jetscii: jetscii returns incorrect result in mysterious circumstances

I’ve reduced this as much as I can, but it disappears if I reduce it any further.

The code is here: https://github.com/brson/searchtest/blob/jetsciibug/src/lib.rs#L16

On the jetsciibug branch of my searchtest repo. The bug shows up in a benchmark, where jetscii returns the wrong result. This happens on both beta and nightly, the versions of which are printed below. This is using RUSTC_BOOTSTRAP to test benchmarking on beta.

This happens with target-cpu=x86_64 but not with target-cpu=skylake. I haven’t tested any other targets.

brian@DESKTOP-UCV672I:~/searchtest⟫ RUSTC_BOOTSTRAP=1 RUSTFLAGS="-C target-cpu=x86-64" cargo +beta bench -- --nocapture
    Finished release [optimized] target(s) in 0.03s
     Running target/release/deps/searchtest-ae1a210660c35b0a

running 1 test
thread 'main' panicked at 'assertion failed: `(left == right)`
  left: `Some(64)`,
 right: `Some(419)`', src/lib.rs:20:9
note: Run with `RUST_BACKTRACE=1` for a backtrace.
test find_from_set_jetscii_bytes_lipsum_emph ... FAILED

failures:

failures:
    find_from_set_jetscii_bytes_lipsum_emph

test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out

error: bench failed
101 brian@DESKTOP-UCV672I:~/searchtest⟫ rustc +beta --version
rustc 1.27.0-beta.5 (84b5a46f8 2018-05-15)
brian@DESKTOP-UCV672I:~/searchtest⟫ RUSTC_BOOTSTRAP=1 RUSTFLAGS="-C target-cpu=x86-64" cargo +nightly bench -- --nocapture
   Compiling searchtest v0.1.0 (file:///home/brian/searchtest)
    Finished release [optimized] target(s) in 1.75s
     Running target/release/deps/searchtest-191788c2bec2b787

running 1 test
thread 'main' panicked at 'assertion failed: `(left == right)`
  left: `Some(64)`,
 right: `Some(419)`', src/lib.rs:20:9
note: Run with `RUST_BACKTRACE=1` for a backtrace.
test find_from_set_jetscii_bytes_lipsum_emph ... FAILED

failures:

failures:
    find_from_set_jetscii_bytes_lipsum_emph

test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out

error: bench failed
101 brian@DESKTOP-UCV672I:~/searchtest⟫ rustc +nightly --version
rustc 1.27.0-nightly (f0fdaba04 2018-05-15)