Improve code generated for `starts_with(<literal char>)` by ranma42 · Pull Request #67249 · rust-lang/rust

ranma42 · 2019-12-12T02:43:47Z

This PR includes two minor improvements to the code generated when checking for string prefix/suffix.

The first commit simplifies the str/str operation, by taking advantage of the raw UTF-8 representation.

The second commit replaces the current str/char matching logic with a char->str encoding and then the previous method.

The resulting code should be equivalent in the generic case (one char is being encoded versus one char being decoded), but it becomes easy to optimize in the case of a literal char, which in most cases a developer might expect to be at least as simple as that of a literal string.

This PR should fix#41993

The comparison can be performed on the raw bytes, as the chars can only match if their UTF8 encoding matches. This avoids the `is_char_boundary` checks and translates to a straight `u8` slice comparison which is optimized to a memcmp or inline comparison where appropriate.

This enables constant folding when matching a literal char. Fixesrust-lang#41993.

rust-highfive · 2019-12-12T02:43:51Z

r? @shepmaster

(rust_highfive has picked a reviewer for you, use r? to override)

Mark-Simulacrum · 2019-12-12T13:51:02Z

cc @kennytm

r? @BurntSushi perhaps?

BurntSushi

The change itself LGTM. Do you have benchmarks showing a difference here? Mostly just to confirm there aren't any regressions. Even with no benefit, I think these changes make the code simpler.

BurntSushi · 2019-12-12T14:02:20Z

 /// Checks whether the pattern matches at the front of the haystack
 #[inline]
 fn is_prefix_of(self, haystack: &'a str) -> bool {
- haystack.is_char_boundary(self.len()) &&


This is interesting. According to git blame, @bluss added this in 2015. But yeah, this looks unnecessary to me and I agree with the change.

BurntSushi · 2019-12-12T14:07:07Z

- false
- }
+ let mut buffer = [0u8; 4];
+ self.encode_utf8(&mut buffer).is_prefix_of(haystack)


I think this can just be simplified to self.encode_utf8(&mut [0; 4]).is_prefix_of(haystack)? And similarly for below.

You are right; I added a cleanup commit.

ranma42 · 2019-12-12T21:20:44Z

I used this code to check the generated assembly.
I benchmarked against the current nightly:

$ rustc --version
rustc 1.41.0-nightly (27d6f55f4 2019-12-11)
$ rustc -C opt-level=3 --test starts_with.rs
$ ./starts_with --bench
running 4 tests
test bench_ends_with_char ... bench: 437 ns/iter (+/- 21)
test bench_ends_with_string ... bench: 1,405 ns/iter (+/- 86)
test bench_starts_with_char ... bench: 713 ns/iter (+/- 19)
test bench_starts_with_string ... bench: 995 ns/iter (+/- 37)
test result: ok. 0 passed; 0 failed; 0 ignored; 4 measured; 0 filtered out
$ ./rust/build/x86_64-apple-darwin/stage2/bin/rustc --version
rustc 1.41.0-dev
$ ./rust/build/x86_64-apple-darwin/stage2/bin/rustc -C opt-level=3 --test starts_with.rs
$ ./starts_with --bench
running 4 tests
test bench_ends_with_char ... bench: 395 ns/iter (+/- 13)
test bench_ends_with_string ... bench: 314 ns/iter (+/- 29)
test bench_starts_with_char ... bench: 315 ns/iter (+/- 12)
test bench_starts_with_string ... bench: 315 ns/iter (+/- 24)
test result: ok. 0 passed; 0 failed; 0 ignored; 4 measured; 0 filtered out

BurntSushi · 2019-12-13T03:44:58Z

@ranma42 Is there a place where those benchmarks can be added in this PR?

ranma42 · 2019-12-16T14:35:22Z

I added the benchmarks in 3de1923 , but I am not completely sure if that is the correct/best way to do it.

BurntSushi · 2019-12-16T14:46:01Z

LGTM. Thanks so much!

@bors r+

bors · 2019-12-16T14:46:03Z

📌 Commit 3de1923 has been approved by BurntSushi

bors · 2019-12-16T14:46:04Z

🌲 The tree is currently closed for pull requests below priority 100, this pull request will be tested once the tree is reopened

…-char, r=BurntSushi Improve code generated for `starts_with(<literal char>)` This PR includes two minor improvements to the code generated when checking for string prefix/suffix. The first commit simplifies the str/str operation, by taking advantage of the raw UTF-8 representation. The second commit replaces the current str/char matching logic with a char->str encoding and then the previous method. The resulting code should be equivalent in the generic case (one char is being encoded versus one char being decoded), but it becomes easy to optimize in the case of a literal char, which in most cases a developer might expect to be at least as simple as that of a literal string. This PR should fixrust-lang#41993

@ghost

Rollup of 8 pull requests Successful merges: - #67249 (Improve code generated for `starts_with(<literal char>)`) - #67308 (Delete flaky test net::tcp::tests::fast_rebind) - #67318 (Improve typeck & lowering docs for slice patterns) - #67322 (use Self alias in place of macros) - #67323 (make transparent enums more ordinary) - #67336 (Fix JS error when loading page with search) - #67344 (.gitignore: Don't ignore a file that exists in the repository) - #67349 (Minor: update Unsize docs for dyn syntax) Failed merges: r? @ghost

ranma42 added 2 commits December 11, 2019 21:24

Prefer encoding the char when checking for string prefix/suffix
1f6d023
This enables constant folding when matching a literal char. Fixesrust-lang#41993.

rust-highfive assigned shepmaster Dec 12, 2019

rust-highfive added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Dec 12, 2019

rust-highfive assigned BurntSushi and unassigned shepmaster Dec 12, 2019

BurntSushi reviewed Dec 12, 2019
View reviewed changes

Minor cleanup in Pattern::{is_prefix_of,is_suffix_of} for char
de7fefa

kennytm approved these changes Dec 12, 2019
View reviewed changes

Add benchmarks for start_with and ends_with
3de1923

bors added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Dec 16, 2019

Centril mentioned this pull request Dec 16, 2019
Rollup of 9 pull requests #67352
Closed

Centril mentioned this pull request Dec 16, 2019
Rollup of 8 pull requests #67356
Merged

bors merged commit 3de1923 into rust-lang:masterDec 16, 2019

ebroto mentioned this pull request Aug 14, 2020
Slow suggestion of single_char_pattern rust-lang/rust-clippy#3813
Closed

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Improve code generated for `starts_with(<literal char>)` - #67249

Improve code generated for `starts_with(<literal char>)`#67249
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