MIPS executables are not maximally portable

[nagisa]

Running on qemu with -M malta -cpu 4Kc, running rustup-init results in run_utf8_validation:

(gdb) bt
#0  0x55a1cd38 in run_utf8_validation ()
    at /buildslave/rust-buildbot/slave/nightly-dist-rustc-cross-rustbuild-linux/build/src/libcore/str/mod.rs:1221
#1  core::str::from_utf8 ()
    at /buildslave/rust-buildbot/slave/nightly-dist-rustc-cross-rustbuild-linux/build/src/libcore/str/mod.rs:233
#2  0x559ed898 in from_utf8 ()
    at /buildslave/rust-buildbot/slave/nightly-dist-rustc-cross-rustbuild-linux/build/src/libcollections/string.rs:470
#3  into_string ()
    at /buildslave/rust-buildbot/slave/nightly-dist-rustc-cross-rustbuild-linux/build/src/libstd/sys/unix/os_str.rs:91
#4  into_string ()
    at /buildslave/rust-buildbot/slave/nightly-dist-rustc-cross-rustbuild-linux/build/src/libstd/ffi/os_str.rs:67
#5  {{closure}} ()
    at /buildslave/rust-buildbot/slave/nightly-dist-rustc-cross-rustbuild-linux/build/src/libstd/env.rs:617
#6  map<std::ffi::os_str::OsString,collections::string::String,closure> ()
    at /buildslave/rust-buildbot/slave/nightly-dist-rustc-cross-rustbuild-linux/build/src/libcore/option.rs:383
#7  std::env::{{impl}}::next ()
    at /buildslave/rust-buildbot/slave/nightly-dist-rustc-cross-rustbuild-linux/build/src/libstd/env.rs:617
#8  0x5561b38c in rustup_init::run_multirust::h4c791998cd88b205 ()
#9  0x5561aa74 in rustup_init::main::hdb8423f0f289797c ()
#10 0x55a086f4 in std::panicking::try::do_call<fn(),()> ()
    at /buildslave/rust-buildbot/slave/nightly-dist-rustc-cross-rustbuild-linux/build/src/libstd/panicking.rs:356
#11 0x55a122fc in panic_unwind::__rust_maybe_catch_panic ()
    at /buildslave/rust-buildbot/slave/nightly-dist-rustc-cross-rustbuild-linux/build/src/libpanic_unwind/lib.rs:97
#12 0x55a09458 in try<(),fn()> ()
    at /buildslave/rust-buildbot/slave/nightly-dist-rustc-cross-rustbuild-linux/build/src/libstd/panicking.rs:332
#13 catch_unwind<fn(),()> ()
    at /buildslave/rust-buildbot/slave/nightly-dist-rustc-cross-rustbuild-linux/build/src/libstd/panic.rs:351
#14 std::rt::lang_start ()
    at /buildslave/rust-buildbot/slave/nightly-dist-rustc-cross-rustbuild-linux/build/src/libstd/rt.rs:57
#15 0x55620de4 in main ()
(gdb) quit

-cpu 4KEc does work.

[jcowgill]

This is probably because the default mips cpu baseline in LLVM is MIPS32r2 and the 4Kc processor does not implement that version of the ISA (it only implemente MIPS32r1). However, 4KEc does implement MIPS32r2.

I'm not sure the default ISA level should be changed. There were quite a lot of instructions added in mips32r2 so I think it would only be a good idea to lower the baseline if someone actually benefits from it (QEMU doesn't really count). AFAIK I don't think any pre-r2 processors have been fabricated for some years now (I think maybe the Loongson-2 series from 2008 were the last ones?)

[sanxiyn]

I think it is somewhat reasonable to synchronize ISA baseline with Linux distributions. FYI, as documented in https://wiki.debian.org/MIPSPort, current Debian stable baseline is MIPS32R2.

[jcowgill]

#48874 may be relevant

[sanxiyn]

#48874 is merged. Closing this, as default ISA level is intentional.

[Jedzia]

For people that don't use MIPS32R2 (like me with R1) and want to compile pre #48874 maybe this helps:

Minimal build for VU+ DUO2 mipsel-unknown-linux-gnu with Rust on Windows #37507 (comment)