linux-kernel - Re: [PATCH 06/20] arch,avr32: Fold atomic

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Date:	Fri, 9 May 2014 22:43:09 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Hans-Christian Egtvedt <egtvedt@...fundet.no>
Cc:	linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
	torvalds@...ux-foundation.org, akpm@...ux-foundation.org,
	mingo@...nel.org, will.deacon@....com, paulmck@...ux.vnet.ibm.com,
	Haavard Skinnemoen <hskinnemoen@...il.com>
Subject: Re: [PATCH 06/20] arch,avr32: Fold atomic_ops

On Fri, May 09, 2014 at 08:32:41PM +0200, Hans-Christian Egtvedt wrote:
> Around Thu 08 May 2014 15:58:46 +0200 or thereabout, Peter Zijlstra wrote:
> > Many of the atomic op implementations are the same except for one
> > instruction; fold the lot into a few CPP macros and reduce LoC.
> 
> The add and sub atomic operations are not 100% the same. Sub has more
> constraints on the integer size than add. Sub only takes a signed 21-bit
> integer, while add can do 32-bit additions IIRC correctly the instructions
> for AVR32.
> 
> This is why you see in atomic_sub_return() that i is typed as "rKs21", while
> in atomic_add_return, i is typed "r".
> 
> Your change limits both atomic operations to work only on signed 21-bit
> integers.

Urgh, fail on me for not seeing that.


> > -	if (__builtin_constant_p(i) && (i >= -1048575) && (i <= 1048576))
> > -		result = atomic_sub_return(-i, v);
> 
> I do not recall why we did it like this any more, I would assume both sub and
> add to be single cycle instructions.

Right and if its a constant the negate is compile time too.

OK, so if I only generate add and provide inline stubs to implement sub
with add this should be good again, right?

Any other instructions I should be careful with? I take it the bit ops
are full 32 bits again?
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