From mboxrd@z Thu Jan  1 00:00:00 1970
From: Joe Perches <joe@perches.com>
Date: Fri, 22 Sep 2017 17:21:21 +0000
Subject: Re: [PATCH] lib/lz4: make arrays static const, reduces object code size
Message-Id: <1506100881.12311.41.camel@perches.com>
List-Id: <kernel-janitors.vger.kernel.org>
References: <20170921221939.20820-1-colin.king@canonical.com>
         <5aab65d9-6da6-c770-b5aa-9edbdda31dec@wanadoo.fr>
         <250fe35e-77d3-a9c0-5613-ce1c65f4cb7f@canonical.com>
         <CAK8P3a1ryR2oXnqrMt+L0_v1G+NggvejX+L31V=QtCBt1cfZWg@mail.gmail.com>
In-Reply-To: <CAK8P3a1ryR2oXnqrMt+L0_v1G+NggvejX+L31V=QtCBt1cfZWg@mail.gmail.com>
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To: Arnd Bergmann <arnd@arndb.de>, Colin Ian King <colin.king@canonical.com>
Cc: Christophe JAILLET <christophe.jaillet@wanadoo.fr>, Sven Schmidt <4sschmid@informatik.uni-hamburg.de>, Andrew Morton <akpm@linux-foundation.org>, kernel-janitors@vger.kernel.org, Linux Kernel Mailing List <linux-kernel@vger.kernel.org>

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@canonical.com> wrote:
> > On 22/09/17 00:09, Christophe JAILLET wrote:
> > > Le 22/09/2017 =E0 00:19, Colin King a =E9crit :
> > > > From: Colin Ian King <colin.king@canonical.com>
> > > >=20
> > > > 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:
> > >=20
> > > 10k? Wouaouh! This is way much more than what you usually win with su=
ch
> > > patches.
> >=20
> > Yes, I had to triple check it because it was an unbelievable win.
> >=20
>=20
> 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:
>=20
>    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[] =3D {1, a};
	...
}

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