Make char::is_ascii_whitespace branchless on 32 and 64-bit targets

[lnicola]

No description provided.

[rust-highfive]

r? @cramertj

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[lnicola]

is_ascii_whitespace/std time:   [267.44 ns 267.82 ns 268.32 ns]                                    
Found 9 outliers among 100 measurements (9.00%)
  3 (3.00%) high mild
  6 (6.00%) high severe
is_ascii_whitespace/pr  time:   [196.86 ns 197.11 ns 197.37 ns]                                   
Found 11 outliers among 100 measurements (11.00%)
  1 (1.00%) low severe
  2 (2.00%) low mild
  3 (3.00%) high mild
  5 (5.00%) high severe

Benchmark code:

use criterion::{black_box, criterion_group, criterion_main, Criterion};

fn bench_is_ascii_whitespace(c: &mut Criterion) {
    let mut group = c.benchmark_group("is_ascii_whitespace");
    group.bench_function("std", |b| {
        b.iter(|| {
            let mut n = 0;
            for i in 0..128u8 {
                if is_whitespace::std_is_ascii_whitespace(black_box(&(i as char))) {
                    n += 1;
                }
            }
            black_box(n);
        })
    });
    group.bench_function("pr", |b| {
        b.iter(|| {
            let mut n = 0;
            for i in 0..128u8 {
                if is_whitespace::pr_is_ascii_whitespace(black_box(&(i as char))) {
                    n += 1;
                }
            }
            black_box(n);
        })
    });
}

criterion_group!(benches, bench_is_ascii_whitespace);
criterion_main!(benches);

Generated code: https://rust.godbolt.org/z/Ws69P3

[lnicola]

Lies, damned lies and statistics

This might not be a real improvement. Here I ran one version first, and then the other (red is std):

[llogiq]

If you look at the generated assembly of the match-based version, you'll find that LLVM already does the u32 as bit set trick, and generates leaner code.

[lnicola]

I haven't looked at the machine code, but the new version is one instruction shorter and (maybe more importantly) branchless.

[llogiq]

Being branchless will not be very beneficial here, because a) In normal text, there isn't that much whitespace (or even other chars < 33), and so the branch will often be taken resulting in only a cmp, jg and xor being run and b) branch prediction has become very good on most architectures (although it'll be interesting to see the result on ARM, which was historically worse than Intel or AMD; unfortunately cargo asm doesn't work on my phone; will look into --emit asm and report back later). Also even if there's 1 more instruction, with the branchy version not all instructions are actually used, so that shouldn't make a difference.

[bors]

☔ The latest upstream changes (presumably #77630) made this pull request unmergeable. Please resolve the merge conflicts.

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