Update i686-linux-android features to match android x86 ABI.

[Nercury]

Based on android's official x86 ABI info, the x86 baseline CPU can be safely updated to pentiumpro, with the addition of MMX, SSE, SSE2, SSE3, SSSE3 features.

r? @alexcrichton

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[nagisa]

Despite Android docs claiming this is supposed to target PentiumPRO (which is what i686 is, in fact), the SSSE3 was introduced with Intel Core. Thus, I think pentium4 (a 2 generation predecessor for Core) is a safe-enough choice and downgrading to pentiumpro is a pessimisation.

[MagaTailor]

32-bit x86 compiler development is pretty much in maintenance mode so this change could probably end up a very small optimisation (as long as all those sse feature sets are available). But yeah, it's cleaner and more consistent to just add +sse3, +ssse3 to the currently exposed cpu.

[Nercury]

I have no idea why Google used PentiumPro as the base CPU, and that might be "pessimisation", but I included a link to official docs in this PR, and I think that having CPU (or any features for that matter) that differ from official requirement will only add to slight confusion in the future for anyone following the link.

[nagisa]

@Nercury they refer to the PentiumPro as the instruction set (i.e. the -mcpu=i686 thing), and not the PentiumPro, the processor generation. As far as instruction sets go, Pentium4 (pre-2004 ones, at least) is the same thing as PentiumPro + mmx + sse + sse2 (p4 has no ssse3 or sse3).

That being said and looking some history up, turns out pentium4 was such a disaster in core design that optimising for it might end up with worse results than optimising for PentiumPro. I guess the only true way to decide on the right choice would be to benchmark.

[Nercury]

@petevine this target is most useful for running in emulator, until we get the x86_64 target up.

[MagaTailor]

@Nercury Absolutely, this change is necessary to correct the target spec - you could even bump the cpu to core, as @nagisa implied, and call it a day. And besides, on a funny note, if you put the word pentiumpro anywhere near i686 that might give people ideas ;)

[alexcrichton]

@bors: r+ 148b5e4

Thanks!

[bors]

⌛ Testing commit 148b5e4 with merge 5743564...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-32cross-opt, auto-linux-64-cargotest, auto-linux-64-cross-armhf, auto-linux-64-cross-armsf, auto-linux-64-cross-freebsd, auto-linux-64-cross-netbsd, auto-linux-64-debug-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-opt-mir, auto-linux-64-opt-rustbuild, auto-linux-64-x-android-t, auto-linux-cross-opt, auto-linux-musl-64-opt, auto-mac-32-opt, auto-mac-64-nopt-t, auto-mac-64-opt, auto-mac-64-opt-rustbuild, auto-mac-cross-ios-opt, auto-win-gnu-32-nopt-t, auto-win-gnu-32-opt, auto-win-gnu-32-opt-rustbuild, auto-win-gnu-64-nopt-t, auto-win-gnu-64-opt, auto-win-msvc-32-cross-opt, auto-win-msvc-32-opt, auto-win-msvc-64-cargotest, auto-win-msvc-64-opt, auto-win-msvc-64-opt-mir, auto-win-msvc-64-opt-rustbuild