linux-kernel - Re: [PATCH] RWSEM: Rewrite rwsem.c and rwsem-spinlock.c more simply.

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Message-Id: <20080325121438.883f7ceb.akpm@linux-foundation.org>
Date:	Tue, 25 Mar 2008 12:14:38 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	"Robert P. J. Day" <rpjday@...shcourse.ca>
Cc:	andi@...stfloor.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] RWSEM: Rewrite rwsem.c and rwsem-spinlock.c more
 simply.

On Tue, 25 Mar 2008 08:42:48 -0400 (EDT)
"Robert P. J. Day" <rpjday@...shcourse.ca> wrote:

> On Tue, 25 Mar 2008, Andi Kleen wrote:
> 
> > > i'm not sure what this means -- which of the transformations in
> > > that patch is considered unsafe?  here's a typical simplification:
> >
> > It is not unsafe, just generates slight worse code.
> >
> > current is inline assembler and the compiler doesn't know that it
> > could cache it in a register because it is not marked pure for
> > various reasons. That is why current is often cached explicitely in
> > a local variable to tell the compiler that.
> 
>   ah, i think i see, thanks.  learn something every day.

A crude measyure is /usr/bin/size.  Your patch increased rwsem-spinlock.o
from 1595 bytes of text up to 1629.

A text size increase isn't necessarily always a bad thing, but it does need
to be monitored, understood, explained, etc.

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