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

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Date:   Fri, 22 Sep 2017 23:39:15 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Joe Perches <joe@...ches.com>
Cc:     Colin Ian King <colin.king@...onical.com>,
        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 9:17 PM, Arnd Bergmann <arnd@...db.de> wrote:
> On Fri, Sep 22, 2017 at 7:21 PM, Joe Perches <joe@...ches.com> wrote:
>> On Fri, 2017-09-22 at 09:48 +0200, Arnd Bergmann wrote:
>>> On Fri, Sep 22, 2017 at 1:11 AM, Colin Ian King
>
>>>    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};
>>         ...
>> }
>
> It depends. With a 10KB different in .text size, my guess is that this
> is a case where gcc does the right optimization in principle, but
> fails to do what was intended in some corner cases.

I found the problem: "gcc -fsanitze=kernel-address --param asan-stack=1"
produces lots of expensive checks here with gcc-5 or higher.

Disabling it makes a big difference:

upstream:
gcc-7.0.0 --fsanitze=kernel-address --param asan-stack=1: 31789 bytes
gcc-7.0.0: 16535 bytes

patched:
gcc-7.0.0 --fsanitze=kernel-address --param asan-stack=1: 20351 bytes
gcc-7.0.0: 14490 bytes

      Arnd