[Python-Dev] Optionally using GMP to implement long if available
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)
> pushl %ebp
> xorl %edx, %edx
> movl %esp, %ebp
> movl 12(%ebp), %eax
> imull 8(%ebp), %eax
> popl %ebp
> 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.
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