I am not opposed to doing this, but just enabling LTO doesn't always automagically give you a performance boost (I am not sure if this is the case here).

I'll look more into this later this week and do some benchmarking and testing.
I was planning to enable LTO/PGO by default for upcoming mozjs68, If I am not mistaken, there were some fixes for this in newer Firefox versions.

@stransky @kalev WDYT about this?

Anyway, thanks for bringing this up!

I'd like to do the same as firefox does, to share bugs/issues. If firefox enables LTO then we should enable it in mozjs as well, I'd say.

PGO+LTO is already enabled in Fedora Firefox since Firefox 64. We have a gcc patch to PGO to generate proper training data for PGO profile. More details are available at http://hubicka.blogspot.com/

There's the related article here: http://hubicka.blogspot.com/2018/12/even-more-fun-with-building-and.html

Thanks for reply, I was aware that LOT+PGO was enabled in Fedora Firefox. But this is effectively SpiderMonkey from Firefox 60 (version way older than where was LTO+PGO enabled).

Aren't you aware of any issues in such an old Firefox if it was built with LTO+PGO?

Thanks!

Thanks for reply, I was aware that LOT+PGO was enabled in Fedora Firefox. But this is effectively SpiderMonkey from Firefox 60 (version way older than where was LTO+PGO enabled).
Aren't you aware of any issues in such an old Firefox if it was built with LTO+PGO?

Only the one described in the referenced article. You can find related patches in Fedora Firefox.

Okay, I've pushed LTO enabled build right now [0] ( @atim credit is given in spec changelog). I did compare the perf by measuring time to run the test suite (not ideal, I know, the difference is tiny, but I wanted to be sure there aren't any regressions at least).

I'll soon fire up build also in F30 if F31 build finishes succesfully for all Fedora arches, but I am planning to let it live in updates-testing for a few days to be absolutely sure nothing got broken.

I have tried to fiddle with PGO a little, but mozjs60 is build in a completely different way than Firefox in Fedora and I wasn't too successful with getting PGO training to run. Firefox 68 ESR was released a few days ago, I am planning to use Firefox Fedora package as a base for mozjs68 , so we'll have LTO+PGO from there.

Having both LTO+PGO enabled in future mozjs68 should bring bigger perf gains.

[0] https://src.fedoraproject.org/rpms/mozjs60/c/5395cffee2cbc4c620fdbe4945e3604d356ddd6a?branch=master

Awesome. Yep, now waiting for mozjs68 and PGO. Thank you.