Re: Runaway cron task on 2.5.63/4 bk?

Linus Torvalds
2003-03-11T22:54:15Z

On Tue, 11 Mar 2003, Andrew Morton wrote:
>
> gcc will generate 64bit * 64bit multiplies without resorting to
> any library code

However, gcc is unable to do-the-right-thing and generate 32x32->64
multiplies, or 32x64->64 multiplies, even though those are both a _lot_
faster than the full 64x64->64 case.

And in quite a _lot_ of cases, that's actually what you want. It might
actually make sense to add a "do_mul()" thing to allow architectures to do
these cases right, since gcc doesn't.

> and you can probably do the division with do_div().

Yes. This is the same issue - gcc will always promote a 64-bit divide to
be _fully_ 64-bit, even if the mixed-size 64/32 -> [64,32] case is much
faster and simpler. Which is why do_div() exists in the first place.

Linus