llvm-project/lldb/examples/python/formatter_bytecode.py

"""
Specification, assembler, disassembler, and interpreter
for LLDB dataformatter bytecode.

See https://lldb.llvm.org/resources/formatterbytecode.html for more details.
"""

from __future__ import annotations

# Work around the fact that one of the local files is called
# types.py, which breaks some versions of python.
import os, sys

path = os.path.abspath(os.path.dirname(__file__))
if path in sys.path:
    sys.path.remove(path)

import re
import io
import textwrap
from dataclasses import dataclass
from typing import BinaryIO, TextIO, Tuple, Union

BINARY_VERSION = 1

# Types
type_String = 1
type_Int = 2
type_UInt = 3
type_Object = 4
type_Type = 5

# Opcodes
opcode = dict()


def define_opcode(n, mnemonic, name):
    globals()["op_" + name] = n
    if mnemonic:
        opcode[mnemonic] = n
    opcode[n] = mnemonic


define_opcode(1, "dup", "dup")
define_opcode(2, "drop", "drop")
define_opcode(3, "pick", "pick")
define_opcode(4, "over", "over")
define_opcode(5, "swap", "swap")
define_opcode(6, "rot", "rot")

define_opcode(0x10, "{", "begin")
define_opcode(0x11, "if", "if")
define_opcode(0x12, "ifelse", "ifelse")
define_opcode(0x13, "return", "return")

define_opcode(0x20, None, "lit_uint")
define_opcode(0x21, None, "lit_int")
define_opcode(0x22, None, "lit_string")
define_opcode(0x23, None, "lit_selector")

define_opcode(0x2A, "as_int", "as_int")
define_opcode(0x2B, "as_uint", "as_uint")
define_opcode(0x2C, "is_null", "is_null")

define_opcode(0x30, "+", "plus")
define_opcode(0x31, "-", "minus")
define_opcode(0x32, "*", "mul")
define_opcode(0x33, "/", "div")
define_opcode(0x34, "%", "mod")
define_opcode(0x35, "<<", "shl")
define_opcode(0x36, ">>", "shr")

define_opcode(0x40, "&", "and")
define_opcode(0x41, "|", "or")
define_opcode(0x42, "^", "xor")
define_opcode(0x43, "~", "not")

define_opcode(0x50, "=", "eq")
define_opcode(0x51, "!=", "neq")
define_opcode(0x52, "<", "lt")
define_opcode(0x53, ">", "gt")
define_opcode(0x54, "=<", "le")
define_opcode(0x55, ">=", "ge")

define_opcode(0x60, "call", "call")

# Function signatures
sig_summary = 0
sig_init = 1
sig_get_num_children = 2
sig_get_child_index = 3
sig_get_child_at_index = 4
sig_update = 5

SIGNATURES = {
    "summary": sig_summary,
    "init": sig_init,
    "get_num_children": sig_get_num_children,
    "get_child_index": sig_get_child_index,
    "get_child_at_index": sig_get_child_at_index,
    "update": sig_update,
}

SIGNATURE_NAMES = "|".join(SIGNATURES.keys())
SIGNATURE_IDS = {v: k for k, v in SIGNATURES.items()}

# Selectors
selector = dict()


def define_selector(n, name):
    globals()["sel_" + name] = n
    selector["@" + name] = n
    selector[n] = "@" + name


define_selector(0, "summary")
define_selector(1, "type_summary")

define_selector(0x10, "get_num_children")
define_selector(0x11, "get_child_at_index")
define_selector(0x12, "get_child_with_name")
define_selector(0x13, "get_child_index")
define_selector(0x15, "get_type")
define_selector(0x16, "get_template_argument_type")
define_selector(0x17, "cast")
define_selector(0x18, "get_synthetic_value")
define_selector(0x19, "get_non_synthetic_value")
define_selector(0x20, "get_value")
define_selector(0x21, "get_value_as_unsigned")
define_selector(0x22, "get_value_as_signed")
define_selector(0x23, "get_value_as_address")

define_selector(0x40, "read_memory_byte")
define_selector(0x41, "read_memory_uint32")
define_selector(0x42, "read_memory_int32")
define_selector(0x43, "read_memory_unsigned")
define_selector(0x44, "read_memory_signed")
define_selector(0x45, "read_memory_address")
define_selector(0x46, "read_memory")

define_selector(0x50, "fmt")
define_selector(0x51, "sprintf")
define_selector(0x52, "strlen")


################################################################################
# Assembler.
################################################################################

_SIGNATURE_LABEL = re.compile(f"@(?:{SIGNATURE_NAMES}):$")


def _tokenize(assembler: str) -> list[str]:
    """Convert string of assembly into tokens."""
    # With one exception, tokens are sequences of non-space characters.
    # The one exception is string literals, which may have spaces.

    # To parse strings, which can contain escaped contents, use a "Friedl
    # unrolled loop". The high level of such a regex is:
    #     open normal* ( special normal* )* close
    # which for string literals is:
    string_literal = r'" [^"\\]* (?: \\. [^"\\]* )* "'

    return re.findall(rf"{string_literal} | \S+", assembler, re.VERBOSE)


def _segment_by_signature(input: list[str]) -> list[Tuple[str, list[str]]]:
    """Segment the input tokens along signature labels."""
    segments = []

    # Loop state
    signature = None
    tokens = []

    def conclude_segment():
        if not tokens:
            raise ValueError(f"empty signature: {signature}")
        segments.append((signature, tokens))

    for token in input:
        if _SIGNATURE_LABEL.match(token):
            if signature:
                conclude_segment()
            signature = token[1:-1]  # strip leading @, trailing :
            tokens = []
        else:
            tokens.append(token)

    if signature:
        conclude_segment()

    return segments


@dataclass
class BytecodeSection:
    """Abstraction of the data serialized to __lldbformatters sections."""

    type_name: str
    flags: int
    signatures: list[Tuple[str, bytes]]

    def validate(self):
        seen = set()
        for sig, _ in self.signatures:
            if sig in seen:
                raise ValueError(f"duplicate signature: {sig}")
            seen.add(sig)

    def _to_binary(self) -> bytes:
        bin = bytearray()
        bin.extend(_to_uleb(len(self.type_name)))
        bin.extend(bytes(self.type_name, encoding="utf-8"))
        bin.extend(_to_byte(self.flags))
        for sig, bc in self.signatures:
            bin.extend(_to_byte(SIGNATURES[sig]))
            bin.extend(_to_uleb(len(bc)))
            bin.extend(bc)

        return bytes(bin)

    def write_binary(self, output: BinaryIO) -> None:
        self.validate()

        bin = self._to_binary()
        output.write(_to_byte(BINARY_VERSION))
        output.write(_to_uleb(len(bin)))
        output.write(self._to_binary())

    class _CBuilder:
        """Helper class for emitting binary data as a C-string literal."""

        entries: list[Tuple[str, str]]

        def __init__(self) -> None:
            self.entries = []

        def add_byte(self, x: int, comment: str) -> None:
            self.add_bytes(_to_byte(x), comment)

        def add_uleb(self, x: int, comment: str) -> None:
            self.add_bytes(_to_uleb(x), comment)

        def add_bytes(self, x: bytes, comment: str) -> None:
            # Construct zero padded hex values with length two.
            string = "".join(f"\\x{b:02x}" for b in x)
            self.add_string(string, comment)

        def add_string(self, string: str, comment: str) -> None:
            self.entries.append((f'"{string}"', comment))

    def write_source(self, output: TextIO) -> None:
        self.validate()

        size = len(self._to_binary())

        b = self._CBuilder()
        b.add_byte(BINARY_VERSION, "version")
        b.add_uleb(size, "remaining record size")
        b.add_uleb(len(self.type_name), "type name size")
        b.add_string(self.type_name, "type name")
        b.add_byte(self.flags, "flags")
        for sig, bc in self.signatures:
            b.add_byte(SIGNATURES[sig], f"sig_{sig}")
            b.add_uleb(len(bc), "program size")
            b.add_bytes(bc, "program")

        print(
            textwrap.dedent(
                """
                #ifdef __APPLE__
                #define FORMATTER_SECTION "__DATA_CONST,__lldbformatters"
                #else
                #define FORMATTER_SECTION ".lldbformatters"
                #endif
                """
            ),
            file=output,
        )
        var_name = re.sub(r"\W", "_", self.type_name)
        print(
            "__attribute__((used, section(FORMATTER_SECTION)))",
            file=output,
        )
        print(f"unsigned char _{var_name}_synthetic[] =", file=output)
        indent = "    "
        for string, comment in b.entries:
            print(f"{indent}// {comment}", file=output)
            print(f"{indent}{string}", file=output)
        print(";", file=output)


def assemble_file(type_name: str, input: TextIO) -> BytecodeSection:
    input_tokens = _tokenize(input.read())
    signatures = []
    for sig, tokens in _segment_by_signature(input_tokens):
        signatures.append((sig, assemble_tokens(tokens)))

    return BytecodeSection(type_name, flags=0, signatures=signatures)


def assemble(assembly: str) -> bytes:
    return assemble_tokens(_tokenize(assembly))


def assemble_tokens(tokens: list[str]) -> bytes:
    """Assemble assembly into bytecode"""
    # This is a stack of all in-flight/unterminated blocks.
    bytecode = [bytearray()]

    def emit(byte):
        bytecode[-1].append(byte)

    tokens.reverse()
    while tokens:
        tok = tokens.pop()
        if tok == "":
            pass
        elif tok == "{":
            bytecode.append(bytearray())
        elif tok == "}":
            block = bytecode.pop()
            emit(op_begin)
            emit(len(block))  # FIXME: uleb
            bytecode[-1].extend(block)
        elif tok[0].isdigit():
            if tok[-1] == "u":
                emit(op_lit_uint)
                emit(int(tok[:-1]))  # FIXME
            else:
                emit(op_lit_int)
                emit(int(tok))  # FIXME
        elif tok[0] == "@":
            emit(op_lit_selector)
            emit(selector[tok])
        elif tok[0] == '"':
            # Remove backslash escaping '"' and '\'.
            s = re.sub(r'\\(["\\])', r"\1", tok[1:-1]).encode()
            emit(op_lit_string)
            emit(len(s))
            bytecode[-1].extend(s)
        else:
            emit(opcode[tok])
    assert len(bytecode) == 1  # unterminated {
    return bytes(bytecode[0])


################################################################################
# Disassembler.
################################################################################


def disassemble_file(input: BinaryIO, output: TextIO) -> None:
    stream = io.BytesIO(input.read())

    version = stream.read(1)[0]
    if version != BINARY_VERSION:
        raise ValueError(f"unknown binary version: {version}")

    record_size = _from_uleb(stream)
    stream.truncate(stream.tell() + record_size)

    name_size = _from_uleb(stream)
    _type_name = stream.read(name_size).decode()
    _flags = stream.read(1)[0]

    while True:
        sig_byte = stream.read(1)
        if not sig_byte:
            break
        sig_name = SIGNATURE_IDS[sig_byte[0]]
        body_size = _from_uleb(stream)
        bc = stream.read(body_size)
        asm, _ = disassemble(bc)
        print(f"@{sig_name}: {asm}", file=output)


def disassemble(bytecode: bytes) -> Tuple[str, list[int]]:
    """Disassemble bytecode into (assembly, token starts)"""
    asm = ""
    all_bytes = list(bytecode)
    all_bytes.reverse()
    blocks = []
    tokens = [0]

    def next_byte():
        """Fetch the next byte in the bytecode and keep track of all
        in-flight blocks"""
        for i in range(len(blocks)):
            blocks[i] -= 1
        tokens.append(len(asm))
        return all_bytes.pop()

    while all_bytes:
        b = next_byte()
        if b == op_begin:
            asm += "{"
            length = next_byte()
            blocks.append(length)
        elif b == op_lit_uint:
            b = next_byte()
            asm += str(b)  # FIXME uleb
            asm += "u"
        elif b == op_lit_int:
            b = next_byte()
            asm += str(b)
        elif b == op_lit_selector:
            b = next_byte()
            asm += selector[b]
        elif b == op_lit_string:
            length = next_byte()
            s = '"'
            for _ in range(length):
                c = chr(next_byte())
                if c in ('"', "\\"):
                    s += "\\"
                s += c
            s += '"'
            asm += s
        else:
            asm += opcode[b]

        while blocks and blocks[-1] == 0:
            asm += " }"
            blocks.pop()

        if all_bytes:
            asm += " "

    if blocks:
        asm += "ERROR"
    return asm, tokens


################################################################################
# Interpreter.
################################################################################


def count_fmt_params(fmt: str) -> int:
    """Count the number of parameters in a format string"""
    from string import Formatter

    f = Formatter()
    n = 0
    for _, name, _, _ in f.parse(fmt):
        if name > n:
            n = name
    return n


def interpret(bytecode: bytes, control: list, data: list, tracing: bool = False):
    """Interpret bytecode"""
    frame = []
    frame.append((0, len(bytecode)))

    def trace():
        """print a trace of the execution for debugging purposes"""

        def fmt(d):
            if isinstance(d, int):
                return str(d)
            if isinstance(d, str):
                return d
            return repr(type(d))

        pc, end = frame[-1]
        asm, tokens = disassemble(bytecode)
        print(
            "=== frame = {1}, data = {2}, opcode = {0}".format(
                opcode[b], frame, [fmt(d) for d in data]
            )
        )
        print(asm)
        print(" " * (tokens[pc]) + "^")

    def next_byte():
        """Fetch the next byte and update the PC"""
        pc, end = frame[-1]
        assert pc < len(bytecode)
        b = bytecode[pc]
        frame[-1] = pc + 1, end
        # At the end of a block?
        while pc >= end:
            frame.pop()
            if not frame:
                return None
            pc, end = frame[-1]
            if pc >= end:
                return None
            b = bytecode[pc]
            frame[-1] = pc + 1, end
        return b

    while frame[-1][0] < len(bytecode):
        b = next_byte()
        if b == None:
            break
        if tracing:
            trace()
        # Data stack manipulation.
        if b == op_dup:
            data.append(data[-1])
        elif b == op_drop:
            data.pop()
        elif b == op_pick:
            data.append(data[data.pop()])
        elif b == op_over:
            data.append(data[-2])
        elif b == op_swap:
            x = data.pop()
            y = data.pop()
            data.append(x)
            data.append(y)
        elif b == op_rot:
            z = data.pop()
            y = data.pop()
            x = data.pop()
            data.append(z)
            data.append(x)
            data.append(y)

        # Control stack manipulation.
        elif b == op_begin:
            length = next_byte()
            pc, end = frame[-1]
            control.append((pc, pc + length))
            frame[-1] = pc + length, end
        elif b == op_if:
            if data.pop():
                frame.append(control.pop())
        elif b == op_ifelse:
            if data.pop():
                control.pop()
                frame.append(control.pop())
            else:
                frame.append(control.pop())
                control.pop()
        elif b == op_return:
            control.clear()
            return data[-1]

        # Literals.
        elif b == op_lit_uint:
            b = next_byte()  # FIXME uleb
            data.append(int(b))
        elif b == op_lit_int:
            b = next_byte()  # FIXME uleb
            data.append(int(b))
        elif b == op_lit_selector:
            b = next_byte()
            data.append(b)
        elif b == op_lit_string:
            length = next_byte()
            s = ""
            while length:
                s += chr(next_byte())
                length -= 1
            data.append(s)

        elif b == op_as_uint:
            pass
        elif b == op_as_int:
            pass
        elif b == op_is_null:
            data.append(1 if data.pop() == None else 0)

        # Arithmetic, logic, etc.
        elif b == op_plus:
            data.append(data.pop() + data.pop())
        elif b == op_minus:
            data.append(-data.pop() + data.pop())
        elif b == op_mul:
            data.append(data.pop() * data.pop())
        elif b == op_div:
            y = data.pop()
            data.append(data.pop() / y)
        elif b == op_mod:
            y = data.pop()
            data.append(data.pop() % y)
        elif b == op_shl:
            y = data.pop()
            data.append(data.pop() << y)
        elif b == op_shr:
            y = data.pop()
            data.append(data.pop() >> y)
        elif b == op_and:
            data.append(data.pop() & data.pop())
        elif b == op_or:
            data.append(data.pop() | data.pop())
        elif b == op_xor:
            data.append(data.pop() ^ data.pop())
        elif b == op_not:
            data.append(not data.pop())
        elif b == op_eq:
            data.append(data.pop() == data.pop())
        elif b == op_neq:
            data.append(data.pop() != data.pop())
        elif b == op_lt:
            data.append(data.pop() > data.pop())
        elif b == op_gt:
            data.append(data.pop() < data.pop())
        elif b == op_le:
            data.append(data.pop() >= data.pop())
        elif b == op_ge:
            data.append(data.pop() <= data.pop())

        # Function calls.
        elif b == op_call:
            sel = data.pop()
            if sel == sel_summary:
                data.append(data.pop().GetSummary())
            elif sel == sel_get_num_children:
                data.append(data.pop().GetNumChildren())
            elif sel == sel_get_child_at_index:
                index = data.pop()
                valobj = data.pop()
                data.append(valobj.GetChildAtIndex(index))
            elif sel == sel_get_child_with_name:
                name = data.pop()
                valobj = data.pop()
                data.append(valobj.GetChildMemberWithName(name))
            elif sel == sel_get_child_index:
                name = data.pop()
                valobj = data.pop()
                data.append(valobj.GetIndexOfChildWithName(name))
            elif sel == sel_get_type:
                data.append(data.pop().GetType())
            elif sel == sel_get_template_argument_type:
                n = data.pop()
                valobj = data.pop()
                data.append(valobj.GetTemplateArgumentType(n))
            elif sel == sel_get_synthetic_value:
                data.append(data.pop().GetSyntheticValue())
            elif sel == sel_get_non_synthetic_value:
                data.append(data.pop().GetNonSyntheticValue())
            elif sel == sel_get_value:
                data.append(data.pop().GetValue())
            elif sel == sel_get_value_as_unsigned:
                data.append(data.pop().GetValueAsUnsigned())
            elif sel == sel_get_value_as_signed:
                data.append(data.pop().GetValueAsSigned())
            elif sel == sel_get_value_as_address:
                data.append(data.pop().GetValueAsAddress())
            elif sel == sel_cast:
                sbtype = data.pop()
                valobj = data.pop()
                data.append(valobj.Cast(sbtype))
            elif sel == sel_strlen:
                s = data.pop()
                data.append(len(s) if s else 0)
            elif sel == sel_fmt:
                fmt = data.pop()
                n = count_fmt_params(fmt)
                args = []
                for i in range(n):
                    args.append(data.pop())
                data.append(fmt.format(*args))
            else:
                print("not implemented: " + selector[sel])
                assert False
    return data[-1]


################################################################################
# Helper functions.
################################################################################


def _to_uleb(value: int) -> bytes:
    """Encode an integer to ULEB128 bytes."""
    if value < 0:
        raise ValueError(f"negative number cannot be encoded to ULEB128: {value}")

    result = bytearray()
    while True:
        byte = value & 0x7F
        value >>= 7
        if value != 0:
            byte |= 0x80
        result.append(byte)
        if value == 0:
            break

    return bytes(result)


def _from_uleb(stream: BinaryIO) -> int:
    """Decode a ULEB128 integer by reading bytes from the stream."""
    result = 0
    shift = 0
    while True:
        byte = stream.read(1)[0]
        result |= (byte & 0x7F) << shift
        shift += 7
        if not (byte & 0x80):
            break

    return result


def _to_byte(n: int) -> bytes:
    return n.to_bytes(1, "big")


def _main():
    import argparse

    parser = argparse.ArgumentParser(
        description="""
    Assembler, disassembler, and interpreter for LLDB dataformatter bytecode.
    See https://lldb.llvm.org/resources/formatterbytecode.html for more details.
    """
    )
    parser.add_argument("input", help="input file")
    mode = parser.add_mutually_exclusive_group()
    mode.add_argument(
        "-c",
        "--assemble",
        action="store_true",
        help="assemble assembly into bytecode",
    )
    mode.add_argument(
        "-d",
        "--disassemble",
        action="store_true",
        help="disassemble bytecode",
    )
    parser.add_argument(
        "-o",
        "--output",
        help="output file (required for --assemble)",
    )
    parser.add_argument(
        "-f",
        "--format",
        choices=("binary", "c"),
        default="binary",
        help="output file format",
    )
    parser.add_argument("-t", "--test", action="store_true", help="run unit tests")

    args = parser.parse_args()
    if args.assemble:
        if not args.output:
            parser.error("--output is required with --assemble")
        with open(args.input) as input:
            section = assemble_file(args.type_name, input)
        if args.format == "binary":
            with open(args.output, "wb") as output:
                section.write_binary(output)
        else:  # args.format == "c"
            with open(args.output, "w") as output:
                section.write_source(output)
    elif args.disassemble:
        if args.output:
            with (
                open(args.input, "rb") as input,
                open(args.output, "w") as output,
            ):
                disassemble_file(input, output)
        else:
            with open(args.input, "rb") as input:
                disassemble_file(input, sys.stdout)


if __name__ == "__main__":
    if not ("-t" in sys.argv or "--test" in sys.argv):
        _main()
        sys.exit()

    ############################################################################
    # Tests.
    ############################################################################
    import unittest

    class TestAssembler(unittest.TestCase):

        def test_assemble(self):
            self.assertEqual(assemble("1u dup").hex(), "200101")
            self.assertEqual(assemble('"1u dup"').hex(), "2206317520647570")
            self.assertEqual(assemble("16 < { dup } if").hex(), "21105210010111")
            self.assertEqual(assemble('{ { " } " } }').hex(), "100710052203207d20")

            def roundtrip(asm):
                self.assertEqual(disassemble(assemble(asm))[0], asm)

            roundtrip("1u dup")
            roundtrip("16 < { dup } if")
            roundtrip('{ { " } " } }')

            # String specific checks.
            roundtrip('1u "2u 3u"')
            roundtrip('"a  b"')
            roundtrip('"a \\" b"')

            self.assertEqual(interpret(assemble("1 1 +"), [], []), 2)
            self.assertEqual(interpret(assemble("2 1 1 + *"), [], []), 4)
            self.assertEqual(
                interpret(assemble('2 1 > { "yes" } { "no" } ifelse'), [], []), "yes"
            )

        def test_assemble_file(self):
            def run_assemble(type_name, asm):
                out = io.BytesIO()
                section = assemble_file(type_name, io.StringIO(asm))
                section.write_binary(out)
                out.seek(0)
                return out

            def run_disassemble(binary):
                out = io.StringIO()
                disassemble_file(binary, out)
                out.seek(0)
                return out

            # assemble -> disassemble -> assemble round-trip: binary is identical.
            asm = "@summary: dup @get_value_as_unsigned call return\n@get_num_children: drop 5u return"
            binary1 = run_assemble("MyType", asm)
            dis = run_disassemble(binary1)
            binary2 = run_assemble("MyType", dis.read())
            self.assertEqual(binary1.getvalue(), binary2.getvalue())

            # disassemble -> assemble -> disassemble round-trip: text is identical.
            dis2 = run_disassemble(binary2)
            self.assertEqual(dis.getvalue(), dis2.getvalue())

            # disassemble output contains expected signatures.
            self.assertIn("@summary:", dis.getvalue())
            self.assertIn("@get_num_children:", dis.getvalue())

            # Duplicate signature is an error.
            with self.assertRaises(ValueError):
                run_assemble("MyType", "@summary: 1u return\n@summary: 2u return")

        def test_write_source(self):
            # Use the Account example from main.cpp as a reference, whose
            # exact byte values are known.
            section = BytecodeSection(
                type_name="Account",
                flags=0,
                signatures=[
                    ("get_num_children", bytes([0x20, 0x01])),
                    ("get_child_at_index", bytes([0x02, 0x20, 0x00, 0x23, 0x11, 0x60])),
                ],
            )
            out = io.StringIO()
            section.write_source(out)
            src = out.getvalue()

            self.assertIn("__attribute__((used, section(FORMATTER_SECTION)))", src)
            self.assertIn("unsigned char _Account_synthetic[] =", src)
            self.assertIn('"\\x01"', src)  # version
            self.assertIn('"\\x15"', src)  # record size (21)
            self.assertIn('"\\x07"', src)  # type name size (7)
            self.assertIn('"Account"', src)  # type name
            self.assertIn('"\\x00"', src)  # flags
            self.assertIn('"\\x02"', src)  # sig_get_num_children
            self.assertIn('"\\x20\\x01"', src)  # program
            self.assertIn('"\\x04"', src)  # sig_get_child_at_index
            self.assertIn('"\\x06"', src)  # program size
            self.assertIn('"\\x02\\x20\\x00\\x23\\x11\\x60"', src)  # program
            self.assertIn("// version", src)
            self.assertIn("// type name", src)
            self.assertIn("// program", src)
            # Semicolon terminates the array initializer.
            self.assertEqual(src.count(";"), 1)

            # Non-identifier characters in the type name are replaced with '_'.
            out2 = io.StringIO()
            BytecodeSection("std::vector<int>", 0, []).write_source(out2)
            self.assertIn("_std__vector_int__synthetic[] =", out2.getvalue())

    unittest.main(argv=[__file__])