linux-kernel - Re: [PATCH] lib/lz4: make arrays static const, reduces object code size

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Date:   Fri, 22 Sep 2017 10:21:21 -0700
From:   Joe Perches <joe@...ches.com>
To:     Arnd Bergmann <arnd@...db.de>,
        Colin Ian King <colin.king@...onical.com>
Cc:     Christophe JAILLET <christophe.jaillet@...adoo.fr>,
        Sven Schmidt <4sschmid@...ormatik.uni-hamburg.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        kernel-janitors@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] lib/lz4: make arrays static const, reduces object code
 size

On Fri, 2017-09-22 at 09:48 +0200, Arnd Bergmann wrote:
> On Fri, Sep 22, 2017 at 1:11 AM, Colin Ian King
> <colin.king@...onical.com> wrote:
> > On 22/09/17 00:09, Christophe JAILLET wrote:
> > > Le 22/09/2017 à 00:19, Colin King a écrit :
> > > > From: Colin Ian King <colin.king@...onical.com>
> > > > 
> > > > Don't populate the read-only arrays dec32table and dec64table on the
> > > > stack, instead make them both static const.  Makes the object code
> > > > smaller by over 10K bytes:
> > > 
> > > 10k? Wouaouh! This is way much more than what you usually win with such
> > > patches.
> > 
> > Yes, I had to triple check it because it was an unbelievable win.
> > 
> 
> I wonder whether this should be reported as a gcc bug. I tried reproducing
> it here with gcc-7.1.1 and gcc-8.0.0, but I only see a 4K difference:
> 
>    text    data     bss     dec     hex filename
>   18220     176       0   18396    47dc build/tmp/lib/lz4/lz4_decompress-after.o
>   22297       0       0   22297    5719 build/tmp/lib/lz4/lz4_decompress-before.o

Perhaps not so much a gcc bug as an opportunity
for gcc to add an additional optimization.

gcc would have to verify that the const array is
not initialized with some variable or argument like:

int foo(int a)
{
	const int array[] = {1, a};
	...
}