Linus Torvalds wrote:
>
> 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.
Often the 64/32 -> [32,32] is all that is needed and that is even
faster if we could get to it.
>
> Linus
>
> -
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