The IR is a bit different (in particular wrt naming) if
debug-assertions-std is enabled. Peculiarly, the issue goes away
if overflow-check-std is also enabled, which is why CI did not
catch this.
slice/ascii: Optimize `eq_ignore_ascii_case` with auto-vectorization
- Refactor the current functionality into a helper function
- Use `as_chunks` to encourage auto-vectorization in the optimized chunk processing function
- Add a codegen test checking for vectorization and no panicking
- Add benches for `eq_ignore_ascii_case`
---
The optimized function is initially only enabled for x86_64 which has `sse2` as part of its baseline, but none of the code is platform specific. Other platforms with SIMD instructions may also benefit from this implementation.
Performance improvements only manifest for slices of 16 bytes or longer, so the optimized path is gated behind a length check for greater than or equal to 16.
Benchmarks - Cases below 16 bytes are unaffected, cases above all show sizeable improvements.
```
before:
str::eq_ignore_ascii_case::bench_large_str_eq 4942.30ns/iter +/- 48.20
str::eq_ignore_ascii_case::bench_medium_str_eq 632.01ns/iter +/- 16.87
str::eq_ignore_ascii_case::bench_str_17_bytes_eq 16.28ns/iter +/- 0.45
str::eq_ignore_ascii_case::bench_str_31_bytes_eq 35.23ns/iter +/- 2.28
str::eq_ignore_ascii_case::bench_str_of_8_bytes_eq 7.56ns/iter +/- 0.22
str::eq_ignore_ascii_case::bench_str_under_8_bytes_eq 2.64ns/iter +/- 0.06
after:
str::eq_ignore_ascii_case::bench_large_str_eq 611.63ns/iter +/- 28.29
str::eq_ignore_ascii_case::bench_medium_str_eq 77.10ns/iter +/- 19.76
str::eq_ignore_ascii_case::bench_str_17_bytes_eq 3.49ns/iter +/- 0.39
str::eq_ignore_ascii_case::bench_str_31_bytes_eq 3.50ns/iter +/- 0.27
str::eq_ignore_ascii_case::bench_str_of_8_bytes_eq 7.27ns/iter +/- 0.09
str::eq_ignore_ascii_case::bench_str_under_8_bytes_eq 2.60ns/iter +/- 0.05
```
This reverts PR <https://github.com/rust-lang/rust/pull/130998> because
the added test seems to be flaky / non-deterministic, and has been
failing in unrelated PRs during merge CI.
Add `ilog10` result range hints
This PR adds hints that the return value of `T::ilog10` will never exceed `T::MAX.ilog10()`.
This works because `ilog10` is a monotonically nondecreasing function, the maximum return value is reached at the max input value.
Similar to how this helps in `slice::Iter::position`, LLVM sometimes loses track of how high this can get, so for `TrustedLen` iterators tell it what the upper bound is.
Refactor the current functionality into a helper function
Use `as_chunks` to encourage auto-vectorization in the optimized chunk processing function
Add a codegen test
Add benches for `eq_ignore_ascii_case`
The optimized function is initially only enabled for x86_64 which has `sse2` as
part of its baseline, but none of the code is platform specific. Other
platforms with SIMD instructions may also benefit from this implementation.
Performance improvements only manifest for slices of 16 bytes or longer, so the
optimized path is gated behind a length check for greater than or equal to 16.
