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

Nick Craig-Wood nick at craig-wood.com
Mon Nov 10 17:26:30 CET 2008

Tim Peters <tim.peters at gmail.com> wrote:

> And for 32x32 -> 64, can't this simply be replaced by "(uint64t)i * j", > where uint64t is as in C99? I'd hope that most compilers would > manage to turn this into the appropriate 32x32-bit hardware multiply.

1. That's C99, not C89, and therefore less portable. 2. On platforms that support it, this is at least 64x64->64 multiplication, potentially much more expensive than the 32x32->64 (or 31x31->62?) flavor you /intend/ to move to. 3. There's no way to know exactly what compilers will do with this short of staring at generated code.

I've relied in the past for the compiler generating a 32*32->64 bit multiply for this code

#include <stdint.h>

uint64_t mul(uint32_t a, uint32_t b) { return a*b; }

Looking at the assembler it produces (x86)

mul: pushl %ebp xorl %edx, %edx movl %esp, %ebp movl 12(%ebp), %eax imull 8(%ebp), %eax popl %ebp ret

Which I'm pretty sure is a 32x32->64 bit mul (though my x86 assembler foo is weak).

I think a compiler would have to be pretty stupid not to take this optimisation... But then there are some pretty stupid compilers out there!

-- Nick Craig-Wood <nick at craig-wood.com> -- http://www.craig-wood.com/nick

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