Baseline performance (9e6ac83acae):
    
$ ./python -m timeit "12345678 == 12345678.0"
5000000 loops, best of 5: 40 nsec per loop
$ ./python -m timeit "1 == 1.0"
10000000 loops, best of 5: 38.8 nsec per loop
$ ./python -m timeit "(1234578987654321).bit_length()"
10000000 loops, best of 5: 39.4 nsec per loop

Upcoming PR:

$ ./python -m timeit "12345678 == 12345678.0"
10000000 loops, best of 5: 34.3 nsec per loop
$ ./python -m timeit "1 == 1.0"
10000000 loops, best of 5: 34.4 nsec per loop
$ ./python -m timeit "(1234578987654321).bit_length()"
10000000 loops, best of 5: 36.4 nsec per loop

Thanks for the idea, and the PR!

To be useful, this would need a bit of tweaking: we can't assume that a digit is always `unsigned long` (in fact, I'd expect that to be rare on 64-bit non-Windows systems, where `unsigned long` typically has 64 bits, and `digit` should be using a 32-bit type), so we'd need to identify and use the most appropriate variant of clz/clzl/clzll somehow.

I think it would also make sense to move the detection of the existence of `__builtin_clz` and friends into the configuration machinery: these builtins aren't just restricted to GCC (clang supports them as well), and that would allow other platforms to provide their own substitutes by modifying `pyport.h`. Ideally, the configuration machinery #defines a HAVE_CLZ variable, and then in longobject.c we do an #ifdef HAVE_CLZ ...

We also need to trade-off the additional complication in the implementation against the benefits: though we do certainly care about the speed, speed at all costs isn't the target here; readability, portability and maintainability of the source counts, too. It'll probably be easier to weigh those factors once there's an implementation that addresses the above points.

I think we can assume that digit is no larger than unsigned long, otherwise PyLong_AsLong() and like wouldn't work.

True enough. It looks as though someone (*cough*) already documented that restriction, too: https://github.com/mdickinson/cpython/blob/v3.6.0/Include/longintrepr.h#L28-L30

Thanks for the review.

I pushed a change to check if clz can be used (`sizeof(digit) <= sizeof(unsigned int)`). Otherwise use clzl. I believe the latter should be the most common, since unsigned long has 32bits. As you say unsigned long long should never be needed.

Btw. mathmodule.c currently duplicates the function: https://github.com/python/cpython/blob/master/Modules/mathmodule.c#L1317. It might be worth factoring it out, but I don't know where to put it.

Oops. Actually clz should commonly be enough. And platforms where clz and clzl are different (<=> unsigned int and unsigned long are different) should be rare.

The change is an optimization, so it requires a benchmark, think you provided. Ok. But the speedup is a few nanoseconds on a function which takes currently 40 nanoseconds. It's not really what I would call significant:

$ ./python -m perf compare_to ref.json patch.json  --table
+---------------------+---------+-----------------------------+
| Benchmark           | ref     | patch                       |
+=====================+=========+=============================+
| int==float 8 digits | 39.2 ns | 37.9 ns: 1.03x faster (-3%) |
+---------------------+---------+-----------------------------+
| int==float 1 digit  | 38.0 ns | 37.9 ns: 1.00x faster (-0%) |
+---------------------+---------+-----------------------------+
| int.bit_length()    | 42.0 ns | 41.2 ns: 1.02x faster (-2%) |
+---------------------+---------+-----------------------------+

(See attached bench_bit_length.py script.)

So I'm not really convinced that the change is useful. Is bit_length() used in hot loops?

> $ ./python -m timeit "12345678 == 12345678.0"
> 5000000 loops, best of 5: 40 nsec per loop

By the way, I check the bytecode to make sure that the compiler doesn't optimize that. I'm suprised that it's not replaced with True!

Is there a reason to perform the test at runtime?

There are not good reasons to optimize this case at compile time. This is very obscure way of writing True.

I don't think a 3% improvement on a micro-benchmark is worth it (this will translate into 0% improvement on real-world workloads).  Are there other uses of _Py_bit_length() that show bigger benefits?

I concur with Antoine and suggest to reject this issue.

In some microbenchmarks this can give up to 15%.

$ ./python.patched -m perf timeit -q --compare-to=./python.default -s "a = list(map(float, range(10000)))" "12345 in a"
Mean +- std dev: [python.default] 1.28 ms +- 0.11 ms -> [python.patched] 1.12 ms +- 0.07 ms: 1.15x faster (-13%)

But even 15% speed up in particular microbenchmarks looks too small to me for such complex change.

Closing.

If this issue is closed by "not a big performance improvement", maybe the XXX in mathmoudle.c should be take off?

"""
/* XXX: This routine does more or less the same thing as
 * bits_in_digit() in Objects/longobject.c.  Someday it would be nice to
 * consolidate them.  On BSD, there's a library function called fls()
 * that we could use, and GCC provides __builtin_clz().
 */
"""

I'm in favor of documenting in comments decisions to not micro-optimize
such code. I did something similar in ceval.c for 1+1.

I reopen the issue since PR 20739 was created.

New changeset 794e7d1ab2d7afe70fe0dd87ca8174ac860413e4 by Niklas Fiekas in branch 'master':
bpo-29782: Consolidate _Py_Bit_Length() (GH-20739)
https://github.com/python/cpython/commit/794e7d1ab2d7afe70fe0dd87ca8174ac860413e4

Thanks Niklas Fiekas for your tenacity :-)