[Python-Dev] Optionally using GMP to implement long if available (original) (raw)

Mark Dickinson dickinsm at gmail.com
Tue Nov 4 12:00:11 CET 2008

[Martin v. Löwis]

Perhaps Tim Peters should also comment here - but if you can come up with a patch that does that in a portable way, I would be in favor. The challenge, ISTM, is to make it still compile on all systems (regardless of how inefficient it might be on minor platforms).

I've just uploaded a patch to the issue tracker that allows Python to use 30-bit digits on platforms having a 64-bit integer type:

http://bugs.python.org/issue4258

[Tim Peters]

Note that while 32x32->64 multiply is supported by x86 HW, that doesn't mean there's a uniform way to get at this HW capability across C compilers. In contrast, (at least) efficient HW 15x15->30 multiply is universally spelled in C via "i*j" :-)

If we're talking about standards and portability, doesn't "i*j" fail on those (nonexistent?) platforms where the 'int' type is only 16-bits? Shouldn't this be something like "(long)i * j" instead?

And for 32x32 -> 64, can't this simply be replaced by "(uint64_t)i * j", where uint64_t is as in C99? I'd hope that most compilers would manage to turn this into the appropriate 32x32-bit hardware multiply.

I agree that very-long-integer optimizations probably don't really belong in Python, but this patch should also provide significant benefits for short and medium-sized integers. I guess I need to go away and do some benchmarking...

Mark

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