linux-kernel - Re: [tip:core/locking] locking, x86: Slightly shorten __ticket_spin

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Message-ID: <4B1675CA.1020504@redhat.com>
Date:	Wed, 02 Dec 2009 16:12:26 +0200
From:	Avi Kivity <avi@...hat.com>
To:	Jan Beulich <JBeulich@...ell.com>
CC:	Ingo Molnar <mingo@...e.hu>, a.p.zijlstra@...llo.nl,
	tglx@...utronix.de, torvalds@...ux-foundation.org,
	mingo@...hat.com, npiggin@...e.de, linux-kernel@...r.kernel.org,
	linux-tip-commits@...r.kernel.org, hpa@...or.com
Subject: Re: [tip:core/locking] locking, x86: Slightly shorten	 __ticket_spin_trylock()

On 12/02/2009 04:06 PM, Jan Beulich wrote:
>>>> Ingo Molnar<mingo@...e.hu>  02.12.09 14:29>>>
>>>>          
>> at first quick sight, this bit looks odd:
>>
>> +       union { int i; bool b; } new;
>>
>> +       return new.b;
>>
>> shouldnt that be short based, to work correctly in the 0-255 CPUs case?
>>      
> No, I can't see why. In both instances, we're using (and had been
> using previously, just with the added movzbl) the outcome of a
> setCC instruction, which produces valid bool (single byte) values.
> It is precisely that reason why I needed to introduce these unions,
> since the upper bytes of the register aren't valid (and shouldn't be
> looked at by the caller).
>
>    

Wouldn't 'u8 ret', as an additional argument be sufficient? gcc still 
ought to be able to use the same register for new and ret if we remove 
the early clobber.

-- 
error compiling committee.c: too many arguments to function

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