Consider this small example (you might have to run sample program multiple
times to see a difference):

$ cat dis-closure.py
import dis

def adder(a, b):
    def add():
        return a + b
    return add

print(dis.dis(adder(1, 2).__code__))

$  ./python.exe dis-closure.py
  5           0 LOAD_DEREF               0 (a) 
              3 LOAD_DEREF               1 (b) 
              6 BINARY_ADD           
              7 RETURN_VALUE         
None
$  ./python.exe dis-closure.py
  5           0 LOAD_DEREF               1 (a) 
              3 LOAD_DEREF               0 (b) 
              6 BINARY_ADD           
              7 RETURN_VALUE         
None

The order of 'co_cellvars' and 'co_freevars' can be different from compile to
compile, thus the bytecode can be different from compile to compile.

This is due to the fact that these variable sets are managed with hashes and
the ordering may come out different when the names in the hashes are given
indexes (via 'dictbytype' in 'compile.c').

I am not sure if these are the only areas that causes bytecode generation
to be non-deterministic. I found this behavior surprising.

I might come to regret asking this, but so what? Is this actually causing you issues, or are you literally just finding "this behavior surprising" and that's it? I mean we could simply sort the tuples, but I don't know what kind of performance hit that would cause.

On Mon, Jul 16, 2012 at 8:11 AM, Brett Cannon <report@bugs.python.org> wrote:

I might come to regret asking this, but so what? Is this actually causing you issues,
or are you literally just finding "this behavior surprising" and that's it?

I noticed it in the context of working on bpo-15352. There are still
a few issue in the
build system where 'importlib.h' isn't regenerated when it needs to be
(e.g. when
the marshaling code is changed). When investigating that issue I noticed that
trivial changes (e.g. touch _bootstrap.py) can cause differences in
'importlib.h'.
Those differences are mainly due to the cell and free var indexes being
reordered.

Since the bytecode generation isn't deterministic, there can be some noisy diffs
in your working copy when mucking around with the files related to the frozen
importlib. Thus if we ever wanted to just always generate importlib.h because
we can't be sure which changed source files should cause importlib.h to be
regenerated, then the non-determinism would be a real annoyance.

That is the only concrete issue I currently am aware of. This issue may not
actually be worth addressing, but I felt it was worthy of discussion.

At least we know the hash randomisation is working :)

Spurious changes when freezing modules seems like a legitimate reason to fix it - the import benchmarks would probably give the compiler enough of a workout to highlight if the sorting is excessively time consuming.

Ditto. I think predictability of bytecode generation is useful, e.g. for make-like tools that examine content, or for unit testing.

OK, so it sounds like we need to do the equivalent of sorting those tuples when generating the bytecode. That would suggest that probably need to tweak Python/compile.c to make it deterministic.

I haven't done any benchmarking (yet), but here is a patch implementing the basic sorting approach.

New patch that addresses feedback from Brett's code review (thanks!).

OK, I ran the Python benchmark suite. I don't have much experience with the benchmark suite so let me know if I did something wrong or need to do more. I ran the following:

python perf.py -b 2n3 ../cpython/python.exe ../cpython/python-with-sorting.exe)

which produces:

Report on Darwin quicksilver 11.4.0 Darwin Kernel Version 11.4.0: Mon Apr 9 19:32:15 PDT 2012; root:xnu-1699.26.8~1/RELEASE_X86_64 x86_64 i386
Total CPU cores: 4

### call_method_slots ###
Min: 1.925941 -> 1.883251: 1.02x faster
Avg: 1.931082 -> 1.887380: 1.02x faster
Significant (t=39.92)
Stddev: 0.01233 -> 0.00526: 2.3439x smaller
Timeline: http://tinyurl.com/8943trh

### call_simple ###
Min: 1.447890 -> 1.367729: 1.06x faster
Avg: 1.451100 -> 1.371689: 1.06x faster
Significant (t=118.46)
Stddev: 0.00497 -> 0.00654: 1.3168x larger
Timeline: http://tinyurl.com/7j7uppg

### fastpickle ###
Min: 3.313799 -> 3.240090: 1.02x faster
Avg: 3.328812 -> 3.248734: 1.02x faster
Significant (t=44.45)
Stddev: 0.00918 -> 0.00883: 1.0404x smaller
Timeline: http://tinyurl.com/6tsk347

### iterative_count ###
Min: 1.277516 -> 1.240536: 1.03x faster
Avg: 1.280186 -> 1.242743: 1.03x faster
Significant (t=38.46)
Stddev: 0.00534 -> 0.00435: 1.2288x smaller
Timeline: http://tinyurl.com/d7vzqjf

### json_load ###
Min: 2.944439 -> 2.877168: 1.02x faster
Avg: 2.950717 -> 2.889950: 1.02x faster
Significant (t=23.96)
Stddev: 0.00737 -> 0.01635: 2.2179x larger
Timeline: http://tinyurl.com/6onbxzf

### regex_compile ###
Min: 2.354580 -> 2.417407: 1.03x slower
Avg: 2.361253 -> 2.421576: 1.03x slower
Significant (t=-44.01)
Stddev: 0.00780 -> 0.00575: 1.3555x smaller
Timeline: http://tinyurl.com/6wryn9v

### richards ###
Min: 0.905023 -> 0.883842: 1.02x faster
Avg: 0.908634 -> 0.888653: 1.02x faster
Significant (t=18.96)
Stddev: 0.00479 -> 0.00571: 1.1911x larger
Timeline: http://tinyurl.com/chgz4ge

### silent_logging ###
Min: 0.432982 -> 0.462848: 1.07x slower
Avg: 0.434553 -> 0.465406: 1.07x slower
Significant (t=-32.15)
Stddev: 0.00390 -> 0.00556: 1.4256x larger
Timeline: http://tinyurl.com/d3penzy

The following not significant results are hidden, use -v to show them:
call_method, call_method_unknown, fastunpickle, float, formatted_logging, json_dump, nbody, normal_startup, nqueens, pidigits, regex_effbot, regex_v8, simple_logging, startup_nosite, threaded_count, unpack_sequence.

You ran the benchmarks correctly, but I was more worried about compilation time than execution time (changing which index is for what really shouldn't make a difference in performance, and the benchmarks show that). If you want to test using the importlib benchmarks, run ./python.exe -m importlib.test.benchmark and compare the output (run with -h to see all the options).

Oh, cool -- I didn't know about the importlib benchmarks. Thanks Brett.

Here are the results (OS X 10.7.4, Dual 1.7GHz Core i5, 4 GB RAM):

$ ./python.exe -m importlib.test.benchmark
Measuring imports/second over 1 second, best out of 3
Entire benchmark run should take about 33 seconds
Using <function __import__ at 0x1063bdd40> as __import__

sys.modules [ 41831 41812 40914 ] best is 41,831
Built-in module [ 20327 20441 19775 ] best is 20,441
Source writing bytecode: small [ 2108 2119 2117 ] best is 2,119
Source w/o bytecode: small [ 5093 5075 5106 ] best is 5,106
Source w/ bytecode: small [ 6021 5915 5997 ] best is 6,021
Source writing bytecode: tabnanny [ 379 379 379 ] best is 379
Source w/o bytecode: tabnanny [ 448 453 456 ] best is 456
Source w/ bytecode: tabnanny [ 2270 2279 2279 ] best is 2,279
Source writing bytecode: decimal [ 28 27 28 ] best is 28
Source w/o bytecode: decimal [ 30 30 30 ] best is 30
Source w/ bytecode: decimal [ 256 254 259 ] best is 259

$ ./python-with-sorting.exe -m importlib.test.benchmark
Measuring imports/second over 1 second, best out of 3
Entire benchmark run should take about 33 seconds
Using <function __import__ at 0x104b01d40> as __import__

sys.modules [ 42369 42203 42348 ] best is 42,369
Built-in module [ 17060 16358 20486 ] best is 20,486
Source writing bytecode: small [ 2118 2112 2127 ] best is 2,127
Source w/o bytecode: small [ 5121 5098 5125 ] best is 5,125
Source w/ bytecode: small [ 6015 5652 6046 ] best is 6,046
Source writing bytecode: tabnanny [ 367 368 366 ] best is 368
Source w/o bytecode: tabnanny [ 439 429 439 ] best is 439
Source w/ bytecode: tabnanny [ 2267 2274 2277 ] best is 2,277
Source writing bytecode: decimal [ 27 27 27 ] best is 27
Source w/o bytecode: decimal [ 28 28 28 ] best is 28
Source w/ bytecode: decimal [ 257 259 259 ] best is 259

Pretty similar results either way.

Yep, so I consider the performance worry dealt with.

Agreed, so definite +1 from me.

So I think that just leaves one other person to verify the patch works as expected and then commit it. I can hopefully later this week, but no sooner than Friday, so if someone can get to it faster that would be great.

I can commit today unless you think we need someone else to independently verify the patch.

BTW, I think this should be committed on 2.7 and 3.2 as well since the same issue can happen when throwing -R. Any objections?

Nah, you are a committer and thus qualify as the second review. =)

As for backporting, it only affects regeneration of bytecode, so I don't see any major harm in it (might surprise some people, but there is a legitimate reason for fixing this).

New changeset 7eb0180c1734 by Meador Inge in branch '2.7':
Issue bpo-15368: make bytecode generation deterministic.
http://hg.python.org/cpython/rev/7eb0180c1734

New changeset 79d54fba49b3 by Meador Inge in branch '3.2':
Issue bpo-15368: make bytecode generation deterministic.
http://hg.python.org/cpython/rev/79d54fba49b3

New changeset 578066372485 by Meador Inge in branch 'default':
Issue bpo-15368: make bytecode generation deterministic.
http://hg.python.org/cpython/rev/578066372485

Committed. Thanks for the reviews y'all.

Meador, you have forgotten to fix the error "PyList_GET_SIZE(src)". src is not list, should be "PyList_GET_SIZE(sorted_keys)".

D'oh! Thanks for catching that Serhiy! Fixing now. Sorry about that.

New changeset 47e074f6ca26 by Meador Inge in branch '2.7':
Issue bpo-15368: fixing variable typo.
http://hg.python.org/cpython/rev/47e074f6ca26

New changeset 1c46f2ede4cb by Meador Inge in branch '3.2':
Issue bpo-15368: fixing variable typo.
http://hg.python.org/cpython/rev/1c46f2ede4cb

New changeset 6502de91610d by Meador Inge in branch 'default':
Issue bpo-15368: fixing variable typo.
http://hg.python.org/cpython/rev/6502de91610d