x86 MSVC does not use SSE(2)

[hanna-kruppe]

@cmr noticed this on IRC. On i686-pc-windows-gnu and in fact on all other i686-* triples (except iOS, which is technically called i386-*), a SSE2-capable microarchitecture is assumed. @dotdash said that this was copied from Clang. Not doing so on MSVC might be an accident, and a rather costly one.

[emberian]

Specifically, in the target spec, we have: https://github.com/rust-lang/rust/blob/master/src/librustc_back/target/i686_pc_windows_msvc.rs#L15, unlike every other i686 target which assumes pentium4 (except for Mac, which assumes yonah, which is newer, so that's fine).

[emberian]

cc @alexcrichton, who wrote that code. Is this an intentional quirk of the MSVC toolchain?

[alexcrichton]

Oh dear no this was not intentional at all, I just assumed everything was i686! Feel free to send a PR to update!