On Sun, Jan 16, 2022 at 12:08 PM Mark Dickinson <dicki...@gmail.com> wrote:
> So gcc is anticipating divisions by 10 and introducing special-case > divide-by-reciprocal-multiply code for that case, and presumably the > profile generated for the PGO backs up this being a common enough case, so > we end up with the above code in the final compilation. > Nope, that's not what's happening. This analysis is backwards, and unfairly attributes to GCC the apparently arbitrary choice to optimise division by 10. But it's not GCC's fault; it's ours. What's *actually* happening is that GCC is simply recording values for n used in calls to divrem1 (via the -fprofile-values option, which is implied by -fprofile-generate, which is used as a result of the --enable-optimizations configure script option). It's then noticing that in our profile task (which consists of a selection of Lib/test/test_*.py test files) we most often do divisions by 10, and so it optimizes that case. To test this hypothesis I added a large number of tests for division by 17 in test_long.py, and then recompiled from scratch (again with --enable-optimizations). Here are the results: root@341b5fd44b23:/home/cpython# ./python -m timeit -n 1000000 -s "x=10**1000; y=10" "x//y" 1000000 loops, best of 5: 1.14 usec per loop root@341b5fd44b23:/home/cpython# ./python -m timeit -n 1000000 -s "x=10**1000; y=17" "x//y" 1000000 loops, best of 5: 306 nsec per loop root@341b5fd44b23:/home/cpython# ./python -m timeit -n 1000000 -s "x=10**1000; y=1" "x//y" 1000000 loops, best of 5: 1.14 usec per loop root@341b5fd44b23:/home/cpython# ./python -m timeit -n 1000000 -s "x=10**1000; y=2" "x//y" 1000000 loops, best of 5: 1.15 usec per loop As expected, division by 17 is now optimised; division by 10 is as slow as division by other small scalars. -- Mark
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