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 Martin,

On 2019-11-27 19:03, Doerr, Martin wrote:
> Hi Claes,
> 
> that kind of surprises me. I'd expect files which rather benefit from -O3 to be far less than those which benefit from -Os.
> Most performance critical code lives inside the code cache and is not dependent on C++ compiler optimizations.
> I'd expect GC code, C2's register allocation and a few runtime files to be the most performance critical C++ code.
> So the list of files for -Os may become long.

that might very well be the end result, and once/if we've gone down this
path long enough to see that -O3 becomes the exception, we can re-
examine the default. Changing the default and then trying to recuperate
would be hard/impossible to do incrementally.

> 
> Yeah, I think we should use native profiling information to find out what's really going on >
> Your idea to change file by file and check for performance regression makes sense to me, though
Hopefully we don't have to do one RFE per file.. :-)

/Claes