building libjvm with -Os for space optimization - was : RE: RFR: 8234525: enable link-time section-gc for linux s390x to remove unused code

[Claes Redestad]

Hi,

we discussed doing the opposite for Mac OS X recently, where builds are
currently set to -Os by default. -O3 helped various networking
(micro)benchmarks by up to 20%.

Rather than doing -Os by default and then cherry-pick things over to -O3
on a case-by-case basis, I'd suggest the opposite: keep -O3 as the
default, start evaluating -Os on a case-by-case basis. This allows for
an incremental approach where we identify things that are definitely not
performance critical, e.g., never shows up in profiles, and switch those
compilation units over to -Os. Check for harmful performance impact and
expected footprint improvement; rinse; repeat.

$.02

/Claes


On 2019-11-27 17:36, Baesken, Matthias wrote:
> Hello Martin,  I checked  building  libjvm.so  with -Os  (instead of -O3) .
> 
> I used gcc-7  on linux x86_64 .
> The size  of  libjvm.so   dropped   from    24M  (normal night make with -O3)  to   18M   ( test make with -Os)   .
>   (adding the link-time gc might  reduce the size by another ~ 10 % ,  but those 2 builds were without the ltgc )
> 
> Cannot say much so far about performance impact .
> 
> Best regards, Matthias
> 
> 
> 
>>
>> Hi Matthias and Erik,
>>
>> I also think this is an interesting option.
>>
>> I like the idea to generate smaller libraries. In addition to that, I could also
>> imagine building with -Os (size optimized) by default and only select -O3 for
>> performance critical files (e.g. C2's register allocation, some gc code, ...).
>>
>> If we want to go into such a direction for all linux platforms and want to use
>> this s390 only change as some kind of pipe cleaner, I think this change is fine
>> and can get pushed.
>> Otherwise, I think building s390 differently and not intending to do the same
>> for other linux platforms would be not so good.
>>
>> We should only make sure the exported symbols are set up properly to avoid
>> that this optimization throws out too much.
>>
>> My 50 Cents.
>>
>> Best regards,
>> Martin
>>
>