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

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Message-ID: <250fe35e-77d3-a9c0-5613-ce1c65f4cb7f@canonical.com>
Date:   Fri, 22 Sep 2017 00:11:14 +0100
From:   Colin Ian King <colin.king@...onical.com>
To:     Christophe JAILLET <christophe.jaillet@...adoo.fr>,
        Sven Schmidt <4sschmid@...ormatik.uni-hamburg.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Arnd Bergmann <arnd@...db.de>
Cc:     kernel-janitors@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] lib/lz4: make arrays static const, reduces object code
 size

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 assume we must thanks FORCE_INLINE in this case.

Yep, I believe so.

> Nice catch!
> 
> CJ
> 
>> Before:
>>     text       data        bss        dec        hex    filename
>>    31500          0          0      31500       7b0c   
>> lib/lz4/lz4_decompress.o
>>
>> After:
>>     text       data        bss        dec        hex    filename
>>    20237        176          0      20413       4fbd   
>> lib/lz4/lz4_decompress.o
>>
>> (gcc version 7.2.0 x86_64)
>>
>> Signed-off-by: Colin Ian King <colin.king@...onical.com>
>> ---
>>   lib/lz4/lz4_decompress.c | 4 ++--
>>   1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/lib/lz4/lz4_decompress.c b/lib/lz4/lz4_decompress.c
>> index bd3574312b82..141734d255e4 100644
>> --- a/lib/lz4/lz4_decompress.c
>> +++ b/lib/lz4/lz4_decompress.c
>> @@ -85,8 +85,8 @@ static FORCE_INLINE int LZ4_decompress_generic(
>>       const BYTE * const lowLimit = lowPrefix - dictSize;
>>         const BYTE * const dictEnd = (const BYTE *)dictStart + dictSize;
>> -    const unsigned int dec32table[] = { 0, 1, 2, 1, 4, 4, 4, 4 };
>> -    const int dec64table[] = { 0, 0, 0, -1, 0, 1, 2, 3 };
>> +    static const unsigned int dec32table[] = { 0, 1, 2, 1, 4, 4, 4, 4 };
>> +    static const int dec64table[] = { 0, 0, 0, -1, 0, 1, 2, 3 };
>>         const int safeDecode = (endOnInput == endOnInputSize);
>>       const int checkOffset = ((safeDecode) && (dictSize < (int)(64 *
>> KB)));
> 
>