to_digit simplification (less jumps)

[gilescope]

I just realised we might be able to make use of the fact that changing case in ascii is easy to help simplify to_digit some more.

It looks a bit cleaner and it looks like it's less jumps and there's less instructions in the generated assembly:

https://godbolt.org/z/84Erh5dhz

The benchmarks don't really tell me much. Maybe a slight improvement on the var radix.

Before:

test char::methods::bench_to_digit_radix_10                     ... bench:      53,819 ns/iter (+/- 8,314)
test char::methods::bench_to_digit_radix_16                     ... bench:      57,265 ns/iter (+/- 10,730)
test char::methods::bench_to_digit_radix_2                      ... bench:      55,077 ns/iter (+/- 5,431)
test char::methods::bench_to_digit_radix_36                     ... bench:      56,549 ns/iter (+/- 3,248)
test char::methods::bench_to_digit_radix_var                    ... bench:      43,848 ns/iter (+/- 3,189)

test char::methods::bench_to_digit_radix_10                     ... bench:      51,707 ns/iter (+/- 10,946)
test char::methods::bench_to_digit_radix_16                     ... bench:      52,835 ns/iter (+/- 2,689)
test char::methods::bench_to_digit_radix_2                      ... bench:      51,012 ns/iter (+/- 2,746)
test char::methods::bench_to_digit_radix_36                     ... bench:      53,210 ns/iter (+/- 8,645)
test char::methods::bench_to_digit_radix_var                    ... bench:      40,386 ns/iter (+/- 4,711)

test char::methods::bench_to_digit_radix_10                     ... bench:      54,088 ns/iter (+/- 5,677)
test char::methods::bench_to_digit_radix_16                     ... bench:      55,972 ns/iter (+/- 17,229)
test char::methods::bench_to_digit_radix_2                      ... bench:      52,083 ns/iter (+/- 2,425)
test char::methods::bench_to_digit_radix_36                     ... bench:      54,132 ns/iter (+/- 1,548)
test char::methods::bench_to_digit_radix_var                    ... bench:      41,250 ns/iter (+/- 5,299)

After:

test char::methods::bench_to_digit_radix_10                     ... bench:      48,907 ns/iter (+/- 19,449)
test char::methods::bench_to_digit_radix_16                     ... bench:      52,673 ns/iter (+/- 8,122)
test char::methods::bench_to_digit_radix_2                      ... bench:      48,509 ns/iter (+/- 2,885)
test char::methods::bench_to_digit_radix_36                     ... bench:      50,526 ns/iter (+/- 4,610)
test char::methods::bench_to_digit_radix_var                    ... bench:      38,618 ns/iter (+/- 3,180)

test char::methods::bench_to_digit_radix_10                     ... bench:      54,202 ns/iter (+/- 6,994)
test char::methods::bench_to_digit_radix_16                     ... bench:      56,585 ns/iter (+/- 8,448)
test char::methods::bench_to_digit_radix_2                      ... bench:      50,548 ns/iter (+/- 1,674)
test char::methods::bench_to_digit_radix_36                     ... bench:      52,749 ns/iter (+/- 2,576)
test char::methods::bench_to_digit_radix_var                    ... bench:      40,215 ns/iter (+/- 3,327)

test char::methods::bench_to_digit_radix_10                     ... bench:      50,233 ns/iter (+/- 22,272)
test char::methods::bench_to_digit_radix_16                     ... bench:      50,841 ns/iter (+/- 19,981)
test char::methods::bench_to_digit_radix_2                      ... bench:      50,386 ns/iter (+/- 4,555)
test char::methods::bench_to_digit_radix_36                     ... bench:      52,369 ns/iter (+/- 2,737)
test char::methods::bench_to_digit_radix_var                    ... bench:      40,417 ns/iter (+/- 2,766)

I removed the likely as it resulted in a few less instructions. (It's not been in there long - I added it in the last to_digit iteration).

[rust-highfive]

r? @kennytm

(rust-highfive has picked a reviewer for you, use r? to override)

[klensy]

There bultin diff function in godbolt: https://godbolt.org/z/jMsv6dKh8

[gilescope]

Good point. Have flicked the code around so the red / green works in the diff and optimised the to upper case:

https://godbolt.org/z/79KrbxMdY

[nagisa]

I don't believe this implementation is correct. E.g. it will return Some(10) for ascii : in base 11.

Some other issues: A is Some(17) in base 36, a is None in base 36, even though both should be returning a Some(10).

(Can you add cases like these as tests, please?)

[gilescope]

Sorry jumped strate to the bench skipping tests - too much excitement.

[gilescope]

Ok am really happy with that: https://godbolt.org/z/n77njGz3T

[gilescope]

@nagisa is this all looking good now from your perspective?

[nagisa]

Squash the commits, please. Otherwise looks pretty nice.

[klensy]

Can you update benches in first message with latest version?

[nagisa]

r? @nagisa

[gilescope]

will do

[gilescope]

Have updated the benchmarks. I'm not really happy with the benchmark definitions though I have failed to come up with an alternative that has less benchmark noise.

[gilescope]

Ok @nagisa think it is good to go.

[nagisa]

@bors r+

[bors]

📌 Commit 9c3d81e has been approved by nagisa

[bors]

⌛ Testing commit 9c3d81e with merge 46ad16b...

[klensy]

@gilescope You can use cargo-benchcmp to compare bench results in more simple way.

[bors]

☀️ Test successful - checks-actions
Approved by: nagisa
Pushing 46ad16b to master...