path: root/Python/compile.c

/*
 * This file compiles an abstract syntax tree (AST) into Python bytecode.
 *
 * The primary entry point is PyAST_Compile(), which returns a
 * PyCodeObject.  The compiler makes several passes to build the code
 * object:
 *   1. Checks for future statements.  See future.c
 *   2. Builds a symbol table.  See symtable.c.
 *   3. Generate code for basic blocks.  See compiler_mod() in this file.
 *   4. Assemble the basic blocks into final code.  See assemble() in
 *      this file.
 *   5. Optimize the byte code (peephole optimizations).  See peephole.c
 *
 * Note that compiler_mod() suggests module, but the module ast type
 * (mod_ty) has cases for expressions and interactive statements.
 *
 * CAUTION: The VISIT_* macros abort the current function when they
 * encounter a problem. So don't invoke them when there is memory
 * which needs to be released. Code blocks are OK, as the compiler
 * structure takes care of releasing those.  Use the arena to manage
 * objects.
 */

#include "Python.h"

#include "Python-ast.h"
#include "node.h"
#include "ast.h"
#include "code.h"
#include "symtable.h"
#include "opcode.h"
#include "wordcode_helpers.h"

#define DEFAULT_BLOCK_SIZE 16
#define DEFAULT_BLOCKS 8
#define DEFAULT_CODE_SIZE 128
#define DEFAULT_LNOTAB_SIZE 16

#define COMP_GENEXP   0
#define COMP_LISTCOMP 1
#define COMP_SETCOMP  2
#define COMP_DICTCOMP 3

struct instr {
    unsigned i_jabs : 1;
    unsigned i_jrel : 1;
    unsigned char i_opcode;
    int i_oparg;
    struct basicblock_ *i_target; /* target block (if jump instruction) */
    int i_lineno;
};

typedef struct basicblock_ {
    /* Each basicblock in a compilation unit is linked via b_list in the
       reverse order that the block are allocated.  b_list points to the next
       block, not to be confused with b_next, which is next by control flow. */
    struct basicblock_ *b_list;
    /* number of instructions used */
    int b_iused;
    /* length of instruction array (b_instr) */
    int b_ialloc;
    /* pointer to an array of instructions, initially NULL */
    struct instr *b_instr;
    /* If b_next is non-NULL, it is a pointer to the next
       block reached by normal control flow. */
    struct basicblock_ *b_next;
    /* b_seen is used to perform a DFS of basicblocks. */
    unsigned b_seen : 1;
    /* b_return is true if a RETURN_VALUE opcode is inserted. */
    unsigned b_return : 1;
    /* depth of stack upon entry of block, computed by stackdepth() */
    int b_startdepth;
    /* instruction offset for block, computed by assemble_jump_offsets() */
    int b_offset;
} basicblock;

/* fblockinfo tracks the current frame block.

A frame block is used to handle loops, try/except, and try/finally.
It's called a frame block to distinguish it from a basic block in the
compiler IR.
*/

enum fblocktype { LOOP, EXCEPT, FINALLY_TRY, FINALLY_END };

struct fblockinfo {
    enum fblocktype fb_type;
    basicblock *fb_block;
};

enum {
    COMPILER_SCOPE_MODULE,
    COMPILER_SCOPE_CLASS,
    COMPILER_SCOPE_FUNCTION,
    COMPILER_SCOPE_ASYNC_FUNCTION,
    COMPILER_SCOPE_LAMBDA,
    COMPILER_SCOPE_COMPREHENSION,
};

/* The following items change on entry and exit of code blocks.
   They must be saved and restored when returning to a block.
*/
struct compiler_unit {
    PySTEntryObject *u_ste;

    PyObject *u_name;
    PyObject *u_qualname;  /* dot-separated qualified name (lazy) */
    int u_scope_type;

    /* The following fields are dicts that map objects to
       the index of them in co_XXX.      The index is used as
       the argument for opcodes that refer to those collections.
    */
    PyObject *u_consts;    /* all constants */
    PyObject *u_names;     /* all names */
    PyObject *u_varnames;  /* local variables */
    PyObject *u_cellvars;  /* cell variables */
    PyObject *u_freevars;  /* free variables */

    PyObject *u_private;        /* for private name mangling */

    Py_ssize_t u_argcount;        /* number of arguments for block */
    Py_ssize_t u_kwonlyargcount; /* number of keyword only arguments for block */
    /* Pointer to the most recently allocated block.  By following b_list
       members, you can reach all early allocated blocks. */
    basicblock *u_blocks;
    basicblock *u_curblock; /* pointer to current block */

    int u_nfblocks;
    struct fblockinfo u_fblock[CO_MAXBLOCKS];

    int u_firstlineno; /* the first lineno of the block */
    int u_lineno;          /* the lineno for the current stmt */
    int u_col_offset;      /* the offset of the current stmt */
    int u_lineno_set;  /* boolean to indicate whether instr
                          has been generated with current lineno */
};

/* This struct captures the global state of a compilation.

The u pointer points to the current compilation unit, while units
for enclosing blocks are stored in c_stack.     The u and c_stack are
managed by compiler_enter_scope() and compiler_exit_scope().

Note that we don't track recursion levels during compilation - the
task of detecting and rejecting excessive levels of nesting is
handled by the symbol analysis pass.

*/

struct compiler {
    PyObject *c_filename;
    struct symtable *c_st;
    PyFutureFeatures *c_future; /* pointer to module's __future__ */
    PyCompilerFlags *c_flags;

    int c_optimize;              /* optimization level */
    int c_interactive;           /* true if in interactive mode */
    int c_nestlevel;
n>ClassDef.bases);
        VISIT_SEQ(st, keyword, s->v.ClassDef.keywords);
        if (s->v.ClassDef.decorator_list)
            VISIT_SEQ(st, expr, s->v.ClassDef.decorator_list);
        if (!symtable_enter_block(st, s->v.ClassDef.name, ClassBlock,
                                  (void *)s, s->lineno, s->col_offset))
            VISIT_QUIT(st, 0);
        tmp = st->st_private;
        st->st_private = s->v.ClassDef.name;
        VISIT_SEQ(st, stmt, s->v.ClassDef.body);
        st->st_private = tmp;
        if (!symtable_exit_block(st, s))
            VISIT_QUIT(st, 0);
        break;
    }
    case Return_kind:
        if (s->v.Return.value) {
            VISIT(st, expr, s->v.Return.value);
            st->st_cur->ste_returns_value = 1;
        }
        break;
    case Delete_kind:
        VISIT_SEQ(st, expr, s->v.Delete.targets);
        break;
    case Assign_kind:
        VISIT_SEQ(st, expr, s->v.Assign.targets);
        VISIT(st, expr, s->v.Assign.value);
        break;
    case AnnAssign_kind:
        if (s->v.AnnAssign.target->kind == Name_kind) {
            expr_ty e_name = s->v.AnnAssign.target;
            long cur = symtable_lookup(st, e_name->v.Name.id);
            if (cur < 0) {
                VISIT_QUIT(st, 0);
            }
            if ((cur & (DEF_GLOBAL | DEF_NONLOCAL))
                && s->v.AnnAssign.simple) {
                PyErr_Format(PyExc_SyntaxError,
                             cur & DEF_GLOBAL ? GLOBAL_ANNOT : NONLOCAL_ANNOT,
                             e_name->v.Name.id);
                PyErr_SyntaxLocationObject(st->st_filename,
                                           s->lineno,
                                           s->col_offset);
                VISIT_QUIT(st, 0);
            }
            if (s->v.AnnAssign.simple &&
                !symtable_add_def(st, e_name->v.Name.id,
                                  DEF_ANNOT | DEF_LOCAL)) {
                VISIT_QUIT(st, 0);
            }
            else {
                if (s->v.AnnAssign.value
                    && !symtable_add_def(st, e_name->v.Name.id, DEF_LOCAL)) {
                    VISIT_QUIT(st, 0);
                }
            }
        }
        else {
            VISIT(st, expr, s->v.AnnAssign.target);
        }
        VISIT(st, expr, s->v.AnnAssign.annotation);
        if (s->v.AnnAssign.value) {
            VISIT(st, expr, s->v.AnnAssign.value);
        }
        break;
    case AugAssign_kind:
        VISIT(st, expr, s->v.AugAssign.target);
        VISIT(st, expr, s->v.AugAssign.value);
        break;
    case For_kind:
        VISIT(st, expr, s->v.For.target);
        VISIT(st, expr, s->v.For.iter);
        VISIT_SEQ(st, stmt, s->v.For.body);
        if (s->v.For.orelse)
            VISIT_SEQ(st, stmt, s->v.For.orelse);
        break;
    case While_kind:
        VISIT(st, expr, s->v.While.test);
        VISIT_SEQ(st, stmt, s->v.While.body);
        if (s->v.While.orelse)
            VISIT_SEQ(st, stmt, s->v.While.orelse);
        break;
    case If_kind:
        /* XXX if 0: and lookup_yield() hacks */
        VISIT(st, expr, s->v.If.test);
        VISIT_SEQ(st, stmt, s->v.If.body);
        if (s->v.If.orelse)
            VISIT_SEQ(st, stmt, s->v.If.orelse);
        break;
    case Raise_kind:
        if (s->v.Raise.exc) {
            VISIT(st, expr, s->v.Raise.exc);
            if (s->v.Raise.cause) {
                VISIT(st, expr, s->v.Raise.cause);
            }
        }
        break;
    case Try_kind:
        VISIT_SEQ(st, stmt, s->v.Try.body);
        VISIT_SEQ(st, stmt, s->v.Try.orelse);
        VISIT_SEQ(st, excepthandler, s->v.Try.handlers);
        VISIT_SEQ(st, stmt, s->v.Try.finalbody);
        break;
    case Assert_kind:
        VISIT(st, expr, s->v.Assert.test);
        if (s->v.Assert.msg)
            VISIT(st, expr, s->v.Assert.msg);
        break;
    case Import_kind:
        VISIT_SEQ(st, alias, s->v.Import.names);
        break;
    case ImportFrom_kind:
        VISIT_SEQ(st, alias, s->v.ImportFrom.names);
        break;
    case Global_kind: {
        int i;
        asdl_seq *seq = s->v.Global.names;
        for (i = 0; i < asdl_seq_LEN(seq); i++) {
            identifier name = (identifier)asdl_seq_GET(seq, i);
            long cur = symtable_lookup(st, name);
            if (cur < 0)
                VISIT_QUIT(st, 0);
            if (cur & (DEF_LOCAL | USE | DEF_ANNOT)) {
                char* msg;
                if (cur & USE) {
                    msg = GLOBAL_AFTER_USE;
                } else if (cur & DEF_ANNOT) {
                    msg = GLOBAL_ANNOT;
                } else {  /* DEF_LOCAL */
                    msg = GLOBAL_AFTER_ASSIGN;
                }
                PyErr_Format(PyExc_SyntaxError,
                             msg, name);
                PyErr_SyntaxLocationObject(st->st_filename,
                                           s->lineno,
                                           s->col_offset);
                VISIT_QUIT(st, 0);
            }
            if (!symtable_add_def(st, name, DEF_GLOBAL))
                VISIT_QUIT(st, 0);
            if (!symtable_record_directive(st, name, s))
                VISIT_QUIT(st, 0);
        }
        break;
    }
    case Nonlocal_kind: {
        int i;
        asdl_seq *seq = s->v.Nonlocal.names;
        for (i = 0; i < asdl_seq_LEN(seq); i++) {
            identifier name = (identifier)asdl_seq_GET(seq, i);
            long cur = symtable_lookup(st, name);
            if (cur < 0)
                VISIT_QUIT(st, 0);
            if (cur & (DEF_LOCAL | USE | DEF_ANNOT)) {
                char* msg;
                if (cur & USE) {
                    msg = NONLOCAL_AFTER_USE;
                } else if (cur & DEF_ANNOT) {
                    msg = NONLOCAL_ANNOT;
                } else {  /* DEF_LOCAL */
                    msg = NONLOCAL_AFTER_ASSIGN;
                }
                PyErr_Format(PyExc_SyntaxError, msg, name);
                PyErr_SyntaxLocationObject(st->st_filename,
                                           s->lineno,
                                           s->col_offset);
                VISIT_QUIT(st, 0);
            }
            if (!symtable_add_def(st, name, DEF_NONLOCAL))
                VISIT_QUIT(st, 0);
            if (!symtable_record_directive(st, name, s))
                VISIT_QUIT(st, 0);
        }
        break;
    }
    case Expr_kind:
        VISIT(st, expr, s->v.Expr.value);
        break;
    case Pass_kind:
    case Break_kind:
    case Continue_kind:
        /* nothing to do here */
        break;
    case With_kind:
        VISIT_SEQ(st, withitem, s->v.With.items);
        VISIT_SEQ(st, stmt, s->v.With.body);
        break;
    case AsyncFunctionDef_kind:
        if (!symtable_add_def(st, s->v.AsyncFunctionDef.name, DEF_LOCAL))
            VISIT_QUIT(st, 0);
        if (s->v.AsyncFunctionDef.args->defaults)
            VISIT_SEQ(st, expr, s->v.AsyncFunctionDef.args->defaults);
        if (s->v.AsyncFunctionDef.args->kw_defaults)
            VISIT_SEQ_WITH_NULL(st, expr,
                                s->v.AsyncFunctionDef.args->kw_defaults);
        if (!symtable_visit_annotations(st, s, s->v.AsyncFunctionDef.args,
                                        s->v.AsyncFunctionDef.returns))
            VISIT_QUIT(st, 0);
        if (s->v.AsyncFunctionDef.decorator_list)
            VISIT_SEQ(st, expr, s->v.AsyncFunctionDef.decorator_list);
        if (!symtable_enter_block(st, s->v.AsyncFunctionDef.name,
                                  FunctionBlock, (void *)s, s->lineno,
                                  s->col_offset))
            VISIT_QUIT(st, 0);
        st->st_cur->ste_coroutine = 1;
        VISIT(st, arguments, s->v.AsyncFunctionDef.args);
        VISIT_SEQ(st, stmt, s->v.AsyncFunctionDef.body);
        if (!symtable_exit_block(st, s))
            VISIT_QUIT(st, 0);
        break;
    case AsyncWith_kind:
        VISIT_SEQ(st, withitem, s->v.AsyncWith.items);
        VISIT_SEQ(st, stmt, s->v.AsyncWith.body);
        break;
    case AsyncFor_kind:
        VISIT(st, expr, s->v.AsyncFor.target);
        VISIT(st, expr, s->v.AsyncFor.iter);
        VISIT_SEQ(st, stmt, s->v.AsyncFor.body);
        if (s->v.AsyncFor.orelse)
            VISIT_SEQ(st, stmt, s->v.AsyncFor.orelse);
        break;
    }
    VISIT_QUIT(st, 1);
}

static int
symtable_visit_expr(struct symtable *st, expr_ty e)
{
    if (++st->recursion_depth > st->recursion_limit) {
        PyErr_SetString(PyExc_RecursionError,
                        "maximum recursion depth exceeded during compilation");
        VISIT_QUIT(st, 0);
    }
    switch (e->kind) {
    case BoolOp_kind:
        VISIT_SEQ(st, expr, e->v.BoolOp.values);
        break;
    case BinOp_kind:
        VISIT(st, expr, e->v.BinOp.left);
        VISIT(st, expr, e->v.BinOp.right);
        break;
    case UnaryOp_kind:
        VISIT(st, expr, e->v.UnaryOp.operand);
        break;
    case Lambda_kind: {
        if (!GET_IDENTIFIER(lambda))
            VISIT_QUIT(st, 0);
        if (e->v.Lambda.args->defaults)
            VISIT_SEQ(st, expr, e->v.Lambda.args->defaults);
        if (e->v.Lambda.args->kw_defaults)
            VISIT_SEQ_WITH_NULL(st, expr, e->v.Lambda.args->kw_defaults);
        if (!symtable_enter_block(st, lambda,
                                  FunctionBlock, (void *)e, e->lineno,
                                  e->col_offset))
            VISIT_QUIT(st, 0);
        VISIT(st, arguments, e->v.Lambda.args);
        VISIT(st, expr, e->v.Lambda.body);
        if (!symtable_exit_block(st, (void *)e))
            VISIT_QUIT(st, 0);
        break;
    }
    case IfExp_kind:
        VISIT(st, expr, e->v.IfExp.test);
        VISIT(st, expr, e->v.IfExp.body);
        VISIT(st, expr, e->v.IfExp.orelse);
        break;
    case Dict_kind:
        VISIT_SEQ_WITH_NULL(st, expr, e->v.Dict.keys);
        VISIT_SEQ(st, expr, e->v.Dict.values);
        break;
    case Set_kind:
        VISIT_SEQ(st, expr, e->v.Set.elts);
        break;
    case GeneratorExp_kind:
        if (!symtable_visit_genexp(st, e))
            VISIT_QUIT(st, 0);
        break;
    case ListComp_kind:
        if (!symtable_visit_listcomp(st, e))
            VISIT_QUIT(st, 0);
        break;
    case SetComp_kind:
        if (!symtable_visit_setcomp(st, e))
            VISIT_QUIT(st, 0);
        break;
    case DictComp_kind:
        if (!symtable_visit_dictcomp(st, e))
            VISIT_QUIT(st, 0);
        break;
    case Yield_kind:
        if (e->v.Yield.value)
            VISIT(st, expr, e->v.Yield.value);
        st->st_cur->ste_generator = 1;
        break;
    case YieldFrom_kind:
        VISIT(st, expr, e->v.YieldFrom.value);
        st->st_cur->ste_generator = 1;
        break;
    case Await_kind:
        VISIT(st, expr, e->v.Await.value);
        st->st_cur->ste_coroutine = 1;
        break;
    case Compare_kind:
        VISIT(st, expr, e->v.Compare.left);
        VISIT_SEQ(st, expr, e->v.Compare.comparators);
        break;
    case Call_kind:
        VISIT(st, expr, e->v.Call.func);
        VISIT_SEQ(st, expr, e->v.Call.args);
        VISIT_SEQ_WITH_NULL(st, keyword, e->v.Call.keywords);
        break;
    case FormattedValue_kind:
        VISIT(st, expr, e->v.FormattedValue.value);
        if (e->v.FormattedValue.format_spec)
            VISIT(st, expr, e->v.FormattedValue.format_spec);
        break;
    case JoinedStr_kind:
        VISIT_SEQ(st, expr, e->v.JoinedStr.values);
        break;
    case Constant_kind:
    case Num_kind:
    case Str_kind:
    case Bytes_kind:
    case Ellipsis_kind:
    case NameConstant_kind:
        /* Nothing to do here. */
        break;
    /* The following exprs can be assignment targets. */
    case Attribute_kind:
        VISIT(st, expr, e->v.Attribute.value);
        break;
    case Subscript_kind:
        VISIT(st, expr, e->v.Subscript.value);
        VISIT(st, slice, e->v.Subscript.slice);
        break;
    case Starred_kind:
        VISIT(st, expr, e->v.Starred.value);
        break;
    case Name_kind:
        if (!symtable_add_def(st, e->v.Name.id,
                              e->v.Name.ctx == Load ? USE : DEF_LOCAL))
            VISIT_QUIT(st, 0);
        /* Special-case super: it counts as a use of __class__ */
        if (e->v.Name.ctx == Load &&
            st->st_cur->ste_type == FunctionBlock &&
            !PyUnicode_CompareWithASCIIString(e->v.Name.id, "super")) {
            if (!GET_IDENTIFIER(__class__) ||
                !symtable_add_def(st, __class__, USE))
                VISIT_QUIT(st, 0);
        }
        break;
    /* child nodes of List and Tuple will have expr_context set */
    case List_kind:
        VISIT_SEQ(st, expr, e->v.List.elts);
        break;
    case Tuple_kind:
        VISIT_SEQ(st, expr, e->v.Tuple.elts);
        break;
    }
    VISIT_QUIT(st, 1);
}

static int
symtable_implicit_arg(struct symtable *st, int pos)
{
    PyObject *id = PyUnicode_FromFormat(".%d", pos);
    if (id == NULL)
        return 0;
    if (!symtable_add_def(st, id, DEF_PARAM)) {
        Py_DECREF(id);
        return 0;
    }
    Py_DECREF(id);
    return 1;
}

static int
symtable_visit_params(struct symtable *st, asdl_seq *args)
{
    int i;

    if (!args)
        return -1;

    for (i = 0; i < asdl_seq_LEN(args); i++) {
        arg_ty arg = (arg_ty)asdl_seq_GET(args, i);
        if (!symtable_add_def(st, arg->arg, DEF_PARAM))
            return 0;
    }

    return 1;
}

static int
symtable_visit_argannotations(struct symtable *st, asdl_seq *args)
{
    int i;

    if (!args)
        return -1;

    for (i = 0; i < asdl_seq_LEN(args); i++) {
        arg_ty arg = (arg_ty)asdl_seq_GET(args, i);
        if (arg->annotation)
            VISIT(st, expr, arg->annotation);
    }

    return 1;
}

static int
symtable_visit_annotations(struct symtable *st, stmt_ty s,
                           arguments_ty a, expr_ty returns)
{
    if (a->args && !symtable_visit_argannotations(st, a->args))
        return 0;
    if (a->vararg && a->vararg->annotation)
        VISIT(st, expr, a->vararg->annotation);
    if (a->kwarg && a->kwarg->annotation)
        VISIT(st, expr, a->kwarg->annotation);
    if (a->kwonlyargs && !symtable_visit_argannotations(st, a->kwonlyargs))
        return 0;
    if (returns)
        VISIT(st, expr, returns);
    return 1;
}

static int
symtable_visit_arguments(struct symtable *st, arguments_ty a)
{
    /* skip default arguments inside function block
       XXX should ast be different?
    */
    if (a->args && !symtable_visit_params(st, a->args))
        return 0;
    if (a->kwonlyargs && !symtable_visit_params(st, a->kwonlyargs))
        return 0;
    if (a->vararg) {
        if (!symtable_add_def(st, a->vararg->arg, DEF_PARAM))
            return 0;
        st->st_cur->ste_varargs = 1;
    }
    if (a->kwarg) {
        if (!symtable_add_def(st, a->kwarg->arg, DEF_PARAM))
            return 0;
        st->st_cur->ste_varkeywords = 1;
    }
    return 1;
}


static int
symtable_visit_excepthandler(struct symtable *st, excepthandler_ty eh)
{
    if (eh->v.ExceptHandler.type)
        VISIT(st, expr, eh->v.ExceptHandler.type);
    if (eh->v.ExceptHandler.name)
        if (!symtable_add_def(st, eh->v.ExceptHandler.name, DEF_LOCAL))
            return 0;
    VISIT_SEQ(st, stmt, eh->v.ExceptHandler.body);
    return 1;
}

static int
symtable_visit_withitem(struct symtable *st, withitem_ty item)
{
    VISIT(st, expr, item->context_expr);
    if (item->optional_vars) {
        VISIT(st, expr, item->optional_vars);
    }
    return 1;
}


static int
symtable_visit_alias(struct symtable *st, alias_ty a)
{
    /* Compute store_name, the name actually bound by the import
       operation.  It is different than a->name when a->name is a
       dotted package name (e.g. spam.eggs)
    */
    PyObject *store_name;
    PyObject *name = (a->asname == NULL) ? a->name : a->asname;
    Py_ssize_t dot = PyUnicode_FindChar(name, '.', 0,
                                        PyUnicode_GET_LENGTH(name), 1);
    if (dot != -1) {
        store_name = PyUnicode_Substring(name, 0, dot);
        if (!store_name)
            return 0;
    }
    else {
        store_name = name;
        Py_INCREF(store_name);
    }
    if (PyUnicode_CompareWithASCIIString(name, "*")) {
        int r = symtable_add_def(st, store_name, DEF_IMPORT);
        Py_DECREF(store_name);
        return r;
    }
    else {
        if (st->st_cur->ste_type != ModuleBlock) {
            int lineno = st->st_cur->ste_lineno;
            int col_offset = st->st_cur->ste_col_offset;
            PyErr_SetString(PyExc_SyntaxError, IMPORT_STAR_WARNING);
            PyErr_SyntaxLocationObject(st->st_filename, lineno, col_offset);
            Py_DECREF(store_name);
            return 0;
        }
        Py_DECREF(store_name);
        return 1;
    }
}


static int
symtable_visit_comprehension(struct symtable *st, comprehension_ty lc)
{
    VISIT(st, expr, lc->target);
    VISIT(st, expr, lc->iter);
    VISIT_SEQ(st, expr, lc->ifs);
    if (lc->is_async) {
        st->st_cur->ste_coroutine = 1;
    }
    return 1;
}


static int
symtable_visit_keyword(struct symtable *st, keyword_ty k)
{
    VISIT(st, expr, k->value);
    return 1;
}


static int
symtable_visit_slice(struct symtable *st, slice_ty s)
{
    switch (s->kind) {
    case Slice_kind:
        if (s->v.Slice.lower)
            VISIT(st, expr, s->v.Slice.lower)
        if (s->v.Slice.upper)
            VISIT(st, expr, s->v.Slice.upper)
        if (s->v.Slice.step)
            VISIT(st, expr, s->v.Slice.step)
        break;
    case ExtSlice_kind:
        VISIT_SEQ(st, slice, s->v.ExtSlice.dims)
        break;
    case Index_kind:
        VISIT(st, expr, s->v.Index.value)
        break;
    }
    return 1;
}

static int
symtable_handle_comprehension(struct symtable *st, expr_ty e,
                              identifier scope_name, asdl_seq *generators,
                              expr_ty elt, expr_ty value)
{
    int is_generator = (e->kind == GeneratorExp_kind);
    int needs_tmp = !is_generator;
    comprehension_ty outermost = ((comprehension_ty)
                                    asdl_seq_GET(generators, 0));
    /* Outermost iterator is evaluated in current scope */
    VISIT(st, expr, outermost->iter);
    /* Create comprehension scope for the rest */
    if (!scope_name ||
        !symtable_enter_block(st, scope_name, FunctionBlock, (void *)e,
                              e->lineno, e->col_offset)) {
        return 0;
    }
    st->st_cur->ste_generator = is_generator;
    if (outermost->is_async) {
        st->st_cur->ste_coroutine = 1;
    }
    /* Outermost iter is received as an argument */
    if (!symtable_implicit_arg(st, 0)) {
        symtable_exit_block(st, (void *)e);
        return 0;
    }
    /* Allocate temporary name if needed */
    if (needs_tmp && !symtable_new_tmpname(st)) {
        symtable_exit_block(st, (void *)e);
        return 0;
    }
    VISIT(st, expr, outermost->target);
    VISIT_SEQ(st, expr, outermost->ifs);
    VISIT_SEQ_TAIL(st, comprehension, generators, 1);
    if (value)
        VISIT(st, expr, value);
    VISIT(st, expr, elt);
    return symtable_exit_block(st, (void *)e);
}

static int
symtable_visit_genexp(struct symtable *st, expr_ty e)
{
    return symtable_handle_comprehension(st, e, GET_IDENTIFIER(genexpr),
                                         e->v.GeneratorExp.generators,
                                         e->v.GeneratorExp.elt, NULL);
}

static int
symtable_visit_listcomp(struct symtable *st, expr_ty e)
{
    return symtable_handle_comprehension(st, e, GET_IDENTIFIER(listcomp),
                                         e->v.ListComp.generators,
                                         e->v.ListComp.elt, NULL);
}

static int
symtable_visit_setcomp(struct symtable *st, expr_ty e)
{
    return symtable_handle_comprehension(st, e, GET_IDENTIFIER(setcomp),
                                         e->v.SetComp.generators,
                                         e->v.SetComp.elt, NULL);
}

static int
symtable_visit_dictcomp(struct symtable *st, expr_ty e)
{
    return symtable_handle_comprehension(st, e, GET_IDENTIFIER(dictcomp),
                                         e->v.DictComp.generators,
                                         e->v.DictComp.key,
                                         e->v.DictComp.value);
}