Baseline JIT patches available for Firefox ESR128 on OpenPOWER

It's been a long hot summer at $DAYJOB and I haven't had much time for much of anything, but I got granted some time this week to take care of an unrelated issue and seized the opportunity to get caught up.

The OpenPOWER Firefox JIT still crashes badly in Wasm and Ion for reasons I have yet to ascertain, but the Baseline Interpreter and Baseline Compiler stages of the JIT continue to work great and are significantly faster than the baseline Interpreter (even in a PGO-LTO build), so I did the needful and finally got them pulled up to the new Extended Support Release which is Firefox 128.

I then spent the last two days bashing out crashes and bugs, including a regression from Firefox's new WebAssembly-based in-browser translation engine. The browser chrome now assumes that WebAssembly is always present, but on JIT-less tier-3 machines (or partially implemented JITs like ours, and possibly where Wasm is disabled in prefs) it isn't, so it hits an uncaught error which then blows up substantial portions of the browser UI like the stop-reload button and context menus. The Fedora official ppc64le build of Firefox 128.0.3 is affected as well; I filed bug 1912623 with a provisional fix. Separately all JIT and JavaScript tests completely pass in multiple permutations of Baseline Interpreter and Baseline Compiler, single- and multi-threaded.

As a sign of confidence I've been dogfooding it for the last 24 hours with my typical massive number of tabs and add-ons and can't get it to crash anymore, so I'm typing this blog post in it and using it to upload its own changesets to Github. Grab the ESR source from Mozilla (either pull a tree with Mercurial or just download an archive) and apply the changesets in numerical order, though after bug 1912623 is fixed you won't need #823094. The necessary .mozconfig for building an LTO-PGO build, which is what I'm using, is also in that issue; it's pretty much the same as earlier ones except for --enable-jit.

Little-endian POWER9 remains the officially supported architecture. This version has not been tested on POWER8 or big-endian POWER9, though the JIT should still statically disable itself even if compiled with it on, so the browser should still otherwise work normally. If this is not the case, I consider that a bug, and will accept a fix (I don't have a POWER8 system here to test against). There are no Power10 specific instructions, but I don't see any reason why it wouldn't work on a Power10 machine or on a SolidSilicon S1 whenever we get one of those.

Comments always solicited, though backtraces and reliable STRs are needed to diagnose any bug, of course. Meanwhile I've got more work cut out for me but at least we're back in the saddle for another go.