linux-kernel - Re: proxy_pda was Re: What was in the x86 merge for .20

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Message-ID: <4579DA6D.1030305@goop.org>
Date:	Fri, 08 Dec 2006 13:34:37 -0800
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Andi Kleen <ak@...e.de>
CC:	Arkadiusz Miskiewicz <arekm@...en.pl>, linux-kernel@...r.kernel.org
Subject: Re: proxy_pda was Re: What was in the x86 merge for .20

Andi Kleen wrote:
> The trouble is when it's CSEd it actually causes worse code because
> a register is tied up. That might not be worth the advantage of having it?
>   

I think so, definitely; without proxy_pda you need to make it asm
volatile+mem clobber, which completely eliminates all optimisation
opportunities; in general the proxy_pda allows gcc to CSE and reorder
pda accesses.  I guess in this case the memory writes inhibited the
overall CSE of current, so its just making do by CSEing the address.

> Hmm, maybe marking it volatile would help? Arkadiusz, does the following patch
> help?
>   

Might work.  But doesn't this make the pointed-at proxy_pda volatile,
not the proxy_pda pointer itself?  Should it be something like (volatile
__T * volatile)?

    J
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