Messages in this thread | | | Date | Tue, 11 Mar 2003 15:02:31 -0800 (PST) | From | Linus Torvalds <> | Subject | Re: Runaway cron task on 2.5.63/4 bk? |
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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
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