[webkit-dev] Proposal: Not supporting x86 w/o SSE2

[Yusuke SUZUKI]

Hello WebKittens,

To mitigate recent side-channel attacks, we added `lfence` in x86 WebKit.
But this is supported in x86 SSE2. So the ToT WebKit is not usable with
very old x86 CPUs which do not have SSE2[1].

According to [2], Mozilla Firefox 49 no longer supports x86 w/o SSE2.
Since Firefox 45 ESR is already End of Life, all the supported Firefox
require SSE2 on x86.
Chromium also does not support x86 if it does not have SSE2 IIRC[3].
So, no major browsers support x86 w/o SSE2 right now.

I think our stakeholders do not maintain non-SSE2 x86.
Even if it works, it is potentially vulnerable, and I think it is not
acceptable for the browser engine.

My proposal here is clarifying that WebKit does not support x86 not having
SSE2.

[1]: https://bugs.webkit.org/show_bug.cgi?id=188145
[2]: https://support.mozilla.org/en-US/kb/your-hardware-no-longer-supported
[3]: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=763290
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