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

Colin Ian King
2017-09-21T23:11:14Z

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@canonical.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@canonical.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)));
>
>