[8u] RFR: 8210352: [hotspot] --with-extra-cxx-flags should not lower optimization

[David Holmes]

On 5/09/2018 7:29 AM, Erik Joelsson wrote:
> On 2018-09-04 14:06, David Holmes wrote:
>> Hi Severin,
>>
>> On 5/09/2018 1:36 AM, Severin Gehwolf wrote:
>>> Hi,
>>>
>>> Could I please get reviews of this JDK 8-only change?
>>>
>>> The issue at hand is that the JDK 8 build seems to be inconsistent with
>>> later JDK builds (9+). When somebody passes
>>> --with-extra-c{,xx}-flags="-O2" to the OpenJDK build on JDK 9+ the JVM
>>> will get compiled with -O3 for a release type build. Not so on JDK 8.
>>> It gets compiled with -O2. The reason we are passing extra C flags via
>>> configure is to: a) get distro-wide optimization done b) allow for
>>> hardened builds. Without the --with-extra-c{,xx}-flags="-O2" flag,
>>> libjsig, libsaproc will get compiled with no optimization. That's what
>>> I'd like to fix. The proposed patch allows one to force -O2 for the JVM
>>> via OPT_EXTRAS make variable if so desired. Omitting OPT_EXTRAS=-O2
>>> allows one to get hotspot libraries optimized with -O2 and better.
>>> Thoughts?
>>
>> I don't understand the motivation for this change. Compatibility with 
>> future versions is not a usual compatibility we care about. I'd also 
>> argue the behaviour in 8 seems more correct. If I set an extra cflag I 
>> expect it to be applied to all compilations - cross-compilation would 
>> be broken if that were not the case. I don't see why -On should be 
>> treated any differently.
>>
> I do agree with you, sort of. All user added flags are applied 
> everywhere. The reason the optimization isn't overridden is because of 
> the order the flags are applied on the command line. In JDK 9 (and in 8 
> for JDK libs) we apply optimization flags after the custom flags from 
> the user. (For Hotspot in 8 it seems the custom flags are added after 
> the optimization flags.) IIRC we did this intentionally in the "new 
> build" with the reasoning that we are pretty particular with the 
> optimization flags in many cases so a global override is probably not a 
> good idea.
> 
> The suggested patch, filtering specific -O flags from the user supplied 
> flags does not look good to me. I would however be ok with changing the 
> ordering of flags for libjvm so that the same behavior as in 9 and later 
> (and as for JDK libs in 8) is achieved.

Okay - but such a change should be considered an enhancement request, 
not a bug fix, and must then go through the enhancement request process 
for 8u. My concern is that this may fix Severin's issue but may break 
others who do custom builds of 8u code. The 8u maintainers would have to 
make the decision to accept this change or not.

David

> 
> /Erik
>> David
>>
>>> Bug: https://bugs.openjdk.java.net/browse/JDK-8210352
>>> webrev: 
>>> http://cr.openjdk.java.net/~sgehwolf/webrevs/JDK-8210352/webrev.01/
>>>
>>> Thanks,
>>> Severin
>>>
>