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

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Message-ID: <20140513205045.GK2485@laptop.programming.kicks-ass.net>
Date:	Tue, 13 May 2014 22:50:45 +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 Tue, May 13, 2014 at 10:40:32PM +0200, Hans-Christian Egtvedt wrote:
> Probably found the reason why we want to use sub with the signed 21-bit
> limit, it uses one less register than the add instruction that can add up to
> 32-bit values.
> 
> Both instructions are 32-bit, to use a 16-bit instruction the immediate is
> very small; 4 bit.
> 
> sub 32-bit, type IV, takes a register and subtracts a 21-bit immediate.
> add 32-bit, type II, adds two register values together.
> 
> So by simplifying you loose this optimization.

OK, let me try if I can come up with anything sane for this.

Re, adding atomic_{or,and,xor}() those should all use the same bits as
add, right, except for the special case using sub.
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