On Mon, May 19, 2014 at 9:45 AM, Uros Bizjak <ubiz...@gmail.com> wrote:
> On Mon, May 19, 2014 at 6:42 PM, H.J. Lu <hjl.to...@gmail.com> wrote:
>
>>>> Uros,
>>>> I am looking into libreoffice size and the data alignment seems to make
>>>> huge
>>>> difference. Data section has grown from 5.8MB to 6.3MB in between GCC 4.8
>>>> and 4.9,
>>>> while clang produces 5.2MB.
>>>>
>>>> The two patches I posted to not align vtables and RTTI reduces it to
>>>> 5.7MB, but
>>>> But perhaps we want to revisit the alignment rules. The optimization
>>>> manuals
>>>> usually care only about performance critical loops. Perhaps we can make
>>>> the
>>>> rules to align only bigger datastructures, or so at least for -O2.
>>>
>>> Based on the above quote, "Misaligned data access can incur
>>> significant performance penalties." and the fact that this particular
>>> alignment rule has some compatibility issues with previous versions of
>>> gcc (these were later fixed by Jakub), I'd rather leave this rule as
>>> is. However, if the access is from the cold section, we can perhaps
>>> avoid extra alignment, while avoiding those compatibility issues.
>>>
>>
>> It is excessive to align
>>
>> struct foo
>> {
>> int x1;
>> int x2;
>> char x3;
>> int x4;
>> int x5;
>> char x6;
>> int x7;
>> int x8;
>> };
>>
>> to 32 bytes and align
>>
>> struct foo
>> {
>> int x1;
>> int x2;
>> char x3;
>> int x4;
>> int x5;
>> char x6;
>> int x7[9];
>> int x8;
>> };
>>
>> to 64 bytes. What performance gain does it provide?
>
> Avoids "significant performance penalties," perhaps?
>
Kirill, do we have performance data for excessive alignment
vs ABI alignment?
--
H.J.
