Performance regression on nightly-2017-11-20

[niklasf]

Source code under test: https://github.com/niklasf/shakmaty

Before:

$ cargo +nightly-2017-11-20 bench
[...]
test perft::tests::bench_deep_perft               ... bench:  29,666,456 ns/iter (+/- 10,492,824)
test perft::tests::bench_shallow_perft            ... bench:   1,153,713 ns/iter (+/- 3,677)
test position::tests::bench_generate_moves        ... bench:         122 ns/iter (+/- 3)
test position::tests::bench_play_unchecked        ... bench:          10 ns/iter (+/- 0)
test position::tests::bench_san_candidates        ... bench:          15 ns/iter (+/- 0)
test san::tests::bench_parse_san_move_complicated ... bench:          10 ns/iter (+/- 0)

After:

$ cargo +nightly-2017-11-21 bench
[...]
test perft::tests::bench_deep_perft               ... bench:  34,374,405 ns/iter (+/- 2,849,721)
test perft::tests::bench_shallow_perft            ... bench:   1,317,448 ns/iter (+/- 1,264)
test position::tests::bench_generate_moves        ... bench:         159 ns/iter (+/- 2)
test position::tests::bench_play_unchecked        ... bench:          14 ns/iter (+/- 0)
test position::tests::bench_san_candidates        ... bench:          15 ns/iter (+/- 0)
test san::tests::bench_parse_san_move_complicated ... bench:           9 ns/iter (+/- 0)

Platform:

binary: rustc
host: x86_64-unknown-linux-gnu
LLVM version: 4.0

I suspect it's related to #45225. Advice on how to debug this or what further information would be useful is welcome.

[Mark-Simulacrum]

Cc @eddyb

[niklasf]

With some optimizations it's almost back to the original level of performance. I did not find anything specific that should be improved in rustc.