true,
                    .int_big_negative => false,
                    else => unreachable,
                };
                return BigIntConst{ .limbs = self.data, .positive = positive };
            }
        };

        pub const Function = struct {
            base: Payload,
            data: *Module.Fn,
        };

        pub const Decl = struct {
            base: Payload,
            data: *Module.Decl,
        };

        pub const Variable = struct {
            base: Payload,
            data: *Module.Var,
        };

        pub const SubValue = struct {
            base: Payload,
            data: Value,
        };

        pub const DeclRefMut = struct {
            pub const base_tag = Tag.decl_ref_mut;

            base: Payload = Payload{ .tag = base_tag },
            data: struct {
                decl: *Module.Decl,
                runtime_index: u32,
            },
        };

        pub const ElemPtr = struct {
            pub const base_tag = Tag.elem_ptr;

            base: Payload = Payload{ .tag = base_tag },
            data: struct {
                array_ptr: Value,
                index: usize,
            },
        };

        pub const FieldPtr = struct {
            pub const base_tag = Tag.field_ptr;

            base: Payload = Payload{ .tag = base_tag },
            data: struct {
                container_ptr: Value,
                field_index: usize,
            },
        };

        pub const Bytes = struct {
            base: Payload,
            data: []const u8,
        };

        pub const Array = struct {
            base: Payload,
            data: []Value,
        };

        pub const Slice = struct {
            base: Payload,
            data: struct {
                ptr: Value,
                len: Value,
            },
        };

        pub const Ty = struct {
            base: Payload,
            data: Type,
        };

        pub const IntType = struct {
            pub const base_tag = Tag.int_type;

            base: Payload = Payload{ .tag = base_tag },
            data: struct {
                bits: u16,
                signed: bool,
            },
        };

        pub const Float_16 = struct {
            pub const base_tag = Tag.float_16;

            base: Payload = .{ .tag = base_tag },
            data: f16,
        };

        pub const Float_32 = struct {
            pub const base_tag = Tag.float_32;

            base: Payload = .{ .tag = base_tag },
            data: f32,
        };

        pub const Float_64 = struct {
            pub const base_tag = Tag.float_64;

            base: Payload = .{ .tag = base_tag },
            data: f64,
        };

        pub const Float_128 = struct {
            pub const base_tag = Tag.float_128;

            base: Payload = .{ .tag = base_tag },
            data: f128,
        };

        pub const Error = struct {
            base: Payload = .{ .tag = .@"error" },
            data: struct {
                /// `name` is owned by `Module` and will be valid for the entire
                /// duration of the compilation.
                /// TODO revisit this when we have the concept of the error tag type
                name: []const u8,
            },
        };

        pub const InferredAlloc = struct {
            pub const base_tag = Tag.inferred_alloc;

            base: Payload = .{ .tag = base_tag },
            data: struct {
                /// The value stored in the inferred allocation. This will go into
                /// peer type resolution. This is stored in a separate list so that
                /// the items are contiguous in memory and thus can be passed to
                /// `Module.resolvePeerTypes`.
                stored_inst_list: std.ArrayListUnmanaged(Air.Inst.Ref) = .{},
            },
        };

        pub const Struct = struct {
            pub const base_tag = Tag.@"struct";

            base: Payload = .{ .tag = base_tag },
            /// Field values. The number and type are according to the struct type.
            data: [*]Value,
        };

        pub const Union = struct {
            pub const base_tag = Tag.@"union";

            base: Payload = .{ .tag = base_tag },
            data: struct {
                tag: Value,
                val: Value,
            },
        };
    };

    /// Big enough to fit any non-BigInt value
    pub const BigIntSpace = struct {
        /// The +1 is headroom so that operations such as incrementing once or decrementing once
        /// are possible without using an allocator.
        limbs: [(@sizeOf(u64) / @sizeOf(std.math.big.Limb)) + 1]std.math.big.Limb,
    };
};