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Date: Tue, 3 Mar 2009 10:17:30 -0300
Message-ID: <82ecf08e0903030517r49367e42ybf21e0020f35f6ce@mail.gmail.com>
Subject: Re: [PATCH] crc32: remove useless __pure modifier from functions
From: Thiago Galesi <thiagogalesi@gmail.com>
To: Will Newton <will.newton@gmail.com>
Cc: Pekka Enberg <penberg@cs.helsinki.fi>, akpm@linux-foundation.org,
       linux-kernel@vger.kernel.org
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>
> Did you see any change in size of your kernel with this annotation? It
> didn't seem to have any effect as far as I could tell.

No, because I didn't test it with the kernel. Anyway, probably size
won't change a lot.

What the compiler does is: if (you're saying) it's a pure function,
and it has been called previously with the same parameters, the return
value is identical, hence, the compiler doesn't need to call the
function again.

Here's the test I did http://duskblue.org/pure_test.tar.gz Pretty
straighforward, but you have to 'make CFLAGS=-O1' to make the
optimization work.

>
> There are a number of functions in lib/ code that could be marked
> __pure or __attribute_const__ but I'm not sure if it's worth the
> effort, for my compiler (gcc 4.2) at least.
>

What const does is similar: it indicates that the function will not
change parameters marked with __const, so it will not need to reload
them after the function is called.

-- 
-
Thiago Galesi
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