memory sanitizer fails at the build-std stage

[g2p]

Looking at CI runs, this failed between 2020-09-14 and 2020-10-07 and is still broken.
This seems to relate to building avx512f instructions, which it might be possible to skip on most machines.
I'll try to investigate further by bisecting nightlies.

• https://github.com/rust-fuzz/cargo-fuzz/runs/1113571401?check_suite_focus=true
• https://github.com/rust-fuzz/cargo-fuzz/runs/1222113851?check_suite_focus=true

[g2p]

Regression appeared on 2020-09-28.

RUST_SRC_REPO=~/src/github.com/rust-lang/rust cargo install cargo-bisect-rustc
echo $'#!/bin/sh\ncargo test run_with_msan_with_crash' >> test-msan.sh
chmod +x test-msan.sh
cargo bisect-rustc --with-src --start 2020-09-14 --end 2020-10-07 --script ./test-msan.sh 

Getting it down to a bors commit was harder since the msan test has false failures with these builds (possibly they are less optimised).

cargo bisect-rustc --preserve --by-commit --with-src --start 2020-09-27 --end 2020-09-28 --script ./test-msan.sh --preserve-target --prompt

This is the culprit:
rust-lang/rust@1d216fe

[g2p]

rust-lang/rust#77259

[g2p]

This can be worked around by reverting #193.

[g2p]

Introduced in rust-lang/stdarch@3fa0f6a
Unsquashed version is at rust-lang/stdarch#907

[g2p]

rust-lang/stdarch#957