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

Mark Dickinson dickinsm at gmail.com
Mon Nov 10 18:42:05 CET 2008

On Mon, Nov 10, 2008 at 4:26 PM, Nick Craig-Wood <nick at craig-wood.com> wrote:

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).

My x86 assembler is also weak (or perhaps I should say nonexistent), but I think this does exactly what the C standard says it should: that is, it returns just the low 32-bits of the product.

Looking at the assembler, I think the imull does a 32-bit by 32-bit multiply and puts its (truncated) result back into the 32-bit register eax. I'd guess that the 64-bit result is being returned to the calling routine in registers edx (high 32 bits) and eax (low 32 bits); this explains why edx has to be zeroed with the 'xorl' instruction.

And if we were really expecting a 64-bit result then there should be an unsigned multiply (mull) there instead of a signed multiply (imull); of course they're the same modulo 2**32, so for a 32-bit result it doesn't matter which is used.

Mark

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