Python/assemble.c

#include "Python.h"
#include "pycore_code.h"            // write_location_entry_start()
#include "pycore_compile.h"
#include "pycore_instruction_sequence.h"
#include "pycore_opcode_utils.h"    // IS_BACKWARDS_JUMP_OPCODE
#include "pycore_opcode_metadata.h" // is_pseudo_target, _PyOpcode_Caches
#include "pycore_symtable.h"        // _Py_SourceLocation

#include <stdbool.h>

#define DEFAULT_CODE_SIZE 128
#define DEFAULT_LNOTAB_SIZE 16
#define DEFAULT_CNOTAB_SIZE 32

#undef SUCCESS
#undef ERROR
#define SUCCESS 0
#define ERROR -1

#define RETURN_IF_ERROR(X)  \
    if ((X) < 0) {          \
        return ERROR;       \
    }

typedef _Py_SourceLocation location;
typedef _PyInstruction instruction;
typedef _PyInstructionSequence instr_sequence;

static inline bool
same_location(location a, location b)
{
    return a.lineno == b.lineno &&
           a.end_lineno == b.end_lineno &&
           a.col_offset == b.col_offset &&
           a.end_col_offset == b.end_col_offset;
}

static int
instr_size(instruction *instr)
{
    int opcode = instr->i_opcode;
    int oparg = instr->i_oparg;
    assert(!IS_PSEUDO_INSTR(opcode));
    assert(OPCODE_HAS_ARG(opcode) || oparg == 0);
    int extended_args = (0xFFFFFF < oparg) + (0xFFFF < oparg) + (0xFF < oparg);
    int caches = _PyOpcode_Caches[opcode];
    return extended_args + 1 + caches;
}

struct assembler {
    PyBytesWriter *a_bytecode_writer; /* writer containing bytecode */
    PyObject *a_bytecode;      /* bytes containing bytecode */
    int a_offset;              /* offset into bytecode */
    PyBytesWriter *a_except_table_writer; /* writer containing exception table */
    PyObject *a_except_table;  /* bytes containing exception table */
    int a_except_table_off;    /* offset into exception table */
    /* Location Info */
    int a_lineno;          /* lineno of last emitted instruction */
    PyBytesWriter *a_linetable_writer; /* writer containing location info */
    PyObject *a_linetable; /* bytes object containing location info */
    int a_location_off;    /* offset of last written location info frame */
};

static int
assemble_init(struct assembler *a, int firstlineno)
{
    memset(a, 0, sizeof(struct assembler));
    a->a_lineno = firstlineno;
    a->a_bytecode_writer = PyBytesWriter_Create(DEFAULT_CODE_SIZE);
    if (a->a_bytecode_writer == NULL) {
        goto error;
    }
    a->a_linetable_writer = PyBytesWriter_Create(DEFAULT_CNOTAB_SIZE);
    if (a->a_linetable_writer == NULL) {
        goto error;
    }
    a->a_except_table_writer = PyBytesWriter_Create(DEFAULT_LNOTAB_SIZE);
    if (a->a_except_table_writer == NULL) {
        goto error;
    }
    return SUCCESS;
error:
    PyBytesWriter_Discard(a->a_bytecode_writer);
    a->a_bytecode_writer = NULL;
    PyBytesWriter_Discard(a->a_linetable_writer);
    a->a_linetable_writer = NULL;
    PyBytesWriter_Discard(a->a_except_table_writer);
    a->a_except_table_writer = NULL;
    return ERROR;
}

static void
assemble_free(struct assembler *a)
{
    PyBytesWriter_Discard(a->a_bytecode_writer);
    PyBytesWriter_Discard(a->a_linetable_writer);
    PyBytesWriter_Discard(a->a_except_table_writer);
    Py_XDECREF(a->a_bytecode);
    Py_XDECREF(a->a_linetable);
    Py_XDECREF(a->a_except_table);
}

static inline void
write_except_byte(struct assembler *a, int byte) {
    unsigned char *p = PyBytesWriter_GetData(a->a_except_table_writer);
    p[a->a_except_table_off++] = byte;
}

#define CONTINUATION_BIT 64

static void
assemble_emit_exception_table_item(struct assembler *a, int value, int msb)
{
    assert ((msb | 128) == 128);
    assert(value >= 0 && value < (1 << 30));
    if (value >= 1 << 24) {
        write_except_byte(a, (value >> 24) | CONTINUATION_BIT | msb);
        msb = 0;
    }
    if (value >= 1 << 18) {
        write_except_byte(a, ((value >> 18)&0x3f) | CONTINUATION_BIT | msb);
        msb = 0;
    }
    if (value >= 1 << 12) {
        write_except_byte(a, ((value >> 12)&0x3f) | CONTINUATION_BIT | msb);
        msb = 0;
    }
    if (value >= 1 << 6) {
        write_except_byte(a, ((value >> 6)&0x3f) | CONTINUATION_BIT | msb);
        msb = 0;
    }
    write_except_byte(a, (value&0x3f) | msb);
}

/* See InternalDocs/exception_handling.md for details of layout */
#define MAX_SIZE_OF_ENTRY 20

static int
assemble_emit_exception_table_entry(struct assembler *a, int start, int end,
                                    int handler_offset,
                                    _PyExceptHandlerInfo *handler)
{
    Py_ssize_t len = PyBytesWriter_GetSize(a->a_except_table_writer);
    if (a->a_except_table_off + MAX_SIZE_OF_ENTRY >= len) {
        RETURN_IF_ERROR(PyBytesWriter_Resize(a->a_except_table_writer, len * 2));
    }
    int size = end-start;
    assert(end > start);
    int target = handler_offset;
    int depth = handler->h_startdepth - 1;
    if (handler->h_preserve_lasti > 0) {
        depth -= 1;
    }
    assert(depth >= 0);
    int depth_lasti = (depth<<1) | handler->h_preserve_lasti;
    assemble_emit_exception_table_item(a, start, (1<<7));
    assemble_emit_exception_table_item(a, size, 0);
    assemble_emit_exception_table_item(a, target, 0);
    assemble_emit_exception_table_item(a, depth_lasti, 0);
    return SUCCESS;
}

static int
assemble_exception_table(struct assembler *a, instr_sequence *instrs)
{
    int ioffset = 0;
    _PyExceptHandlerInfo handler;
    handler.h_label = -1;
    handler.h_startdepth = -1;
    handler.h_preserve_lasti = -1;
    int start = -1;
    for (int i = 0; i < instrs->s_used; i++) {
        instruction *instr = &instrs->s_instrs[i];
        if (instr->i_except_handler_info.h_label != handler.h_label) {
            if (handler.h_label >= 0) {
                int handler_offset = instrs->s_instrs[handler.h_label].i_offset;
                RETURN_IF_ERROR(
                    assemble_emit_exception_table_entry(a, start, ioffset,
                                                        handler_offset,
                                                        &handler));
            }
            start = ioffset;
            handler = instr->i_except_handler_info;
        }
        ioffset += instr_size(instr);
    }
    if (handler.h_label >= 0) {
        int handler_offset = instrs->s_instrs[handler.h_label].i_offset;
        RETURN_IF_ERROR(assemble_emit_exception_table_entry(a, start, ioffset,
                                                            handler_offset,
                                                            &handler));
    }
    return SUCCESS;
}


/* Code location emitting code. See locations.md for a description of the format. */

#define MSB 0x80

static void
write_location_byte(struct assembler* a, int val)
{
    uint8_t *linetable = PyBytesWriter_GetData(a->a_linetable_writer);
    linetable[a->a_location_off] = val & 255;
    a->a_location_off++;
}


static uint8_t *
location_pointer(struct assembler* a)
{
    uint8_t *linetable = PyBytesWriter_GetData(a->a_linetable_writer);
    return linetable + a->a_location_off;
}

static void
write_location_first_byte(struct assembler* a, int code, int length)
{
    a->a_location_off += write_location_entry_start(
        location_pointer(a), code, length);
}

static void
write_location_varint(struct assembler* a, unsigned int val)
{
    uint8_t *ptr = location_pointer(a);
    a->a_location_off += write_varint(ptr, val);
}


static void
write_location_signed_varint(struct assembler* a, int val)
{
    uint8_t *ptr = location_pointer(a);
    a->a_location_off += write_signed_varint(ptr, val);
}

static void
write_location_info_short_form(struct assembler* a, int length, int column, int end_column)
{
    assert(length > 0 &&  length <= 8);
    int column_low_bits = column & 7;
    int column_group = column >> 3;
    assert(column < 80);
    assert(end_column >= column);
    assert(end_column - column < 16);
    write_location_first_byte(a, PY_CODE_LOCATION_INFO_SHORT0 + column_group, length);
    write_location_byte(a, (column_low_bits << 4) | (end_column - column));
}

static void
write_location_info_oneline_form(struct assembler* a, int length, int line_delta, int column, int end_column)
{
    assert(length > 0 &&  length <= 8);
    assert(line_delta >= 0 && line_delta < 3);
    assert(column < 128);
    assert(end_column < 128);
    write_location_first_byte(a, PY_CODE_LOCATION_INFO_ONE_LINE0 + line_delta, length);
    write_location_byte(a, column);
    write_location_byte(a, end_column);
}

static void
write_location_info_long_form(struct assembler* a, location loc, int length)
{
    assert(length > 0 &&  length <= 8);
    write_location_first_byte(a, PY_CODE_LOCATION_INFO_LONG, length);
    write_location_signed_varint(a, loc.lineno - a->a_lineno);
    assert(loc.end_lineno >= loc.lineno);
    write_location_varint(a, loc.end_lineno - loc.lineno);
    write_location_varint(a, loc.col_offset + 1);
    write_location_varint(a, loc.end_col_offset + 1);
}

static void
write_location_info_none(struct assembler* a, int length)
{
    write_location_first_byte(a, PY_CODE_LOCATION_INFO_NONE, length);
}

static void
write_location_info_no_column(struct assembler* a, int length, int line_delta)
{
    write_location_first_byte(a, PY_CODE_LOCATION_INFO_NO_COLUMNS, length);
    write_location_signed_varint(a, line_delta);
}

#define THEORETICAL_MAX_ENTRY_SIZE 25 /* 1 + 6 + 6 + 6 + 6 */


static int
write_location_info_entry(struct assembler* a, location loc, int isize)
{
    Py_ssize_t len = PyBytesWriter_GetSize(a->a_linetable_writer);
    if (a->a_location_off + THEORETICAL_MAX_ENTRY_SIZE >= len) {
        assert(len > THEORETICAL_MAX_ENTRY_SIZE);
        RETURN_IF_ERROR(PyBytesWriter_Resize(a->a_linetable_writer, len * 2));
    }
    if (loc.lineno == NO_LOCATION.lineno) {
        write_location_info_none(a, isize);
        return SUCCESS;
    }
    int line_delta = loc.lineno - a->a_lineno;
    int column = loc.col_offset;
    int end_column = loc.end_col_offset;
    if (column < 0 || end_column < 0) {
        if (loc.end_lineno == loc.lineno || loc.end_lineno < 0) {
            write_location_info_no_column(a, isize, line_delta);
            a->a_lineno = loc.lineno;
            return SUCCESS;
        }
    }
    else if (loc.end_lineno == loc.lineno) {
        if (line_delta == 0 && column < 80 && end_column - column < 16 && end_column >= column) {
            write_location_info_short_form(a, isize, column, end_column);
            return SUCCESS;
        }
        if (line_delta >= 0 && line_delta < 3 && column < 128 && end_column < 128) {
            write_location_info_oneline_form(a, isize, line_delta, column, end_column);
            a->a_lineno = loc.lineno;
            return SUCCESS;
        }
    }
    write_location_info_long_form(a, loc, isize);
    a->a_lineno = loc.lineno;
    return SUCCESS;
}

static int
assemble_emit_location(struct assembler* a, location loc, int isize)
{
    if (isize == 0) {
        return SUCCESS;
    }
    while (isize > 8) {
        RETURN_IF_ERROR(write_location_info_entry(a, loc, 8));
        isize -= 8;
    }
    return write_location_info_entry(a, loc, isize);
}

static int
assemble_location_info(struct assembler *a, instr_sequence *instrs,
                       int firstlineno)
{
    a->a_lineno = firstlineno;
    location loc = NO_LOCATION;
    for (int i = instrs->s_used-1; i >= 0; i--) {
        instruction *instr = &instrs->s_instrs[i];
        if (same_location(instr->i_loc, NEXT_LOCATION)) {
            if (IS_TERMINATOR_OPCODE(instr->i_opcode)) {
                instr->i_loc = NO_LOCATION;
            }
            else {
                assert(i < instrs->s_used-1);
                instr->i_loc = instr[1].i_loc;
            }
        }
    }
    int size = 0;
    for (int i = 0; i < instrs->s_used; i++) {
        instruction *instr = &instrs->s_instrs[i];
        if (!same_location(loc, instr->i_loc)) {
                RETURN_IF_ERROR(assemble_emit_location(a, loc, size));
                loc = instr->i_loc;
                size = 0;
        }
        size += instr_size(instr);
    }
    RETURN_IF_ERROR(assemble_emit_location(a, loc, size));
    return SUCCESS;
}

static void
write_instr(_Py_CODEUNIT *codestr, instruction *instr, int ilen)
{
    int opcode = instr->i_opcode;
    assert(!IS_PSEUDO_INSTR(opcode));
    int oparg = instr->i_oparg;
    assert(OPCODE_HAS_ARG(opcode) || oparg == 0);
    int caches = _PyOpcode_Caches[opcode];
    switch (ilen - caches) {
        case 4:
            codestr->op.code = EXTENDED_ARG;
            codestr->op.arg = (oparg >> 24) & 0xFF;
            codestr++;
            _Py_FALLTHROUGH;
        case 3:
            codestr->op.code = EXTENDED_ARG;
            codestr->op.arg = (oparg >> 16) & 0xFF;
            codestr++;
            _Py_FALLTHROUGH;
        case 2:
            codestr->op.code = EXTENDED_ARG;
            codestr->op.arg = (oparg >> 8) & 0xFF;
            codestr++;
            _Py_FALLTHROUGH;
        case 1:
            codestr->op.code = opcode;
            codestr->op.arg = oparg & 0xFF;
            codestr++;
            break;
        default:
            Py_UNREACHABLE();
    }
    while (caches--) {
        codestr->op.code = CACHE;
        codestr->op.arg = 0;
        codestr++;
    }
}

/* assemble_emit_instr()
   Extend the bytecode with a new instruction.
   Update lnotab if necessary.
*/

static int
assemble_emit_instr(struct assembler *a, instruction *instr)
{
    Py_ssize_t len = PyBytesWriter_GetSize(a->a_bytecode_writer);
    _Py_CODEUNIT *code;

    int size = instr_size(instr);
    if (a->a_offset + size >= len / (int)sizeof(_Py_CODEUNIT)) {
        if (len > PY_SSIZE_T_MAX / 2) {
            PyErr_NoMemory();
            return ERROR;
        }
        RETURN_IF_ERROR(PyBytesWriter_Resize(a->a_bytecode_writer, len * 2));
    }
    code = (_Py_CODEUNIT *)PyBytesWriter_GetData(a->a_bytecode_writer) + a->a_offset;
    a->a_offset += size;
    write_instr(code, instr, size);
    return SUCCESS;
}

static int
assemble_emit(struct assembler *a, instr_sequence *instrs,
              int first_lineno, PyObject *const_cache)
{
    RETURN_IF_ERROR(assemble_init(a, first_lineno));

    for (int i = 0; i < instrs->s_used; i++) {
        instruction *instr = &instrs->s_instrs[i];
        RETURN_IF_ERROR(assemble_emit_instr(a, instr));
    }

    RETURN_IF_ERROR(assemble_location_info(a, instrs, a->a_lineno));

    RETURN_IF_ERROR(assemble_exception_table(a, instrs));

    a->a_except_table = PyBytesWriter_FinishWithSize(a->a_except_table_writer,
                                                     a->a_except_table_off);
    a->a_except_table_writer = NULL;
    if (a->a_except_table == NULL) {
        return ERROR;
    }
    RETURN_IF_ERROR(_PyCompile_ConstCacheMergeOne(const_cache, &a->a_except_table));

    a->a_linetable = PyBytesWriter_FinishWithSize(a->a_linetable_writer,
                                                  a->a_location_off);
    a->a_linetable_writer = NULL;
    if (a->a_linetable == NULL) {
        return ERROR;
    }
    RETURN_IF_ERROR(_PyCompile_ConstCacheMergeOne(const_cache, &a->a_linetable));

    a->a_bytecode = PyBytesWriter_FinishWithSize(a->a_bytecode_writer,
                                                 a->a_offset * sizeof(_Py_CODEUNIT));
    a->a_bytecode_writer = NULL;
    if (a->a_bytecode == NULL) {
        return ERROR;
    }
    RETURN_IF_ERROR(_PyCompile_ConstCacheMergeOne(const_cache, &a->a_bytecode));
    return SUCCESS;
}

static PyObject *
dict_keys_inorder(PyObject *dict, Py_ssize_t offset)
{
    PyObject *tuple, *k, *v;
    Py_ssize_t pos = 0, size = PyDict_GET_SIZE(dict);

    tuple = PyTuple_New(size);
    if (tuple == NULL)
        return NULL;
    while (PyDict_Next(dict, &pos, &k, &v)) {
        Py_ssize_t i = PyLong_AsSsize_t(v);
        if (i == -1 && PyErr_Occurred()) {
            Py_DECREF(tuple);
            return NULL;
        }
        assert((i - offset) < size);
        assert((i - offset) >= 0);
        PyTuple_SET_ITEM(tuple, i - offset, Py_NewRef(k));
    }
    return tuple;
}

// This is in codeobject.c.
extern void _Py_set_localsplus_info(int, PyObject *, unsigned char,
                                   PyObject *, PyObject *);

static int
compute_localsplus_info(_PyCompile_CodeUnitMetadata *umd, int nlocalsplus,
                        int flags, PyObject *names, PyObject *kinds)
{
    PyObject *k, *v;
    Py_ssize_t pos = 0;

    // Set the locals kinds.  Arg vars fill the first portion of the list.
    struct {
        int count;
        _PyLocals_Kind kind;
    }  argvarkinds[6] = {
        {(int)umd->u_posonlyargcount, CO_FAST_ARG_POS},
        {(int)umd->u_argcount, CO_FAST_ARG_POS | CO_FAST_ARG_KW},
        {(int)umd->u_kwonlyargcount, CO_FAST_ARG_KW},
        {!!(flags & CO_VARARGS), CO_FAST_ARG_VAR | CO_FAST_ARG_POS},
        {!!(flags & CO_VARKEYWORDS), CO_FAST_ARG_VAR | CO_FAST_ARG_KW},
        {-1, 0},  // the remaining local vars
    };
    int max = 0;
    for (int i = 0; i < 6; i++) {
        max = argvarkinds[i].count < 0
            ? INT_MAX
            : max + argvarkinds[i].count;
        while (pos < max && PyDict_Next(umd->u_varnames, &pos, &k, &v)) {
            int offset = PyLong_AsInt(v);
            if (offset == -1 && PyErr_Occurred()) {
                return ERROR;
            }
            assert(offset >= 0);
            assert(offset < nlocalsplus);

            _PyLocals_Kind kind = CO_FAST_LOCAL | argvarkinds[i].kind;

            if (umd->u_fasthidden != NULL) {
                int hidden = PySet_Contains(umd->u_fasthidden, k);
                RETURN_IF_ERROR(hidden);
                if (hidden) {
                    kind |= CO_FAST_HIDDEN;
                }
            }

            int has_key = PyDict_Contains(umd->u_cellvars, k);
            RETURN_IF_ERROR(has_key);
            if (has_key) {
                kind |= CO_FAST_CELL;
            }

            _Py_set_localsplus_info(offset, k, kind, names, kinds);
        }
    }
    int nlocals = (int)PyDict_GET_SIZE(umd->u_varnames);

    // This counter mirrors the fix done in fix_cell_offsets().
    int numdropped = 0, cellvar_offset = -1;
    pos = 0;
    while (PyDict_Next(umd->u_cellvars, &pos, &k, &v)) {
        int has_name = PyDict_Contains(umd->u_varnames, k);
        RETURN_IF_ERROR(has_name);
        if (has_name) {
            // Skip cells that are already covered by locals.
            numdropped += 1;
            continue;
        }

        cellvar_offset = PyLong_AsInt(v);
        if (cellvar_offset == -1 && PyErr_Occurred()) {
            return ERROR;
        }
        assert(cellvar_offset >= 0);
        cellvar_offset += nlocals - numdropped;
        assert(cellvar_offset < nlocalsplus);
        _Py_set_localsplus_info(cellvar_offset, k, CO_FAST_CELL, names, kinds);
    }

    pos = 0;
    while (PyDict_Next(umd->u_freevars, &pos, &k, &v)) {
        int offset = PyLong_AsInt(v);
        if (offset == -1 && PyErr_Occurred()) {
            return ERROR;
        }
        assert(offset >= 0);
        offset += nlocals - numdropped;
        assert(offset < nlocalsplus);
        /* XXX If the assertion below fails it is most likely because a freevar
           was added to u_freevars with the wrong index due to not taking into
           account cellvars already present, see gh-128632. */
        assert(offset > cellvar_offset);
        _Py_set_localsplus_info(offset, k, CO_FAST_FREE, names, kinds);
    }
    return SUCCESS;
}

static PyCodeObject *
makecode(_PyCompile_CodeUnitMetadata *umd, struct assembler *a, PyObject *const_cache,
         PyObject *constslist, int maxdepth, int nlocalsplus, int code_flags,
         PyObject *filename)
{
    PyCodeObject *co = NULL;
    PyObject *names = NULL;
    PyObject *consts = NULL;
    PyObject *localsplusnames = NULL;
    PyObject *localspluskinds = NULL;
    names = dict_keys_inorder(umd->u_names, 0);
    if (!names) {
        goto error;
    }
    if (_PyCompile_ConstCacheMergeOne(const_cache, &names) < 0) {
        goto error;
    }

    consts = PyList_AsTuple(constslist); /* PyCode_New requires a tuple */
    if (consts == NULL) {
        goto error;
    }
    if (_PyCompile_ConstCacheMergeOne(const_cache, &consts) < 0) {
        goto error;
    }

    assert(umd->u_posonlyargcount < INT_MAX);
    assert(umd->u_argcount < INT_MAX);
    assert(umd->u_kwonlyargcount < INT_MAX);
    int posonlyargcount = (int)umd->u_posonlyargcount;
    int posorkwargcount = (int)umd->u_argcount;
    assert(INT_MAX - posonlyargcount - posorkwargcount > 0);
    int kwonlyargcount = (int)umd->u_kwonlyargcount;

    localsplusnames = PyTuple_New(nlocalsplus);
    if (localsplusnames == NULL) {
        goto error;
    }
    localspluskinds = PyBytes_FromStringAndSize(NULL, nlocalsplus);
    if (localspluskinds == NULL) {
        goto error;
    }
    if (compute_localsplus_info(
            umd, nlocalsplus, code_flags,
            localsplusnames, localspluskinds) == ERROR)
    {
        goto error;
    }

    struct _PyCodeConstructor con = {
        .filename = filename,
        .name = umd->u_name,
        .qualname = umd->u_qualname ? umd->u_qualname : umd->u_name,
        .flags = code_flags,

        .code = a->a_bytecode,
        .firstlineno = umd->u_firstlineno,
        .linetable = a->a_linetable,

        .consts = consts,
        .names = names,

        .localsplusnames = localsplusnames,
        .localspluskinds = localspluskinds,

        .argcount = posonlyargcount + posorkwargcount,
        .posonlyargcount = posonlyargcount,
        .kwonlyargcount = kwonlyargcount,

        .stacksize = maxdepth,

        .exceptiontable = a->a_except_table,
    };

   if (_PyCode_Validate(&con) < 0) {
        goto error;
    }

    if (_PyCompile_ConstCacheMergeOne(const_cache, &localsplusnames) < 0) {
        goto error;
    }
    con.localsplusnames = localsplusnames;

    co = _PyCode_New(&con);
    if (co == NULL) {
        goto error;
    }

error:
    Py_XDECREF(names);
    Py_XDECREF(consts);
    Py_XDECREF(localsplusnames);
    Py_XDECREF(localspluskinds);
    return co;
}


// The offset (in code units) of the END_SEND from the SEND in the `yield from` sequence.
#define END_SEND_OFFSET 6

static int
resolve_jump_offsets(instr_sequence *instrs)
{
    /* Compute the size of each instruction and fixup jump args.
     * Replace instruction index with position in bytecode.
     */

    for (int i = 0; i < instrs->s_used; i++) {
        instruction *instr = &instrs->s_instrs[i];
        if (OPCODE_HAS_JUMP(instr->i_opcode)) {
            instr->i_target = instr->i_oparg;
        }
    }

    int extended_arg_recompile;

    do {
        int totsize = 0;
        for (int i = 0; i < instrs->s_used; i++) {
            instruction *instr = &instrs->s_instrs[i];
            instr->i_offset = totsize;
            int isize = instr_size(instr);
            totsize += isize;
        }
        extended_arg_recompile = 0;

        int offset = 0;
        for (int i = 0; i < instrs->s_used; i++) {
            instruction *instr = &instrs->s_instrs[i];
            int isize = instr_size(instr);
            /* jump offsets are computed relative to
             * the instruction pointer after fetching
             * the jump instruction.
             */
            offset += isize;
            if (OPCODE_HAS_JUMP(instr->i_opcode)) {
                instruction *target = &instrs->s_instrs[instr->i_target];
                instr->i_oparg = target->i_offset;
                if (instr->i_opcode == END_ASYNC_FOR) {
                    // sys.monitoring needs to be able to find the matching END_SEND
                    // but the target is the SEND, so we adjust it here.
                    instr->i_oparg = offset - instr->i_oparg - END_SEND_OFFSET;
                }
                else if (instr->i_oparg < offset) {
                    assert(IS_BACKWARDS_JUMP_OPCODE(instr->i_opcode));
                    instr->i_oparg = offset - instr->i_oparg;
                }
                else {
                    assert(!IS_BACKWARDS_JUMP_OPCODE(instr->i_opcode));
                    instr->i_oparg = instr->i_oparg - offset;
                }
                if (instr_size(instr) != isize) {
                    extended_arg_recompile = 1;
                }
            }
        }
    /* XXX: This is an awful hack that could hurt performance, but
        on the bright side it should work until we come up
        with a better solution.

        The issue is that in the first loop instr_size() is
        called, and it requires i_oparg be set appropriately.
        There is a bootstrap problem because i_oparg is
        calculated in the second loop above.

        So we loop until we stop seeing new EXTENDED_ARGs.
        The only EXTENDED_ARGs that could be popping up are
        ones in jump instructions.  So this should converge
        fairly quickly.
    */
    } while (extended_arg_recompile);
    return SUCCESS;
}

static int
resolve_unconditional_jumps(instr_sequence *instrs)
{
    /* Resolve directions of unconditional jumps */

    for (int i = 0; i < instrs->s_used; i++) {
        instruction *instr = &instrs->s_instrs[i];
        bool is_forward = (instr->i_oparg > i);
        switch(instr->i_opcode) {
            case JUMP:
                assert(is_pseudo_target(JUMP, JUMP_FORWARD));
                assert(is_pseudo_target(JUMP, JUMP_BACKWARD));
                instr->i_opcode = is_forward ? JUMP_FORWARD : JUMP_BACKWARD;
                break;
            case JUMP_NO_INTERRUPT:
                assert(is_pseudo_target(JUMP_NO_INTERRUPT, JUMP_FORWARD));
                assert(is_pseudo_target(JUMP_NO_INTERRUPT, JUMP_BACKWARD_NO_INTERRUPT));
                instr->i_opcode = is_forward ?
                    JUMP_FORWARD : JUMP_BACKWARD_NO_INTERRUPT;
                break;
            default:
                if (OPCODE_HAS_JUMP(instr->i_opcode) &&
                    IS_PSEUDO_INSTR(instr->i_opcode)) {
                    Py_UNREACHABLE();
                }
        }
    }
    return SUCCESS;
}

PyCodeObject *
_PyAssemble_MakeCodeObject(_PyCompile_CodeUnitMetadata *umd, PyObject *const_cache,
                           PyObject *consts, int maxdepth, instr_sequence *instrs,
                           int nlocalsplus, int code_flags, PyObject *filename)
{
    if (_PyInstructionSequence_ApplyLabelMap(instrs) < 0) {
        return NULL;
    }
    if (resolve_unconditional_jumps(instrs) < 0) {
        return NULL;
    }
    if (resolve_jump_offsets(instrs) < 0) {
        return NULL;
    }
    PyCodeObject *co = NULL;

    struct assembler a;
    int res = assemble_emit(&a, instrs, umd->u_firstlineno, const_cache);
    if (res == SUCCESS) {
        co = makecode(umd, &a, const_cache, consts, maxdepth, nlocalsplus,
                      code_flags, filename);
    }
    assemble_free(&a);
    return co;
}