CPython 3.2 eval deep dive

CPython 3.2
eval
Shinya Kawanaka (@mayahjp)

Sunday, March 27, 2011

• : (Shinya Kawanaka)

• : mayah (or MAYAH)

• twitter: @mayahjp
• Python: ( )


• CPython eval

•


eval

Γ⊦e→f
Γ ⊦ e1 → v1, ..., Γ ⊦ en → vn
Γ ⊦ f(v1, ..., vn) → v
Γ ⊦ e(e1, ..., en) → v

( )


AST

• AST
eval

• …… Python


Python
→ ALMOST YES
• ceval.c
PyEval_EvalCode co code (AST)
globals locals

PyObject *
PyEval_EvalCode(PyObject *co, PyObject *globals, PyObject *locals)
{
return PyEval_EvalCodeEx(co,
globals, locals,
(PyObject **)NULL, 0,
NULL, NULL);
}


AST

• AST
• PyEvalCode PyCodeObject
AST

•

• PyObject

• PyObject

•

• PyEvalCode PyObject* globals locals
map


AST / PyCodeObject (code.h)
• PyCodeObject code.h

• opcode
... PyObject
typedef struct {
PyObject_HEAD
... <snip /> ...
PyObject *co_code; /* instruction opcodes */
PyObject *co_consts; /* list (constants used) */
PyObject *co_names; /* list of strings (names used) */
PyObject *co_varnames; /* tuple of strings (local variable names) */
PyObject *co_freevars; /* tuple of strings (free variable names) */
PyObject *co_cellvars; /* tuple of strings (cell variable names) */
... <snip /> ...
} PyCodeObject;

opcode (opcode.h)
• opcode.h opcode

• define

#define MAKE_CLOSURE 134 /* same as MAKE_FUNCTION */
#define LOAD_CLOSURE 135 /* Load free variable from closure */
#define LOAD_DEREF 136 /* Load and dereference from closure cell */
#define STORE_DEREF 137 /* Store into cell */
#define DELETE_DEREF 138 /* Delete closure cell */


opcode
grep
•

• →

• ceval.c eval switch
case

• AST


AST
• PyCodeObject
• co_code opcode

• co_consts AST

• AST


PyObject (object.h)
• GC

• ob_type PyObject

typedef struct _object {
... <snip /> ...
Py_ssize_t ob_refcnt;
struct _typeobject *ob_type;
} PyObject;

typedef struct {
PyObject ob_base;
Py_ssize_t ob_size; /* Number of items in variable part */
} PyVarObject;


_typeobject (object.h)
•

•

typedef struct _typeobject {
PyObject_VAR_HEAD
... <snip /> ...
destructor tp_dealloc;
printfunc tp_print;
getattrfunc tp_getattr;
setattrfunc tp_setattr;
void *tp_reserved; /* formerly known as tp_compare */
reprfunc tp_repr;


•

• _typeobject (PyTypeObject) tp_dict

• PyObject

• primitive
primitive PyObject

• int


PyLongObject (longobject.h)
• Py Object
PyIntObject

• → grep

• PyLongObject

• 64bit long ...


PyLongObject (contd.)
• _longobject

•

• digit ( ) uint32

• 64bit

struct _longobject {
PyObject_VAR_HEAD
digit ob_digit[1];
};


• PyObject

• PyObject

•

• primitive
primitive PyObject

• PyLongObject


...
• map

• PyObject dict

• eval

• eval


• eval

•


PyEval_EvalCodeEx (ceval.c)
•

• +α (PyFrameObject) (PyFrame_New())

•

• PyFrameObject PyEval_EvalFrameEx

• PyEval_EvalFrameEx AST eval


PyFrameObject (frameobject.h)
•

typedef struct _frame {
PyObject_VAR_HEAD
struct _frame *f_back; /* previous frame, or NULL */
PyCodeObject *f_code; /* code segment */
PyObject *f_builtins; /* builtin symbol table (PyDictObject) */
PyObject *f_globals; /* global symbol table (PyDictObject) */
PyObject *f_locals; /* local symbol table (any mapping) */
PyObject **f_valuestack; /* points after the last local */
....
}


PyFrame_New (frameobject.c)
•

•

•

• code

•

• malloc


• ( )

• local kw (keyword)

• SETLOCAL OK

for (i = 0; i < n; i++) {
x = args[i];
Py_INCREF(x);
SETLOCAL(i, x);
}


PyEval_EvalCodeEx (contd.)
•

• PyEval_EvalFrameEx

• goto fail

• fail __del__

• C stack


PyEval_EvalCode

• pythonrun.c run_mod()

• AST eval

static PyObject * run_mod(mod_ty mod, const char *filename, PyObject
*globals, PyObject *locals, PyCompilerFlags *flags, PyArena *arena)
{
PyCodeObject *co;
PyObject *v;
co = PyAST_Compile(mod, filename, flags, arena);
if (co == NULL) return NULL;
v = PyEval_EvalCode((PyObject*)co, globals, locals);
Py_DECREF(co);
return v;
}


fast_function (ceval.c)
• eval

•

• eval call_function

•


PyEval_EvalFrameEx (ceval.c)
•

•

register int opcode; /* Current opcode */
register int oparg; /* Current opcode argument, if any */
register enum why_code why; /* Reason for block stack unwind */
register int err; /* Error status -- nonzero if error */
register PyObject *x; /* Result object -- NULL if error */
register PyObject *v; /* Temporary objects popped off stack */
register PyObject *w;
register PyObject *u;
register PyObject *t;


PyEval_EvalFrameEx (contd.)
• Main
switch on opcode

• operation x NULL
err 0 why WHY_NOT

operation


• NOP (No operation)
TARGET(NOP)
FAST_DISPATCH();

•

#define TARGET(op)
case op:

#define DISPATCH() continue

#define FAST_DISPATCH() goto fast_next_opcode


• NOP LOAD_CONST ( )
x = GETITEM(consts, oparg);
Py_INCREF(x);
PUSH(x);
FAST_DISPATCH();

• PUSH(x) stack machine

#deﬁne BASIC_PUSH(v) (*stack_pointer++ = (v))
#deﬁne PUSH(v) BASIC_PUSH(v)


stack machine
• stack machine (BINARY_ADD) 2
operands POP → POP → PUSH

•→
TARGET(BINARY_ADD) // only main
w = POP();
v = TOP();
x = PyNumber_Add(v, w);
Py_DECREF(v);
Py_DECREF(w);
SET_TOP(x);
if (x != NULL) DISPATCH();
break;


• stack machine

• if

• if compare & branch

•

• python AST compile


JUMP_IF_TRUE [FALSE]
(_OR_POP)
• stack machine jump

• JUMP_IF_TRUE [FALSE] (_OR_POP)
TARGET(POP_JUMP_IF_TRUE)
w = POP();
if (w == Py_False) {
Py_DECREF(w);
FAST_DISPATCH();
}
if (w == Py_True) {
Py_DECREF(w);
JUMPTO(oparg); // this will set the next instruction!
FAST_DISPATCH();


JUMPTO
• first_instr code

• offset
#define JUMPTO(x) (next_instr = first_instr + (x))

• first_instr
first_instr = (unsigned char*) PyBytes_AS_STRING(co->co_code);

• co_code


CALL_FUNCTION
• stack pointer call_function

TARGET(CALL_FUNCTION) {
PyObject **sp;
PCALL(PCALL_ALL); // for proﬁling
sp = stack_pointer;
x = call_function(&sp, oparg);
stack_pointer = sp;
PUSH(x);
if (x != NULL) DISPATCH();
break;
}


call_function
• oparg

• 16bit
(
static PyObject *
call_function(PyObject ***pp_stack, int oparg)
{
int na = oparg & 0xff;
int nk = (oparg>>8) & 0xff;
int n = na + 2 * nk;
PyObject **pfunc = (*pp_stack) - n - 1;
PyObject *func = *pfunc;
PyObject *x, *w;


call_function (contd.)
•

• CPyFunction

PyObject *callargs;
callargs = load_args(pp_stack, na);
READ_TIMESTAMP(*pintr0);
C_TRACE(x, PyCFunction_Call(func,callargs,NULL));
READ_TIMESTAMP(*pintr1);
Py_XDECREF(callargs);


TIME UP
•


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