[Python-Dev] 64 bit units in PyLong

[Mark Dickinson]

On Mon, Jul 3, 2017 at 5:52 AM, Siyuan Ren <netheril96 at gmail.com> wrote:
> The current PyLong implementation represents arbitrary precision integers in
> units of 15 or 30 bits. I presume the purpose is to avoid overflow in
> addition , subtraction and multiplication. But compilers these days offer
> intrinsics that allow one to access the overflow flag, and to obtain the
> result of 64 bit multiplication as a 128 bit number. Or at least on x86-64,
> which is the dominant platform.  Any reason why it is not done?

Portability matters, so any use of these intrinsics would likely also
have to be accompanied by fallback code that doesn't depend on them,
as well as some buildsystem complexity to figure out whether those
intrinsics are supported or not. And then the Objects/longobject.c
would suffer in terms of simplicity and readability, so there would
have to be some clear gains to offset that. Note that the typical
Python workload does not involve thousand-digit integers: what would
matter would be performance of smaller integers, and it seems
conceivable that 64-bit limbs would speed up those operations simply
because so many more integers would become single-limb and so there
would be more opportunities to take fast paths, but there would need
to be benchmarks demonstrating that.

Oh, and you'd have to rewrite the power algorithm, which currently
depends on the size of a limb in bytes being a multiple of 5. :-)

-- 
Mark