Our x86-32 target names are inconsistent

[RalfJung]

The typical naming scheme we use for x86-32 targets is:

• i686 means Pentium 4 (yes that makes no sense but, well, it's too disruptive to change now and i786 didn't catch on as a name anywhere), which in particular have SSE2
• i586 means "original Pentium" (no SSE)

We have some targets that violate this:

• i686-linux-android is actually PentiumPro with SSE, but that seems sufficiently close to Pentium 4 that it's still the right name IMO (in particular, it does have more vector extensions than Pentium 4; not sure why we are using "Pentium Pro" as the baseline) (resolved by i686-linux-android: increase CPU baseline to Pentium 4 (without an actual change #136885)
• i386-apple-ios uses Penryn as baseline, way newer than Pentium 4, so of course it has SSE (this target is ancient, it predates Rust 1.0)
• i586-pc-nto-qnx700 uses Pentium 4 as baseline (resolved by Make x86 QNX target name consistent with other Rust targets #137324)
• i686-unknown-hurd-gnu uses PentiumPro without SSE as baseline (resolved by i686-unknown-hurd-gnu: bump baseline CPU to Pentium 4 #136700)
• i686-unknown-redox uses PentiumPro without SSE as baseline (resolved by Replace i686-unknown-redox target with i586-unknown-redox #136698)

If we want to establish the pattern that "i686 has SSE and the rest does not", then the last four of these should be renamed. (These are all tier 3 targets.) I wonder if there is a specific reason that these names were picked diverging from our usual naming scheme, or is it just an oversight because our naming scheme is admittedly not very self-explaining?

• The Apple target is ancient and I assume was picked for consistency with how Apple calls this -- not sure if that should overwrite our own naming scheme.
• For Hurd and Redox, we have no other targets that use PentiumPro without SSE as baseline, so it's a bit unclear what one would even use -- they are somewhere between i586 (original Pentium) and what we call i686 (Pentium 4). The most consistent outcome here would be to use Pentium 4 as the baseline like we use for all other OSes; not sure why Hurd and Redox should be special.
• The i586-pc-nto-qnx700 one however should almost certainly be called i686.

Pinging the listed target maintainers and some other folks:
Cc @bjorn3 @workingjubilee @badboy @deg4uss3r @madsmtm @sthibaul @jackpot51 @flba-eb @gh-tr @jonathanpallant @japaric

[jonathanpallant]

For QNX Neutrino 7.0.0, that is the name that Blackberry QNX use for their C toolchain:

~/qnx700/host/darwin/x86_64/usr/bin/i586-pc-nto-qnx7.0.0-ld

I think the primary question you all need to ask yourselves is:

• Is it better for targets to be internally consistent (i.e. within rustc), or is it better for each target be consistent with other toolchains for that target

I've already had this argument about arm64e-apple-darwin (which in my view should have been aarch64-apple-darwin-something), and I lost. But maybe you'll have another ticket for the Arm 64-bit targets.

[RalfJung]

I was wondering about the arm64e prefix... but I didn't plan to check all our targets, now. I am too scared of what I may find. ;)

[bjorn3]

arm64ec-pc-windows-msvc is similar in weird naming, though it's behavior is much less cursed than in C where it literally makes target detection using macros think it is running on x86 with x86 vendor intrinsics emulated by the compiler.

[sthibaul]

For the last 2, we have no other targets that use PentiumPro without SSE as baseline

I don't remember the details, but basically we targetted i686 as defined in Debian:

https://wiki.debian.org/ArchitectureSpecificsMemo#i386-1

(and also gcc)

The most consistent outcome here would be to use Pentium 4 as the baseline like we use for all other OSes

? That would assume SSE2, which is wrong? #82435 (only pentium-M has SSE2)

Adding MMX and SSE should however be fine enough, even if the very original pentiumpro didn't have them.

[workingjubilee]

Debian also advises that you not use a CPU less than Pentium 4.

[sthibaul]

Debian also advises that you not use a CPU less than Pentium 4.

This is just a recommendation.

Debian can actually be used on way less-powered i686 systems if you stick to a minimal system. But you cannot expect to be able to do what you were used to be doing 20 years ago with the Debian distribution at the time: code has gotten much larger.

[workingjubilee]

Debian states that a Pentium Pro is the minimum:

However, Debian GNU/Linux bookworm will not run on 586 (Pentium) or earlier processors.

(or however else you choose to interpret "not Pentium, but post-Pentium Pentiums, thus...")

[sthibaul]

Yes, Pentium Pro, not Pentium4

[workingjubilee]

For note: Currently, the Debian-distributed rustc patches our i686-unknown-linux-gnu to use Pentium Pros instead of Pentium 4s. This results in them getting bugs and test failures in Rust code because the code generation doesn't use SSE2 registers, thus is unsound due to various bugs in LLVM (and while I am told GCC is better, I'm not entirely sure). They then report this to crate maintainers. The crate maintainers, having deliberately developed against i686-unknown-linux-gnu as per rustc's definition, get very confused. They then get annoyed, when they find out this is entirely caused by Debian's insistence on divergence from rustc's definitions.

Unfortunately using real CPU names in targets like perhaps pentium4-unknown-linux-gnu didn't catch on, and neither did tuples like i786-unknown-linux-gnu (even if they are technically recognized by Autotools).

[sthibaul]

this is entirely caused by Debian's insistence on divergence from rustc's definitions

You mean caused by rust's insistence on assuming that i686 has SSE2.