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From: Manfred Spraul on 20 May 2010 14:20
On 05/20/2010 09:00 AM, Nick Piggin wrote:
> All the ipc ids and sequences are signed integers, so power of 2
> sequence multiplier does not work so well. Convert it to use shifts,
> which improves generated code particularly in ipc_lock/ipc_lock_check
> fast paths.
>
>
Have you checked the asm output?
I would expect that gcc auto-optimizes constant divides by power-of-two.

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Manfred
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From: Nick Piggin on 20 May 2010 21:40
On Thu, May 20, 2010 at 08:16:24PM +0200, Manfred Spraul wrote:
> On 05/20/2010 09:00 AM, Nick Piggin wrote:
> >All the ipc ids and sequences are signed integers, so power of 2
> >sequence multiplier does not work so well. Convert it to use shifts,
> >which improves generated code particularly in ipc_lock/ipc_lock_check
> >fast paths.
> >
> Have you checked the asm output?

Yep, it's quite improved (although this isn't a particularly hot
path I thought it is a good cleanup).


> I would expect that gcc auto-optimizes constant divides by power-of-two.

It can't with signed I guess.
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