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Date: Tue, 11 Mar 2003 15:35:56 -0800
From: george anzinger <george@mvista.com>
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To: Linus Torvalds <torvalds@transmeta.com>
CC: Andrew Morton <akpm@digeo.com>, felipe_alfaro@linuxmail.org,
       cobra@compuserve.com, linux-kernel@vger.kernel.org
Subject: Re: Runaway cron task on 2.5.63/4 bk?
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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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-- 
George Anzinger   george@mvista.com
High-res-timers:  http://sourceforge.net/projects/high-res-timers/
Preemption patch: http://www.kernel.org/pub/linux/kernel/people/rml

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