checked_ilog: improve performance

[FedericoStra]

Addresses #115874.

(This PR replicates the original #115875, which I accidentally closed by deleting my forked repository...)

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[joshtriplett]

r? libs

[the8472]

The implementation looks correct and we do have exhaustive tests for u16 so that part should be fine. And multiplication should be quite obviously better than division. But since you added benchmark sanyway can you add the before/after benchmark results to PR comment for future reference?

[tspiteri]

I ran the benchmarks included in the fourth commit. Basically, for "before" I cherry picked just the fourth commit onto the master branch, and for the "after" I rebased the whole PR onto the master. Results below:

Before:

    num::int_log::u128_log_geometric        82.91µs/iter   +/- 1.20µs
    num::int_log::u128_log_random            2.10ms/iter   +/- 2.74µs
    num::int_log::u128_log_random_small      1.42ms/iter  +/- 14.30µs
    num::int_log::u16_log_geometric        740.00ns/iter   +/- 0.00ns
    num::int_log::u16_log_random           634.11µs/iter  +/- 10.48µs
    num::int_log::u16_log_random_small     502.78µs/iter   +/- 3.06µs
    num::int_log::u32_log_geometric          4.29µs/iter  +/- 11.00ns
    num::int_log::u32_log_random           757.75µs/iter   +/- 2.19µs
    num::int_log::u32_log_random_small     203.69µs/iter   +/- 2.36µs
    num::int_log::u64_log_geometric         26.20µs/iter +/- 124.00ns
    num::int_log::u64_log_random             1.09ms/iter   +/- 3.65µs
    num::int_log::u64_log_random_small     251.95µs/iter   +/- 3.23µs
    num::int_log::u8_log_geometric         180.00ns/iter   +/- 6.00ns
    num::int_log::u8_log_random            192.59µs/iter   +/- 2.61µs
    num::int_log::u8_log_random_small      192.83µs/iter   +/- 2.88µs

After:

    num::int_log::u128_log_geometric        26.60µs/iter  +/- 65.00ns
    num::int_log::u128_log_random            1.83ms/iter  +/- 11.76µs
    num::int_log::u128_log_random_small    936.31µs/iter   +/- 3.43µs
    num::int_log::u16_log_geometric        296.00ns/iter  +/- 17.00ns
    num::int_log::u16_log_random           466.24µs/iter   +/- 2.04µs
    num::int_log::u16_log_random_small     407.55µs/iter   +/- 9.85µs
    num::int_log::u32_log_geometric        812.00ns/iter   +/- 6.00ns
    num::int_log::u32_log_random           451.27µs/iter   +/- 2.17µs
    num::int_log::u32_log_random_small     468.65µs/iter   +/- 2.36µs
    num::int_log::u64_log_geometric          4.99µs/iter  +/- 30.00ns
    num::int_log::u64_log_random           593.27µs/iter   +/- 1.83µs
    num::int_log::u64_log_random_small     179.98µs/iter   +/- 3.62µs
    num::int_log::u8_log_geometric         129.00ns/iter   +/- 3.00ns
    num::int_log::u8_log_random            475.02µs/iter   +/- 1.58µs
    num::int_log::u8_log_random_small      474.46µs/iter   +/- 1.39µs

[the8472]

Looks great. Thanks.

@bors r+ rollup

[bors]

📌 Commit 3de51c9 has been approved by the8472

It is now in the queue for this repository.

[bors]

⌛ Testing commit 3de51c9 with merge 981835d...

[rust-log-analyzer]

The job x86_64-gnu-aux failed! Check out the build log: (web) (plain)

Click to see the possible cause of the failure (guessed by this bot)

[bors]

💔 Test failed - checks-actions

[the8472]

Looks like a flaky test in miri, other PRs have run into that too.

@bors retry