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

[Erik Joelsson]

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.

/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
>>