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

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Date:   Fri, 22 Sep 2017 09:48:25 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     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, 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

I tried x86 defconfig and allmodconfig on a slightly patched mainline kernel.

        Arnd