Firefox ESR78 subtly breaks with -O3

[mid-kid]

I've had to add this:

www-client/firefox /-O3/-O2

To my package.cflags, or else violentmonkey wouldn't work.
It broke with something about being unable to open the addon's local storage due to internal errors. You can see errors pop up in about:debugging after installing violentmonkey.

EDIT: This is with GCC 10, I haven't attempted -O3 builds with Clang. Don't really want to, either.

[gottaeat]

i'm not on gentoo but i have this exact same problem. when i build firefox using gcc 10.2.0 and -O3 or -O2 + -flto, violentmonkey userscripts simply disappear after a reboot of the browser.

same problem does not exist on a completely standalone nognu llvm 11.0.0, building with crosslto (thinlto) + O3 and polly does not break anything.

[mid-kid]

Apparently, if firefox is USE=pgo/MOZ_PGO=1, it will force adding -O3 to the CXXFLAGS. This breaks the browser in much the same way.
To fix this, I had to create /etc/portage/bashrc.d/49-firefox-pgo-fix.sh:

FirefoxPgoFix() {
	if [ "$CATEGORY/$PN" = 'www-client/firefox' ]; then
		cd "$S"
		sed -i -e '/MOZ_PGO_OPTIMIZE_FLAGS=/d' \
			old-configure old-configure.in \
			js/src/old-configure js/src/old-configure.in
	fi
}

BashrcdPhase configure FirefoxPgoFix

(This will cause python/mozbuild/mozbuild/frontend/context.py to pick up MOZ_OPTIMIZE_FLAGS correctly, instead of the pgo one)

Update: Even with -O2 with this doesn't work anymore...

[mid-kid]

@mssx86 Are you sure this issue doesn't happen without -flto? Last I tried I think it still did, but I'd like to save myself a couple of hours in rebuilding the browser again...

[gottaeat]

@mssx86 Are you sure this issue doesn't happen without -flto? Last I tried I think it still did, but I'd like to save myself a couple of hours in rebuilding the browser again...

yes, userscripts break with just -O3 when built with gcc, just like how it does with -O2 -flto.

[mid-kid]

Gotcha, I guess I've hit this bug now, too. I'll report it to Gentoo upstream since USE=lto is upstream, maybe they know where to forward it to. I believe this can be fixed by Firefox upstream.

[mid-kid]

Created Gentoo bug: https://bugs.gentoo.org/758446

Posting error message image here for reference, this is the error that a bunch of addons are getting:

Also want to add, that playing .mp3 files in the browser, or using google's audio captcha results in choppy audio. I'm using USE=pulseaudio, -lto and -pgo with '-O2' in CFLAGS, and I'm not sure if the issue is firefox itself or a different package, or if these issues are even related. Will investigate this as well when I have time...

Update: Rebuilding with USE=clang, otherwise keeping the same settings fixes the audio issue as well, so I'm sure it's related...

[gottaeat]

your bug report caused the mozilla team at gentoo to set all firefox ebuild's to be built with clang only. let's hope something comes out of it.

[mid-kid]

That's mildly annoying (especially for the many people still using GCC 9 that are unaffected by this) but having it be confirmed on the latest release is reassuring it'll get a fix.
And hey, it's been reported to upstream now: https://bugzilla.mozilla.org/show_bug.cgi?id=1682191

[mid-kid]

This bug has been worked around in gentoo by disabling -ftree-loop-vectorize: https://gitweb.gentoo.org/repo/gentoo.git/commit/?id=09886ee677ea97b80f1469dc7534ff6aea6247cb

You can re-emerge firefox to have this fixed, now. Figuring out the specifics of this bug to provide a proper resolution is out of the scope of this issue, and there's upstream bug reports for that, so I'm closing it.