with should be as fast as try/finally

[jyasskin]

BPO	2179
Nosy	@rhettinger, @jcea, @amauryfa, @ncoghlan, @abalkin, @tiran, @benjaminp
Files	faster_with.patch: proof of concept faster_with.patch: complete

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2008-03-07.14:17:51.663>
created_at = <Date 2008-02-24.23:10:40.175>
labels = ['interpreter-core', 'type-feature']
title = 'with should be as fast as try/finally'
updated_at = <Date 2008-03-09.17:03:40.472>
user = 'https://bugs.python.org/jyasskin'

bugs.python.org fields:

activity = <Date 2008-03-09.17:03:40.472>
actor = 'jyasskin'
assignee = 'none'
closed = True
closed_date = <Date 2008-03-07.14:17:51.663>
closer = 'ncoghlan'
components = ['Interpreter Core']
creation = <Date 2008-02-24.23:10:40.175>
creator = 'jyasskin'
dependencies = []
files = ['9589', '9590']
hgrepos = []
issue_num = 2179
keywords = ['patch']
message_count = 10.0
messages = ['62951', '63165', '63167', '63197', '63200', '63323', '63353', '63355', '63356', '63425']
nosy_count = 10.0
nosy_names = ['nnorwitz', 'collinwinter', 'rhettinger', 'jcea', 'amaury.forgeotdarc', 'ncoghlan', 'belopolsky', 'christian.heimes', 'jyasskin', 'benjamin.peterson']
pr_nums = []
priority = 'normal'
resolution = 'accepted'
stage = None
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue2179'
versions = ['Python 2.6']

[jyasskin]

Currently, 'with' costs about .2us over try/finally:

$ ./python.exe -m timeit -s 'import thread; lock =
thread.allocate_lock()' 'lock.acquire()' 'try: pass' 'finally:
lock.release()'
1000000 loops, best of 3: 0.617 usec per loop
$ ./python.exe -m timeit -s 'import thread; lock =
thread.allocate_lock()' 'with lock: pass'
1000000 loops, best of 3: 0.774 usec per loop

Since it's doing the same thing (and calling the same C functions to do
the lock manipulation), it shouldn't take more time. The bytecodes
associated with the two options look like:

2. with lock:

3.  pass
   

2 0 LOAD_GLOBAL 0 (lock)
3 DUP_TOP
4 LOAD_ATTR 1 (exit)
7 STORE_FAST 0 (_[1])
10 LOAD_ATTR 2 (enter)
13 CALL_FUNCTION 0
16 POP_TOP
17 SETUP_FINALLY 4 (to 24)

3 20 POP_BLOCK
21 LOAD_CONST 0 (None)
>> 24 LOAD_FAST 0 ([1])
27 DELETE_FAST 0 ([1])
30 WITH_CLEANUP
31 END_FINALLY
32 LOAD_CONST 0 (None)
35 RETURN_VALUE

6. lock.acquire()
7. try:

8.  pass
   

9. finally:

10. lock.release()
    

6 0 LOAD_GLOBAL 0 (lock)
3 LOAD_ATTR 1 (acquire)
6 CALL_FUNCTION 0
9 POP_TOP

7 10 SETUP_FINALLY 4 (to 17)

8 13 POP_BLOCK
14 LOAD_CONST 0 (None)

10 >> 17 LOAD_GLOBAL 0 (lock)
20 LOAD_ATTR 2 (release)
23 CALL_FUNCTION 0
26 POP_TOP
27 END_FINALLY
28 LOAD_CONST 0 (None)
31 RETURN_VALUE

The major difference I see is the extra local variable (_[1]) used by
with. It looks like the compiler ought to be able to save that on the
stack instead, and save 3 opcodes. Neal Norwitz suggested that, if that
optimization is impossible, WITH_CLEANUP could be modified to take the
variable as a parameter, which would let it absorb the LOAD_FAST and
DELETE_FAST instructions.

I've added everyone on the previous bug to the nosy list. Sorry if you
don't care. :)

[ncoghlan]

A closer approximation of what the with statement is doing would be:

exit = lock.release()
lock.acquire()
try:
    pass
finally:
    exit()

The problem with trying to store the result of the retrieval of __exit__
on the stack is that we need to leave the context manager itself on top
of the stack for the next LOAD_ATTR opcode (when we're retrieving the
__enter__ method).

However, changing WITH_CLEANUP to take an argument indicating which
local variable holds the bound __exit__ method sounds like it might work.

[ncoghlan]

Scratch the parentheses on that first line of sample code in my previous
comment (isn't copy and paste wonderful?)

[jyasskin]

Here's a proof-of-concept patch that keeps the __exit__ method on the
stack. It uses ROT_TWO to stuff it under the context object instead of
storing it into a temporary. (Thanks Nick for pointing out that problem
before I had to waste time on it.) test_with passes, although I need to
update several more things and maybe fix a refleak.

The patch changes the compilation of:

    def with_(l):
        with l:
            pass

from

4 0 LOAD_FAST 0 (l)
3 DUP_TOP
4 LOAD_ATTR 0 (exit)
7 STORE_FAST 1 (_[1])
10 LOAD_ATTR 1 (enter)
13 CALL_FUNCTION 0
16 POP_TOP
17 SETUP_FINALLY 4 (to 24)

5 20 POP_BLOCK
21 LOAD_CONST 0 (None)
>> 24 LOAD_FAST 1 ([1])
27 DELETE_FAST 1 ([1])
30 WITH_CLEANUP
31 END_FINALLY
32 LOAD_CONST 0 (None)
35 RETURN_VALUE

to

4 0 LOAD_FAST 0 (l)
3 DUP_TOP
4 LOAD_ATTR 0 (exit)
7 ROT_TWO
8 LOAD_ATTR 1 (enter)
11 CALL_FUNCTION 0
14 POP_TOP
15 SETUP_FINALLY 4 (to 22)

5 18 POP_BLOCK
19 LOAD_CONST 0 (None)
>> 22 WITH_CLEANUP
23 END_FINALLY
24 LOAD_CONST 0 (None)
27 RETURN_VALUE

And speeds it up from:

$ ./python.exe -m timeit -s 'import thread; lock =
thread.allocate_lock()' 'with lock: pass'
1000000 loops, best of 3: 0.832 usec per loop

to:

$ ./python.exe -m timeit -s 'import thread; lock =
thread.allocate_lock()' 'with lock: pass'
1000000 loops, best of 3: 0.762 usec per loop

That's only half of the way to parity with try/finally:

$ ./python.exe -m timeit -s 'import thread; lock =
thread.allocate_lock()' 'lock.acquire()' 'try: pass' 'finally:
lock.release()'
1000000 loops, best of 3: 0.638 usec per loop

What's strange is that calling __enter__ and __exit__ in a try/finally
block brings the speed back to the faster 'with' speed, even though they
call the same C functions:

$ ./python.exe -m timeit -s 'import thread; lock =
thread.allocate_lock()' 'lock.__enter__()' 'try: pass' 'finally:
lock.__exit__()'
1000000 loops, best of 3: 0.754 usec per loop

Any ideas?

[jyasskin]

Now with documentation, a working test_compile, and one less refleak.

[amauryfa]

What's strange is that calling __enter__ and __exit__ in a
try/finally block brings the speed back to the faster 'with' speed,
even though they call the same C functions

Looking carefully at the code, there are two reasons for this:

• LockType has no methods! try "dir(thread.LockType)". Instead, LockType
  defines a tp_getattr which does a *linear* search in a PyMethodDef
  array. First items are served faster...
• After converting this to use the usual tp_methods slot, there is still
  a performance difference, due to the fact that release() is a
  METH_NOARGS method, while __exit__() uses METH_VARARGS.

Phew.

[ncoghlan]

Patch applied cleanly for me and all tests pass.

It also looked good on a visual scan over the diff text.

[ncoghlan]

I went ahead and committed the change to the bytecode generation as r61290.

The deficiencies in the lock implementation should probably be raised as
a separate issue, so I'm closing this one.

[amauryfa]

Hm, my tests do not see any speedup with this patch.
I used VS2005 on win2K, and VS2008 on winXP.
Timings are very similar before and after this patch.

Maybe the optimization is only useful with gcc?

[jyasskin]

Thanks Nick and Amaury!

Amaury, what times are you seeing? It could be a just-gcc speedup, but I
wouldn't have thought so since the patch eliminates 2 times around the
eval loop. It could be that the overhead of WITH_CLEANUP is high enough
to drown out those iterations. You had mentioned optimizing the
PyObject_CallFunctionObjArgs() call before. If you're still seeing with
significantly slower than try, that's probably the right place to look.

Here are my current timings. To avoid the lock issues, I wrote
simple_cm.py containing:

class CM(object):
    def __enter__(self):
        pass
    def __exit__(self, *args):
        pass

$ ./python.exe -m timeit -s 'import simple_cm; cm = simple_cm.CM()'
'with cm: pass'
1000000 loops, best of 3: 0.885 usec per loop
$ ./python.exe -m timeit -s 'import simple_cm; cm = simple_cm.CM()'
'cm.__enter__()' 'try: pass' 'finally: cm.__exit__()'
1000000 loops, best of 3: 0.858 usec per loop

If __exit__ doesn't contain *args (making it not a context manager), the
try/finally time goes down to:
$ ./python.exe -m timeit -s 'import simple_cm; cm = simple_cm.CM()'
'cm.__enter__()' 'try: pass' 'finally: cm.__exit__()'
1000000 loops, best of 3: 0.619 usec per loop

I think in theory, with could be slightly faster than finally with the
same argument list, but it's pretty close now.