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bpo-40222: "Zero cost" exception handling (GH-25729)

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"Zero cost" exception handling.

* Uses a lookup table to determine how to handle exceptions.
* Removes SETUP_FINALLY and POP_TOP block instructions, eliminating (most of) the runtime overhead of try statements.
* Reduces the size of the frame object by about 60%.
This commit is contained in:
Mark Shannon 2021-05-07 15:19:19 +01:00 • committed by GitHub
parent b32c8e9795
commit adcd220556
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GPG key ID: 4AEE18F83AFDEB23
30 changed files with 6614 additions and 5687 deletions
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371
Python/ceval.c
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@ -95,6 +95,7 @@ static PyObject * special_lookup(PyThreadState *, PyObject *, _Py_Identifier *);
static int check_args_iterable(PyThreadState *, PyObject *func, PyObject *vararg);
static void format_kwargs_error(PyThreadState *, PyObject *func, PyObject *kwargs);
static void format_awaitable_error(PyThreadState *, PyTypeObject *, int, int);
static PyTryBlock get_exception_handler(PyCodeObject *, int);
#define NAME_ERROR_MSG \
"name '%.200s' is not defined"
@ -1448,34 +1449,6 @@ eval_frame_handle_pending(PyThreadState *tstate)
GETLOCAL(i) = value; \
Py_XDECREF(tmp); } while (0)
#define UNWIND_BLOCK(b) \
while (STACK_LEVEL() > (b)->b_level) { \
PyObject *v = POP(); \
Py_XDECREF(v); \
}
#define UNWIND_EXCEPT_HANDLER(b) \
do { \
PyObject *type, *value, *traceback; \
_PyErr_StackItem *exc_info; \
assert(STACK_LEVEL() >= (b)->b_level + 3); \
while (STACK_LEVEL() > (b)->b_level + 3) { \
value = POP(); \
Py_XDECREF(value); \
} \
exc_info = tstate->exc_info; \
type = exc_info->exc_type; \
value = exc_info->exc_value; \
traceback = exc_info->exc_traceback; \
exc_info->exc_type = POP(); \
exc_info->exc_value = POP(); \
exc_info->exc_traceback = POP(); \
Py_XDECREF(type); \
Py_XDECREF(value); \
Py_XDECREF(traceback); \
} while(0)
/* macros for opcode cache */
#define OPCACHE_CHECK() \
do { \
@ -1738,7 +1711,6 @@ _PyEval_EvalFrameDefault(PyThreadState *tstate, PyFrameObject *f, int throwflag)
assert(!_PyErr_Occurred(tstate));
#endif
main_loop:
for (;;) {
assert(stack_pointer >= f->f_valuestack); /* else underflow */
assert(STACK_LEVEL() <= co->co_stacksize); /* else overflow */
@ -1754,9 +1726,7 @@ main_loop:
if (_Py_atomic_load_relaxed(eval_breaker)) {
opcode = _Py_OPCODE(*next_instr);
if (opcode != SETUP_FINALLY &&
opcode != SETUP_WITH &&
opcode != BEFORE_ASYNC_WITH &&
if (opcode != BEFORE_ASYNC_WITH &&
opcode != YIELD_FROM) {
/* Few cases where we skip running signal handlers and other
pending calls:
@ -1800,14 +1770,14 @@ main_loop:
tstate->c_traceobj,
tstate, f,
&trace_info);
/* Reload possibly changed frame fields */
JUMPTO(f->f_lasti);
stack_pointer = f->f_valuestack+f->f_stackdepth;
f->f_stackdepth = -1;
if (err) {
/* trace function raised an exception */
goto error;
}
/* Reload possibly changed frame fields */
JUMPTO(f->f_lasti);
stack_pointer = f->f_valuestack+f->f_stackdepth;
f->f_stackdepth = -1;
NEXTOPARG();
}
@ -2425,7 +2395,6 @@ main_loop:
case TARGET(RETURN_VALUE): {
retval = POP();
assert(f->f_iblock == 0);
assert(EMPTY());
f->f_state = FRAME_RETURNED;
f->f_stackdepth = 0;
@ -2664,14 +2633,6 @@ main_loop:
case TARGET(POP_EXCEPT): {
PyObject *type, *value, *traceback;
_PyErr_StackItem *exc_info;
PyTryBlock *b = PyFrame_BlockPop(f);
if (b->b_type != EXCEPT_HANDLER) {
_PyErr_SetString(tstate, PyExc_SystemError,
"popped block is not an except handler");
goto error;
}
assert(STACK_LEVEL() >= (b)->b_level + 3 &&
STACK_LEVEL() <= (b)->b_level + 4);
exc_info = tstate->exc_info;
type = exc_info->exc_type;
value = exc_info->exc_value;
@ -2685,15 +2646,48 @@ main_loop:
DISPATCH();
}
case TARGET(POP_BLOCK): {
PyFrame_BlockPop(f);
DISPATCH();
case TARGET(POP_EXCEPT_AND_RERAISE): {
PyObject *lasti = PEEK(4);
if (PyLong_Check(lasti)) {
f->f_lasti = PyLong_AsLong(lasti);
assert(!_PyErr_Occurred(tstate));
}
else {
_PyErr_SetString(tstate, PyExc_SystemError, "lasti is not an int");
goto error;
}
PyObject *type, *value, *traceback;
_PyErr_StackItem *exc_info;
type = POP();
value = POP();
traceback = POP();
Py_DECREF(POP()); /* lasti */
_PyErr_Restore(tstate, type, value, traceback);
exc_info = tstate->exc_info;
type = exc_info->exc_type;
value = exc_info->exc_value;
traceback = exc_info->exc_traceback;
exc_info->exc_type = POP();
exc_info->exc_value = POP();
exc_info->exc_traceback = POP();
Py_XDECREF(type);
Py_XDECREF(value);
Py_XDECREF(traceback);
goto exception_unwind;
}
case TARGET(RERAISE): {
assert(f->f_iblock > 0);
if (oparg) {
f->f_lasti = f->f_blockstack[f->f_iblock-1].b_handler;
PyObject *lasti = PEEK(oparg+3);
if (PyLong_Check(lasti)) {
f->f_lasti = PyLong_AsLong(lasti);
assert(!_PyErr_Occurred(tstate));
}
else {
assert(PyLong_Check(lasti));
_PyErr_SetString(tstate, PyExc_SystemError, "lasti is not an int");
goto error;
}
}
PyObject *exc = POP();
PyObject *val = POP();
@ -2705,19 +2699,17 @@ main_loop:
case TARGET(END_ASYNC_FOR): {
PyObject *exc = POP();
PyObject *val = POP();
PyObject *tb = POP();
assert(PyExceptionClass_Check(exc));
if (PyErr_GivenExceptionMatches(exc, PyExc_StopAsyncIteration)) {
PyTryBlock *b = PyFrame_BlockPop(f);
assert(b->b_type == EXCEPT_HANDLER);
Py_DECREF(exc);
UNWIND_EXCEPT_HANDLER(b);
Py_DECREF(val);
Py_DECREF(tb);
Py_DECREF(POP());
JUMPBY(oparg);
DISPATCH();
}
else {
PyObject *val = POP();
PyObject *tb = POP();
_PyErr_Restore(tstate, exc, val, tb);
goto exception_unwind;
}
@ -4022,12 +4014,6 @@ main_loop:
DISPATCH();
}
case TARGET(SETUP_FINALLY): {
PyFrame_BlockSetup(f, SETUP_FINALLY, INSTR_OFFSET() + oparg,
STACK_LEVEL());
DISPATCH();
}
case TARGET(BEFORE_ASYNC_WITH): {
_Py_IDENTIFIER(__aenter__);
_Py_IDENTIFIER(__aexit__);
@ -4053,17 +4039,7 @@ main_loop:
DISPATCH();
}
case TARGET(SETUP_ASYNC_WITH): {
PyObject *res = POP();
/* Setup the finally block before pushing the result
of __aenter__ on the stack. */
PyFrame_BlockSetup(f, SETUP_FINALLY, INSTR_OFFSET() + oparg,
STACK_LEVEL());
PUSH(res);
DISPATCH();
}
case TARGET(SETUP_WITH): {
case TARGET(BEFORE_WITH): {
_Py_IDENTIFIER(__enter__);
_Py_IDENTIFIER(__exit__);
PyObject *mgr = TOP();
@ -4081,23 +4057,20 @@ main_loop:
Py_DECREF(mgr);
res = _PyObject_CallNoArg(enter);
Py_DECREF(enter);
if (res == NULL)
if (res == NULL) {
goto error;
/* Setup the finally block before pushing the result
of __enter__ on the stack. */
PyFrame_BlockSetup(f, SETUP_FINALLY, INSTR_OFFSET() + oparg,
STACK_LEVEL());
}
PUSH(res);
DISPATCH();
}
case TARGET(WITH_EXCEPT_START): {
/* At the top of the stack are 7 values:
/* At the top of the stack are 8 values:
- (TOP, SECOND, THIRD) = exc_info()
- (FOURTH, FIFTH, SIXTH) = previous exception for EXCEPT_HANDLER
- SEVENTH: the context.__exit__ bound method
We call SEVENTH(TOP, SECOND, THIRD).
- (FOURTH, FIFTH, SIXTH) = previous exception
- SEVENTH: lasti of exception in exc_info()
- EIGHTH: the context.__exit__ bound method
We call EIGHTH(TOP, SECOND, THIRD).
Then we push again the TOP exception and the __exit__
return value.
*/
@ -4109,7 +4082,8 @@ main_loop:
tb = THIRD();
assert(!Py_IsNone(exc));
assert(!PyLong_Check(exc));
exit_func = PEEK(7);
assert(PyLong_Check(PEEK(7)));
exit_func = PEEK(8);
PyObject *stack[4] = {NULL, exc, val, tb};
res = PyObject_Vectorcall(exit_func, stack + 1,
3 | PY_VECTORCALL_ARGUMENTS_OFFSET, NULL);
@ -4120,6 +4094,37 @@ main_loop:
DISPATCH();
}
case TARGET(PUSH_EXC_INFO): {
PyObject *type = TOP();
PyObject *value = SECOND();
PyObject *tb = THIRD();
_PyErr_StackItem *exc_info = tstate->exc_info;
SET_THIRD(exc_info->exc_traceback);
SET_SECOND(exc_info->exc_value);
if (exc_info->exc_type != NULL) {
SET_TOP(exc_info->exc_type);
}
else {
Py_INCREF(Py_None);
SET_TOP(Py_None);
}
Py_INCREF(tb);
PUSH(tb);
exc_info->exc_traceback = tb;
Py_INCREF(value);
PUSH(value);
assert(PyExceptionInstance_Check(value));
exc_info->exc_value = value;
Py_INCREF(type);
PUSH(type);
assert(PyExceptionClass_Check(type));
exc_info->exc_type = type;
DISPATCH();
}
case TARGET(LOAD_METHOD): {
/* Designed to work in tandem with CALL_METHOD. */
PyObject *name = GETITEM(names, oparg);
@ -4455,64 +4460,54 @@ error:
}
exception_unwind:
f->f_state = FRAME_UNWINDING;
/* Unwind stacks if an exception occurred */
while (f->f_iblock > 0) {
/* Pop the current block. */
PyTryBlock *b = &f->f_blockstack[--f->f_iblock];
/* We can't use f->f_lasti here, as RERAISE may have set it */
int lasti = INSTR_OFFSET()-1;
PyTryBlock from_table = get_exception_handler(co, lasti);
if (from_table.b_handler < 0) {
// No handlers, so exit.
break;
}
if (b->b_type == EXCEPT_HANDLER) {
UNWIND_EXCEPT_HANDLER(b);
continue;
assert(STACK_LEVEL() >= from_table.b_level);
while (STACK_LEVEL() > from_table.b_level) {
PyObject *v = POP();
Py_XDECREF(v);
}
PyObject *exc, *val, *tb;
int handler = from_table.b_handler;
if (from_table.b_type) {
PyObject *lasti = PyLong_FromLong(f->f_lasti);
if (lasti == NULL) {
goto exception_unwind;
}
UNWIND_BLOCK(b);
if (b->b_type == SETUP_FINALLY) {
PyObject *exc, *val, *tb;
int handler = b->b_handler;
_PyErr_StackItem *exc_info = tstate->exc_info;
/* Beware, this invalidates all b->b_* fields */
PyFrame_BlockSetup(f, EXCEPT_HANDLER, f->f_lasti, STACK_LEVEL());
PUSH(exc_info->exc_traceback);
PUSH(exc_info->exc_value);
if (exc_info->exc_type != NULL) {
PUSH(exc_info->exc_type);
}
else {
Py_INCREF(Py_None);
PUSH(Py_None);
}
_PyErr_Fetch(tstate, &exc, &val, &tb);
/* Make the raw exception data
available to the handler,
so a program can emulate the
Python main loop. */
_PyErr_NormalizeException(tstate, &exc, &val, &tb);
if (tb != NULL)
PyException_SetTraceback(val, tb);
else
PyException_SetTraceback(val, Py_None);
Py_INCREF(exc);
exc_info->exc_type = exc;
Py_INCREF(val);
exc_info->exc_value = val;
exc_info->exc_traceback = tb;
if (tb == NULL)
tb = Py_None;
Py_INCREF(tb);
PUSH(tb);
PUSH(val);
PUSH(exc);
JUMPTO(handler);
if (trace_info.cframe.use_tracing) {
trace_info.instr_prev = INT_MAX;
}
/* Resume normal execution */
f->f_state = FRAME_EXECUTING;
goto main_loop;
}
} /* unwind stack */
/* End the loop as we still have an error */
break;
PUSH(lasti);
}
_PyErr_Fetch(tstate, &exc, &val, &tb);
/* Make the raw exception data
available to the handler,
so a program can emulate the
Python main loop. */
_PyErr_NormalizeException(tstate, &exc, &val, &tb);
if (tb != NULL)
PyException_SetTraceback(val, tb);
else
PyException_SetTraceback(val, Py_None);
if (tb == NULL) {
tb = Py_None;
Py_INCREF(Py_None);
}
PUSH(tb);
PUSH(val);
PUSH(exc);
JUMPTO(handler);
if (trace_info.cframe.use_tracing) {
trace_info.instr_prev = INT_MAX;
}
/* Resume normal execution */
f->f_state = FRAME_EXECUTING;
f->f_lasti = handler;
NEXTOPARG();
goto dispatch_opcode;
} /* main loop */
assert(retval == NULL);
@ -4777,6 +4772,102 @@ fail:
}
/* Exception table parsing code.
* See Objects/exception_table_notes.txt for details.
*/
static inline unsigned char *
parse_varint(unsigned char *p, int *result) {
int val = p[0] & 63;
while (p[0] & 64) {
p++;
val = (val << 6) | (p[0] & 63);
}
*result = val;
return p+1;
}
static inline unsigned char *
scan_back_to_entry_start(unsigned char *p) {
for (; (p[0]&128) == 0; p--);
return p;
}
static inline unsigned char *
skip_to_next_entry(unsigned char *p) {
for (; (p[0]&128) == 0; p++);
return p;
}
static inline unsigned char *
parse_range(unsigned char *p, int *start, int*end)
{
p = parse_varint(p, start);
int size;
p = parse_varint(p, &size);
*end = *start + size;
return p;
}
static inline void
parse_block(unsigned char *p, PyTryBlock *block) {
int depth_and_lasti;
p = parse_varint(p, &block->b_handler);
p = parse_varint(p, &depth_and_lasti);
block->b_level = depth_and_lasti >> 1;
block->b_type = depth_and_lasti & 1;
}
#define MAX_LINEAR_SEARCH 40
static PyTryBlock
get_exception_handler(PyCodeObject *code, int index)
{
PyTryBlock res;
unsigned char *start = (unsigned char *)PyBytes_AS_STRING(code->co_exceptiontable);
unsigned char *end = start + PyBytes_GET_SIZE(code->co_exceptiontable);
/* Invariants:
* start_table == end_table OR
* start_table points to a legal entry and end_table points
* beyond the table or to a legal entry that is after index.
*/
if (end - start > MAX_LINEAR_SEARCH) {
int offset;
parse_varint(start, &offset);
if (offset > index) {
res.b_handler = -1;
return res;
}
do {
unsigned char * mid = start + ((end-start)>>1);
mid = scan_back_to_entry_start(mid);
parse_varint(mid, &offset);
if (offset > index) {
end = mid;
}
else {
start = mid;
}
} while (end - start > MAX_LINEAR_SEARCH);
}
unsigned char *scan = start;
while (scan < end) {
int start_offset, size;
scan = parse_varint(scan, &start_offset);
if (start_offset > index) {
break;
}
scan = parse_varint(scan, &size);
if (start_offset + size > index) {
parse_block(scan, &res);
return res;
}
scan = skip_to_next_entry(scan);
}
res.b_handler = -1;
return res;
}
PyFrameObject *
_PyEval_MakeFrameVector(PyThreadState *tstate,