StackMachine

pub fn StackMachine(comptime options: Options) type {
    const addr_type = switch (options.addr_size) {
        2 => u16,
        4 => u32,
        8 => u64,
        else => @compileError("Unsupported address size of " ++ options.addr_size),
    };

    const addr_type_signed = switch (options.addr_size) {
        2 => i16,
        4 => i32,
        8 => i64,
        else => @compileError("Unsupported address size of " ++ options.addr_size),
    };

    return struct {
        const Self = @This();

        const Operand = union(enum) {
            generic: addr_type,
            register: u8,
            type_size: u8,
            branch_offset: i16,
            base_register: struct {
                base_register: u8,
                offset: i64,
            },
            composite_location: struct {
                size: u64,
                offset: i64,
            },
            block: []const u8,
            register_type: struct {
                register: u8,
                type_offset: addr_type,
            },
            const_type: struct {
                type_offset: addr_type,
                value_bytes: []const u8,
            },
            deref_type: struct {
                size: u8,
                type_offset: addr_type,
            },
        };

        const Value = union(enum) {
            generic: addr_type,

            // Typed value with a maximum size of a register
            regval_type: struct {
                // Offset of DW_TAG_base_type DIE
                type_offset: addr_type,
                type_size: u8,
                value: addr_type,
            },

            // Typed value specified directly in the instruction stream
            const_type: struct {
                // Offset of DW_TAG_base_type DIE
                type_offset: addr_type,
                // Backed by the instruction stream
                value_bytes: []const u8,
            },

            pub fn asIntegral(self: Value) !addr_type {
                return switch (self) {
                    .generic => |v| v,

                    // TODO: For these two prongs, look up the type and assert it's integral?
                    .regval_type => |regval_type| regval_type.value,
                    .const_type => |const_type| {
                        const value: u64 = switch (const_type.value_bytes.len) {
                            1 => mem.readInt(u8, const_type.value_bytes[0..1], native_endian),
                            2 => mem.readInt(u16, const_type.value_bytes[0..2], native_endian),
                            4 => mem.readInt(u32, const_type.value_bytes[0..4], native_endian),
                            8 => mem.readInt(u64, const_type.value_bytes[0..8], native_endian),
                            else => return error.InvalidIntegralTypeSize,
                        };

                        return std.math.cast(addr_type, value) orelse error.TruncatedIntegralType;
                    },
                };
            }
        };

        stack: std.ArrayListUnmanaged(Value) = .empty,

        pub fn reset(self: *Self) void {
            self.stack.clearRetainingCapacity();
        }

        pub fn deinit(self: *Self, allocator: std.mem.Allocator) void {
            self.stack.deinit(allocator);
        }

        fn generic(value: anytype) Operand {
            const int_info = @typeInfo(@TypeOf(value)).int;
            if (@sizeOf(@TypeOf(value)) > options.addr_size) {
                return .{ .generic = switch (int_info.signedness) {
                    .signed => @bitCast(@as(addr_type_signed, @truncate(value))),
                    .unsigned => @truncate(value),
                } };
            } else {
                return .{ .generic = switch (int_info.signedness) {
                    .signed => @bitCast(@as(addr_type_signed, @intCast(value))),
                    .unsigned => @intCast(value),
                } };
            }
        }

        pub fn readOperand(stream: *std.io.FixedBufferStream([]const u8), opcode: u8, context: Context) !?Operand {
            const reader = stream.reader();
            return switch (opcode) {
                OP.addr => generic(try reader.readInt(addr_type, options.endian)),
                OP.call_ref => switch (context.format) {
                    .@"32" => generic(try reader.readInt(u32, options.endian)),
                    .@"64" => generic(try reader.readInt(u64, options.endian)),
                },
                OP.const1u,
                OP.pick,
                => generic(try reader.readByte()),
                OP.deref_size,
                OP.xderef_size,
                => .{ .type_size = try reader.readByte() },
                OP.const1s => generic(try reader.readByteSigned()),
                OP.const2u,
                OP.call2,
                => generic(try reader.readInt(u16, options.endian)),
                OP.call4 => generic(try reader.readInt(u32, options.endian)),
                OP.const2s => generic(try reader.readInt(i16, options.endian)),
                OP.bra,
                OP.skip,
                => .{ .branch_offset = try reader.readInt(i16, options.endian) },
                OP.const4u => generic(try reader.readInt(u32, options.endian)),
                OP.const4s => generic(try reader.readInt(i32, options.endian)),
                OP.const8u => generic(try reader.readInt(u64, options.endian)),
                OP.const8s => generic(try reader.readInt(i64, options.endian)),
                OP.constu,
                OP.plus_uconst,
                OP.addrx,
                OP.constx,
                OP.convert,
                OP.reinterpret,
                => generic(try leb.readUleb128(u64, reader)),
                OP.consts,
                OP.fbreg,
                => generic(try leb.readIleb128(i64, reader)),
                OP.lit0...OP.lit31 => |n| generic(n - OP.lit0),
                OP.reg0...OP.reg31 => |n| .{ .register = n - OP.reg0 },
                OP.breg0...OP.breg31 => |n| .{ .base_register = .{
                    .base_register = n - OP.breg0,
                    .offset = try leb.readIleb128(i64, reader),
                } },
                OP.regx => .{ .register = try leb.readUleb128(u8, reader) },
                OP.bregx => blk: {
                    const base_register = try leb.readUleb128(u8, reader);
                    const offset = try leb.readIleb128(i64, reader);
                    break :blk .{ .base_register = .{
                        .base_register = base_register,
                        .offset = offset,
                    } };
                },
                OP.regval_type => blk: {
                    const register = try leb.readUleb128(u8, reader);
                    const type_offset = try leb.readUleb128(addr_type, reader);
                    break :blk .{ .register_type = .{
                        .register = register,
                        .type_offset = type_offset,
                    } };
                },
                OP.piece => .{
                    .composite_location = .{
                        .size = try leb.readUleb128(u8, reader),
                        .offset = 0,
                    },
                },
                OP.bit_piece => blk: {
                    const size = try leb.readUleb128(u8, reader);
                    const offset = try leb.readIleb128(i64, reader);
                    break :blk .{ .composite_location = .{
                        .size = size,
                        .offset = offset,
                    } };
                },
                OP.implicit_value, OP.entry_value => blk: {
                    const size = try leb.readUleb128(u8, reader);
                    if (stream.pos + size > stream.buffer.len) return error.InvalidExpression;
                    const block = stream.buffer[stream.pos..][0..size];
                    stream.pos += size;
                    break :blk .{
                        .block = block,
                    };
                },
                OP.const_type => blk: {
                    const type_offset = try leb.readUleb128(addr_type, reader);
                    const size = try reader.readByte();
                    if (stream.pos + size > stream.buffer.len) return error.InvalidExpression;
                    const value_bytes = stream.buffer[stream.pos..][0..size];
                    stream.pos += size;
                    break :blk .{ .const_type = .{
                        .type_offset = type_offset,
                        .value_bytes = value_bytes,
                    } };
                },
                OP.deref_type,
                OP.xderef_type,
                => .{
                    .deref_type = .{
                        .size = try reader.readByte(),
                        .type_offset = try leb.readUleb128(addr_type, reader),
                    },
                },
                OP.lo_user...OP.hi_user => return error.UnimplementedUserOpcode,
                else => null,
            };
        }

        pub fn run(
            self: *Self,
            expression: []const u8,
            allocator: std.mem.Allocator,
            context: Context,
            initial_value: ?usize,
        ) Error!?Value {
            if (initial_value) |i| try self.stack.append(allocator, .{ .generic = i });
            var stream = std.io.fixedBufferStream(expression);
            while (try self.step(&stream, allocator, context)) {}
            if (self.stack.items.len == 0) return null;
            return self.stack.items[self.stack.items.len - 1];
        }

        /// Reads an opcode and its operands from `stream`, then executes it
        pub fn step(
            self: *Self,
            stream: *std.io.FixedBufferStream([]const u8),
            allocator: std.mem.Allocator,
            context: Context,
        ) Error!bool {
            if (@sizeOf(usize) != @sizeOf(addr_type) or options.endian != native_endian)
                @compileError("Execution of non-native address sizes / endianness is not supported");

            const opcode = try stream.reader().readByte();
            if (options.call_frame_context and !isOpcodeValidInCFA(opcode)) return error.InvalidCFAOpcode;
            const operand = try readOperand(stream, opcode, context);
            switch (opcode) {

                // 2.5.1.1: Literal Encodings
                OP.lit0...OP.lit31,
                OP.addr,
                OP.const1u,
                OP.const2u,
                OP.const4u,
                OP.const8u,
                OP.const1s,
                OP.const2s,
                OP.const4s,
                OP.const8s,
                OP.constu,
                OP.consts,
                => try self.stack.append(allocator, .{ .generic = operand.?.generic }),

                OP.const_type => {
                    const const_type = operand.?.const_type;
                    try self.stack.append(allocator, .{ .const_type = .{
                        .type_offset = const_type.type_offset,
                        .value_bytes = const_type.value_bytes,
                    } });
                },

                OP.addrx,
                OP.constx,
                => {
                    if (context.compile_unit == null) return error.IncompleteExpressionContext;
                    if (context.debug_addr == null) return error.IncompleteExpressionContext;
                    const debug_addr_index = operand.?.generic;
                    const offset = context.compile_unit.?.addr_base + debug_addr_index;
                    if (offset >= context.debug_addr.?.len) return error.InvalidExpression;
                    const value = mem.readInt(usize, context.debug_addr.?[offset..][0..@sizeOf(usize)], native_endian);
                    try self.stack.append(allocator, .{ .generic = value });
                },

                // 2.5.1.2: Register Values
                OP.fbreg => {
                    if (context.compile_unit == null) return error.IncompleteExpressionContext;
                    if (context.compile_unit.?.frame_base == null) return error.IncompleteExpressionContext;

                    const offset: i64 = @intCast(operand.?.generic);
                    _ = offset;

                    switch (context.compile_unit.?.frame_base.?.*) {
                        .exprloc => {
                            // TODO: Run this expression in a nested stack machine
                            return error.UnimplementedOpcode;
                        },
                        .loclistx => {
                            // TODO: Read value from .debug_loclists
                            return error.UnimplementedOpcode;
                        },
                        .sec_offset => {
                            // TODO: Read value from .debug_loclists
                            return error.UnimplementedOpcode;
                        },
                        else => return error.InvalidFrameBase,
                    }
                },
                OP.breg0...OP.breg31,
                OP.bregx,
                => {
                    if (context.thread_context == null) return error.IncompleteExpressionContext;

                    const base_register = operand.?.base_register;
                    var value: i64 = @intCast(mem.readInt(usize, (try abi.regBytes(
                        context.thread_context.?,
                        base_register.base_register,
                        context.reg_context,
                    ))[0..@sizeOf(usize)], native_endian));
                    value += base_register.offset;
                    try self.stack.append(allocator, .{ .generic = @intCast(value) });
                },
                OP.regval_type => {
                    const register_type = operand.?.register_type;
                    const value = mem.readInt(usize, (try abi.regBytes(
                        context.thread_context.?,
                        register_type.register,
                        context.reg_context,
                    ))[0..@sizeOf(usize)], native_endian);
                    try self.stack.append(allocator, .{
                        .regval_type = .{
                            .type_offset = register_type.type_offset,
                            .type_size = @sizeOf(addr_type),
                            .value = value,
                        },
                    });
                },

                // 2.5.1.3: Stack Operations
                OP.dup => {
                    if (self.stack.items.len == 0) return error.InvalidExpression;
                    try self.stack.append(allocator, self.stack.items[self.stack.items.len - 1]);
                },
                OP.drop => {
                    _ = self.stack.pop();
                },
                OP.pick, OP.over => {
                    const stack_index = if (opcode == OP.over) 1 else operand.?.generic;
                    if (stack_index >= self.stack.items.len) return error.InvalidExpression;
                    try self.stack.append(allocator, self.stack.items[self.stack.items.len - 1 - stack_index]);
                },
                OP.swap => {
                    if (self.stack.items.len < 2) return error.InvalidExpression;
                    mem.swap(Value, &self.stack.items[self.stack.items.len - 1], &self.stack.items[self.stack.items.len - 2]);
                },
                OP.rot => {
                    if (self.stack.items.len < 3) return error.InvalidExpression;
                    const first = self.stack.items[self.stack.items.len - 1];
                    self.stack.items[self.stack.items.len - 1] = self.stack.items[self.stack.items.len - 2];
                    self.stack.items[self.stack.items.len - 2] = self.stack.items[self.stack.items.len - 3];
                    self.stack.items[self.stack.items.len - 3] = first;
                },
                OP.deref,
                OP.xderef,
                OP.deref_size,
                OP.xderef_size,
                OP.deref_type,
                OP.xderef_type,
                => {
                    if (self.stack.items.len == 0) return error.InvalidExpression;
                    const addr = try self.stack.items[self.stack.items.len - 1].asIntegral();
                    const addr_space_identifier: ?usize = switch (opcode) {
                        OP.xderef,
                        OP.xderef_size,
                        OP.xderef_type,
                        => blk: {
                            _ = self.stack.pop();
                            if (self.stack.items.len == 0) return error.InvalidExpression;
                            break :blk try self.stack.items[self.stack.items.len - 1].asIntegral();
                        },
                        else => null,
                    };

                    // Usage of addr_space_identifier in the address calculation is implementation defined.
                    // This code will need to be updated to handle any architectures that utilize this.
                    _ = addr_space_identifier;

                    const size = switch (opcode) {
                        OP.deref,
                        OP.xderef,
                        => @sizeOf(addr_type),
                        OP.deref_size,
                        OP.xderef_size,
                        => operand.?.type_size,
                        OP.deref_type,
                        OP.xderef_type,
                        => operand.?.deref_type.size,
                        else => unreachable,
                    };

                    if (context.memory_accessor) |memory_accessor| {
                        if (!switch (size) {
                            1 => memory_accessor.load(u8, addr) != null,
                            2 => memory_accessor.load(u16, addr) != null,
                            4 => memory_accessor.load(u32, addr) != null,
                            8 => memory_accessor.load(u64, addr) != null,
                            else => return error.InvalidExpression,
                        }) return error.InvalidExpression;
                    }

                    const value: addr_type = std.math.cast(addr_type, @as(u64, switch (size) {
                        1 => @as(*const u8, @ptrFromInt(addr)).*,
                        2 => @as(*const u16, @ptrFromInt(addr)).*,
                        4 => @as(*const u32, @ptrFromInt(addr)).*,
                        8 => @as(*const u64, @ptrFromInt(addr)).*,
                        else => return error.InvalidExpression,
                    })) orelse return error.InvalidExpression;

                    switch (opcode) {
                        OP.deref_type,
                        OP.xderef_type,
                        => {
                            self.stack.items[self.stack.items.len - 1] = .{
                                .regval_type = .{
                                    .type_offset = operand.?.deref_type.type_offset,
                                    .type_size = operand.?.deref_type.size,
                                    .value = value,
                                },
                            };
                        },
                        else => {
                            self.stack.items[self.stack.items.len - 1] = .{ .generic = value };
                        },
                    }
                },
                OP.push_object_address => {
                    // In sub-expressions, `push_object_address` is not meaningful (as per the
                    // spec), so treat it like a nop
                    if (!context.entry_value_context) {
                        if (context.object_address == null) return error.IncompleteExpressionContext;
                        try self.stack.append(allocator, .{ .generic = @intFromPtr(context.object_address.?) });
                    }
                },
                OP.form_tls_address => {
                    return error.UnimplementedOpcode;
                },
                OP.call_frame_cfa => {
                    if (context.cfa) |cfa| {
                        try self.stack.append(allocator, .{ .generic = cfa });
                    } else return error.IncompleteExpressionContext;
                },

                // 2.5.1.4: Arithmetic and Logical Operations
                OP.abs => {
                    if (self.stack.items.len == 0) return error.InvalidExpression;
                    const value: isize = @bitCast(try self.stack.items[self.stack.items.len - 1].asIntegral());
                    self.stack.items[self.stack.items.len - 1] = .{
                        .generic = @abs(value),
                    };
                },
                OP.@"and" => {
                    if (self.stack.items.len < 2) return error.InvalidExpression;
                    const a = try self.stack.pop().?.asIntegral();
                    self.stack.items[self.stack.items.len - 1] = .{
                        .generic = a & try self.stack.items[self.stack.items.len - 1].asIntegral(),
                    };
                },
                OP.div => {
                    if (self.stack.items.len < 2) return error.InvalidExpression;
                    const a: isize = @bitCast(try self.stack.pop().?.asIntegral());
                    const b: isize = @bitCast(try self.stack.items[self.stack.items.len - 1].asIntegral());
                    self.stack.items[self.stack.items.len - 1] = .{
                        .generic = @bitCast(try std.math.divTrunc(isize, b, a)),
                    };
                },
                OP.minus => {
                    if (self.stack.items.len < 2) return error.InvalidExpression;
                    const b = try self.stack.pop().?.asIntegral();
                    self.stack.items[self.stack.items.len - 1] = .{
                        .generic = try std.math.sub(addr_type, try self.stack.items[self.stack.items.len - 1].asIntegral(), b),
                    };
                },
                OP.mod => {
                    if (self.stack.items.len < 2) return error.InvalidExpression;
                    const a: isize = @bitCast(try self.stack.pop().?.asIntegral());
                    const b: isize = @bitCast(try self.stack.items[self.stack.items.len - 1].asIntegral());
                    self.stack.items[self.stack.items.len - 1] = .{
                        .generic = @bitCast(@mod(b, a)),
                    };
                },
                OP.mul => {
                    if (self.stack.items.len < 2) return error.InvalidExpression;
                    const a: isize = @bitCast(try self.stack.pop().?.asIntegral());
                    const b: isize = @bitCast(try self.stack.items[self.stack.items.len - 1].asIntegral());
                    self.stack.items[self.stack.items.len - 1] = .{
                        .generic = @bitCast(@mulWithOverflow(a, b)[0]),
                    };
                },
                OP.neg => {
                    if (self.stack.items.len == 0) return error.InvalidExpression;
                    self.stack.items[self.stack.items.len - 1] = .{
                        .generic = @bitCast(
                            try std.math.negate(
                                @as(isize, @bitCast(try self.stack.items[self.stack.items.len - 1].asIntegral())),
                            ),
                        ),
                    };
                },
                OP.not => {
                    if (self.stack.items.len == 0) return error.InvalidExpression;
                    self.stack.items[self.stack.items.len - 1] = .{
                        .generic = ~try self.stack.items[self.stack.items.len - 1].asIntegral(),
                    };
                },
                OP.@"or" => {
                    if (self.stack.items.len < 2) return error.InvalidExpression;
                    const a = try self.stack.pop().?.asIntegral();
                    self.stack.items[self.stack.items.len - 1] = .{
                        .generic = a | try self.stack.items[self.stack.items.len - 1].asIntegral(),
                    };
                },
                OP.plus => {
                    if (self.stack.items.len < 2) return error.InvalidExpression;
                    const b = try self.stack.pop().?.asIntegral();
                    self.stack.items[self.stack.items.len - 1] = .{
                        .generic = try std.math.add(addr_type, try self.stack.items[self.stack.items.len - 1].asIntegral(), b),
                    };
                },
                OP.plus_uconst => {
                    if (self.stack.items.len == 0) return error.InvalidExpression;
                    const constant = operand.?.generic;
                    self.stack.items[self.stack.items.len - 1] = .{
                        .generic = try std.math.add(addr_type, try self.stack.items[self.stack.items.len - 1].asIntegral(), constant),
                    };
                },
                OP.shl => {
                    if (self.stack.items.len < 2) return error.InvalidExpression;
                    const a = try self.stack.pop().?.asIntegral();
                    const b = try self.stack.items[self.stack.items.len - 1].asIntegral();
                    self.stack.items[self.stack.items.len - 1] = .{
                        .generic = std.math.shl(usize, b, a),
                    };
                },
                OP.shr => {
                    if (self.stack.items.len < 2) return error.InvalidExpression;
                    const a = try self.stack.pop().?.asIntegral();
                    const b = try self.stack.items[self.stack.items.len - 1].asIntegral();
                    self.stack.items[self.stack.items.len - 1] = .{
                        .generic = std.math.shr(usize, b, a),
                    };
                },
                OP.shra => {
                    if (self.stack.items.len < 2) return error.InvalidExpression;
                    const a = try self.stack.pop().?.asIntegral();
                    const b: isize = @bitCast(try self.stack.items[self.stack.items.len - 1].asIntegral());
                    self.stack.items[self.stack.items.len - 1] = .{
                        .generic = @bitCast(std.math.shr(isize, b, a)),
                    };
                },
                OP.xor => {
                    if (self.stack.items.len < 2) return error.InvalidExpression;
                    const a = try self.stack.pop().?.asIntegral();
                    self.stack.items[self.stack.items.len - 1] = .{
                        .generic = a ^ try self.stack.items[self.stack.items.len - 1].asIntegral(),
                    };
                },

                // 2.5.1.5: Control Flow Operations
                OP.le,
                OP.ge,
                OP.eq,
                OP.lt,
                OP.gt,
                OP.ne,
                => {
                    if (self.stack.items.len < 2) return error.InvalidExpression;
                    const a = self.stack.pop().?;
                    const b = self.stack.items[self.stack.items.len - 1];

                    if (a == .generic and b == .generic) {
                        const a_int: isize = @bitCast(a.asIntegral() catch unreachable);
                        const b_int: isize = @bitCast(b.asIntegral() catch unreachable);
                        const result = @intFromBool(switch (opcode) {
                            OP.le => b_int <= a_int,
                            OP.ge => b_int >= a_int,
                            OP.eq => b_int == a_int,
                            OP.lt => b_int < a_int,
                            OP.gt => b_int > a_int,
                            OP.ne => b_int != a_int,
                            else => unreachable,
                        });

                        self.stack.items[self.stack.items.len - 1] = .{ .generic = result };
                    } else {
                        // TODO: Load the types referenced by these values, find their comparison operator, and run it
                        return error.UnimplementedTypedComparison;
                    }
                },
                OP.skip, OP.bra => {
                    const branch_offset = operand.?.branch_offset;
                    const condition = if (opcode == OP.bra) blk: {
                        if (self.stack.items.len == 0) return error.InvalidExpression;
                        break :blk try self.stack.pop().?.asIntegral() != 0;
                    } else true;

                    if (condition) {
                        const new_pos = std.math.cast(
                            usize,
                            try std.math.add(isize, @as(isize, @intCast(stream.pos)), branch_offset),
                        ) orelse return error.InvalidExpression;

                        if (new_pos < 0 or new_pos > stream.buffer.len) return error.InvalidExpression;
                        stream.pos = new_pos;
                    }
                },
                OP.call2,
                OP.call4,
                OP.call_ref,
                => {
                    const debug_info_offset = operand.?.generic;
                    _ = debug_info_offset;

                    // TODO: Load a DIE entry at debug_info_offset in a .debug_info section (the spec says that it
                    //       can be in a separate exe / shared object from the one containing this expression).
                    //       Transfer control to the DW_AT_location attribute, with the current stack as input.

                    return error.UnimplementedExpressionCall;
                },

                // 2.5.1.6: Type Conversions
                OP.convert => {
                    if (self.stack.items.len == 0) return error.InvalidExpression;
                    const type_offset = operand.?.generic;

                    // TODO: Load the DW_TAG_base_type entries in context.compile_unit and verify both types are the same size
                    const value = self.stack.items[self.stack.items.len - 1];
                    if (type_offset == 0) {
                        self.stack.items[self.stack.items.len - 1] = .{ .generic = try value.asIntegral() };
                    } else {
                        // TODO: Load the DW_TAG_base_type entry in context.compile_unit, find a conversion operator
                        //       from the old type to the new type, run it.
                        return error.UnimplementedTypeConversion;
                    }
                },
                OP.reinterpret => {
                    if (self.stack.items.len == 0) return error.InvalidExpression;
                    const type_offset = operand.?.generic;

                    // TODO: Load the DW_TAG_base_type entries in context.compile_unit and verify both types are the same size
                    const value = self.stack.items[self.stack.items.len - 1];
                    if (type_offset == 0) {
                        self.stack.items[self.stack.items.len - 1] = .{ .generic = try value.asIntegral() };
                    } else {
                        self.stack.items[self.stack.items.len - 1] = switch (value) {
                            .generic => |v| .{
                                .regval_type = .{
                                    .type_offset = type_offset,
                                    .type_size = @sizeOf(addr_type),
                                    .value = v,
                                },
                            },
                            .regval_type => |r| .{
                                .regval_type = .{
                                    .type_offset = type_offset,
                                    .type_size = r.type_size,
                                    .value = r.value,
                                },
                            },
                            .const_type => |c| .{
                                .const_type = .{
                                    .type_offset = type_offset,
                                    .value_bytes = c.value_bytes,
                                },
                            },
                        };
                    }
                },

                // 2.5.1.7: Special Operations
                OP.nop => {},
                OP.entry_value => {
                    const block = operand.?.block;
                    if (block.len == 0) return error.InvalidSubExpression;

                    // TODO: The spec states that this sub-expression needs to observe the state (ie. registers)
                    //       as it was upon entering the current subprogram. If this isn't being called at the
                    //       end of a frame unwind operation, an additional ThreadContext with this state will be needed.

                    if (isOpcodeRegisterLocation(block[0])) {
                        if (context.thread_context == null) return error.IncompleteExpressionContext;

                        var block_stream = std.io.fixedBufferStream(block);
                        const register = (try readOperand(&block_stream, block[0], context)).?.register;
                        const value = mem.readInt(usize, (try abi.regBytes(context.thread_context.?, register, context.reg_context))[0..@sizeOf(usize)], native_endian);
                        try self.stack.append(allocator, .{ .generic = value });
                    } else {
                        var stack_machine: Self = .{};
                        defer stack_machine.deinit(allocator);

                        var sub_context = context;
                        sub_context.entry_value_context = true;
                        const result = try stack_machine.run(block, allocator, sub_context, null);
                        try self.stack.append(allocator, result orelse return error.InvalidSubExpression);
                    }
                },

                // These have already been handled by readOperand
                OP.lo_user...OP.hi_user => unreachable,
                else => {
                    //std.debug.print("Unknown DWARF expression opcode: {x}\n", .{opcode});
                    return error.UnknownExpressionOpcode;
                },
            }

            return stream.pos < stream.buffer.len;
        }
    };
}