Mozilla's x86 Linux builds now require SSE2

[Henri Sivonen]

https://bugzilla.mozilla.org/show_bug.cgi?id=1274196 has landed for
Firefox 53. It makes build for 32-bit x86 Linux made on Mozilla's
infrastructure require SSE2. It does not change the defaults for
builds made elsewhere.

This effectively means that x86+SSE2 is tier-1 but x86 potentially
without SSE2 is in the same tier-3 bucket as other CPU architectures
that Mozilla doesn't provide builds for but Linux distros do.

Mozilla-made builds that require for 32-bit x86 Linux check for SSE2
on startup and exit with an error message to stderr if SSE2 is not
found. Depending on their distribution, the affected users may have
the recourse of obtaining a build that doesn't require SSE2 from their
Linux distribution. For the duration of the ESR 52 cycle, the users
could, alternatively, switch to ESR, but the error message does not
point this out.

For updates, the capability to report the presence of SSE2 went into
Firefox 51, so the updater can be smart about showing a tombstone SUMO
link vs. updating.

Notably, this means that 387 floating-point math is no longer tier-1
now that all tier-1 x86 platforms require SSE2 and, therefore, can use
standard-based SSE2 floating-point math instead of legacy 387
floating-point math. x86 Mac and x86 Android never shipped on non-SSE2
CPUs, so SSE2 has been part of their baseline since the beginning.
Mozilla's Windows builds have required SSE2 since Firefox 49. SSE2 is
also a mandatory part of x86_64.

Since the 32-bit x86 targets that are tier-1 for Rust require SSE2,
this enables us to use rustc with a tier-1 target configuration in
Mozilla-made Firefox builds for 32-bit x86 Linux.

--
Henri Sivonen
hsiv...@hsivonen.fi
https://hsivonen.fi/

[Jim Blandy]

But, how can we so casually drop support for processors manufactured as
recently as 2003? Wasn't there any discussion of this decision?

Just kidding, I'm glad we can finally put the x87 behind us, and I know
there was *lots* of discussion...

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