Chromium hitting SIGILL crashes on aarch64 after #144938 enabled outlined atomics by default

[zmodem]

Continuing the discussion here as requested on #144938

After this change, we're getting reports from developers about hitting SIGILL in Chromium's render process on some Android devices.

Disabling outline-atomics seems to fix the crashes.

We don't have a good understanding of the root cause yet, but for a start, we'd like to be able to disable the feature.

Passing -Ctarget-feature=-outline-atomics works, but causes warnings:

warning: unstable feature specified for `-Ctarget-feature`: `outline-atomics`
  |
  = note: this feature is not stably supported; its behavior can change in the future

It would be good to have a knob for turning off the feature without warnings. Alternatively it would be good to have a knob to turn off that kind of warning.

(We'll hack it out for now: zmodem@b01caa5)

[zmodem]

It's entirely possible that this is specific to our setup of course. Our tracking bug is https://crbug.com/477565811

[zmodem]

The current theory is that we're crashing due to lack of BTI landing pads in the outlined functions.

[nico]

@tgross35 Trunk currently produces broken code on chips supporting BTI.

[tgross35]

Thanks for looking into this more. It would be best if we can fix the symbols to work with BTI, which rust-lang/compiler-builtins#1063 should do.

(If you happen to be able to test/review that patch, that would be rather helpful)