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time: use bit manipulation instead of modulo #4480

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merged 2 commits into from
Feb 15, 2022
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samueltardieu
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time: resolve TODO

Motivation

Existing TODO comment in src/time/driver/wheel/level.rs.

Solution

level_range() always return a strictly positive power of 2. If b is a
strictly positive power of 2, a - (a % b) is equal to a & !(b - 1).

`level_range()` always return a strictly positive power of 2.
@Darksonn Darksonn added A-tokio Area: The main tokio crate M-time Module: tokio/time labels Feb 8, 2022
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@hawkw hawkw left a comment

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this looks correct to me. it might be worth commenting on the bit manipulation?

tokio/src/time/driver/wheel/level.rs Show resolved Hide resolved
@hawkw hawkw merged commit 28b983c into tokio-rs:master Feb 15, 2022
hawkw added a commit that referenced this pull request Feb 15, 2022
# 1.16.2 (February 15, 2022)

This release updates the minimum supported Rust version (MSRV) to 1.49,
the `mio` dependency to v0.8, and the (optional) `parking_lot`
dependency to v0.12. Additionally, it contains several bug fixes, as
well as internal refactoring and performance improvements.

### Fixed

- time: prevent panicking in `sleep` with large durations ([#4495])
- time: eliminate potential panics in `Instant` arithmetic on platforms
  where `Instant::now` is not monotonic ([#4461])
- io: fix `DuplexStream` not participating in cooperative yielding
  ([#4478])
- rt: fix potential double panic when dropping a `JoinHandle` ([#4430])

### Changed

- update minimum supported Rust version to 1.49 ([#4457])
- update `parking_lot` dependency to v0.12.0 ([#4459])
- update `mio` dependency to v0.8 ([#4449])
- rt: remove an unnecessary lock in the blocking pool ([#4436])
- rt: remove an unnecessary enum in the basic scheduler ([#4462])
- time: use bit manipulation instead of modulo to improve performance
  ([#4480])
- net: use `std::future::Ready` instead of our own `Ready` future
  ([#4271])
- replace deprecated `atomic::spin_loop_hint` with `hint::spin_loop`
  ([#4491])
- fix miri failures in intrusive linked lists ([#4397])

### Documented

- io: add an example for `tokio::process::ChildStdin` ([#4479])

### Unstable

The following changes only apply when building with `--cfg
tokio_unstable`:

- task: fix missing location information in `tracing` spans generated by
  `spawn_local` ([#4483])
- task: add `JoinSet` for managing sets of tasks ([#4335])
- metrics: fix compilation error on MIPS ([#4475])
- metrics: fix compilation error on arm32v7 ([#4453])

[#4495]: #4495
[#4461]: #4461
[#4478]: #4478
[#4430]: #4430
[#4457]: #4457
[#4459]: #4459
[#4449]: #4449
[#4462]: #4462
[#4436]: #4436
[#4480]: #4480
[#4271]: #4271
[#4491]: #4491
[#4397]: #4397
[#4479]: #4479
[#4483]: #4483
[#4335]: #4335
[#4475]: #4475
[#4453]: #4453
hawkw added a commit that referenced this pull request Feb 16, 2022
# 1.17.0 (February 16, 2022)

This release updates the minimum supported Rust version (MSRV) to 1.49,
the `mio` dependency to v0.8, and the (optional) `parking_lot`
dependency to v0.12. Additionally, it contains several bug fixes, as
well as internal refactoring and performance improvements.

### Fixed

- time: prevent panicking in `sleep` with large durations ([#4495])
- time: eliminate potential panics in `Instant` arithmetic on platforms
  where `Instant::now` is not monotonic ([#4461])
- io: fix `DuplexStream` not participating in cooperative yielding
  ([#4478])
- rt: fix potential double panic when dropping a `JoinHandle` ([#4430])

### Changed

- update minimum supported Rust version to 1.49 ([#4457])
- update `parking_lot` dependency to v0.12.0 ([#4459])
- update `mio` dependency to v0.8 ([#4449])
- rt: remove an unnecessary lock in the blocking pool ([#4436])
- rt: remove an unnecessary enum in the basic scheduler ([#4462])
- time: use bit manipulation instead of modulo to improve performance
  ([#4480])
- net: use `std::future::Ready` instead of our own `Ready` future
  ([#4271])
- replace deprecated `atomic::spin_loop_hint` with `hint::spin_loop`
  ([#4491])
- fix miri failures in intrusive linked lists ([#4397])

### Documented

- io: add an example for `tokio::process::ChildStdin` ([#4479])

### Unstable

The following changes only apply when building with `--cfg
tokio_unstable`:

- task: fix missing location information in `tracing` spans generated by
  `spawn_local` ([#4483])
- task: add `JoinSet` for managing sets of tasks ([#4335])
- metrics: fix compilation error on MIPS ([#4475])
- metrics: fix compilation error on arm32v7 ([#4453])

[#4495]: #4495
[#4461]: #4461
[#4478]: #4478
[#4430]: #4430
[#4457]: #4457
[#4459]: #4459
[#4449]: #4449
[#4462]: #4462
[#4436]: #4436
[#4480]: #4480
[#4271]: #4271
[#4491]: #4491
[#4397]: #4397
[#4479]: #4479
[#4483]: #4483
[#4335]: #4335
[#4475]: #4475
[#4453]: #4453
@rneswold
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Does this actually improve performance? Or is the performance the same because the compiler already applied your recommendation? Compilers are very smart and could do this optimization for us without having to obfuscating the code.

In the Rust playground:

#![crate_type = "staticlib"]

#[no_mangle]
pub fn f(x: u32) -> u32 {
    x - (x % 11)
}

shows it loading a register with 11 and performing a division. But in this:

#![crate_type = "staticlib"]

#[no_mangle]
pub fn f(x: u32) -> u32 {
    x - (x % 4)
}

the compiler instead masks the value with 3 before subtracting, so it performed the optimization you described.

@Darksonn
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I don't know, but it would require that the compiler can tell that level_range returns a power of two.

@rneswold
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I see.

I'm not a fan of having comments enforce a constraint (line 39: "Being a power of 2 is important.")

How about, at line 40:

const LEVELS: usize = 6;
const LEVEL_MULT: usize = 1 << LEVELS;

@Darksonn
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Feel free to submit a PR that makes the code easier to read.

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4 participants