speed up `String::push` and `String::insert` #124810

lincot · 2024-05-06T16:39:28Z

Addresses the concerns described in #116235.

The performance gain comes mainly from avoiding temporary buffers.

Complex pattern matching in encode_utf8 (introduced in #67569) has been simplified to a comparison and an exhaustive match in the encode_utf8_raw_unchecked helper function. It takes a slice of MaybeUninit<u8> because otherwise we'd have to construct a normal slice to uninitialized data, which is not desirable, I guess.

Several functions still have that unneeded zeroing, but a single instruction is not that important, I guess.

@rustbot label T-libs C-optimization A-str

rustbot · 2024-05-06T16:39:35Z

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @scottmcm (or someone else) some time within the next two weeks.

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scottmcm · 2024-05-13T05:16:23Z

library/core/src/char/methods.rs

+#[unstable(feature = "char_internals", reason = "exposed only for libstd", issue = "none")]
+#[doc(hidden)]
+#[inline]
+pub unsafe fn encode_utf8_raw_unchecked(code: u32, dst: &mut [MaybeUninit<u8>]) -> &mut [u8] {


Pondering: How useful is it to be dealing in slices for this? Could this return, say, (usize, [u8; 4]) and thus not ever need to worry about the indirections? That would presumably resolve the zeroing issue, since it'd just be shifting together a 32-bit number (since [u8; 4] is passes as i32 in our LLVM ABI).

Currently, some functions use the encode_utf8 API, which requires a slice. Using a [u8; 4] makes it act as a number indeed, but it still needs to be xored with itself and is later moved to a buffer for bcmp in this case.

scottmcm · 2024-05-13T05:18:36Z

library/alloc/src/string.rs

-            _ => self.vec.extend_from_slice(ch.encode_utf8(&mut [0; 4]).as_bytes()),
+        let len = self.len();
+        let ch_len = ch.len_utf8();
+        self.reserve(ch_len);


Related to the previous, I wonder about making this .reserve(4), and just always copying the 4 bytes into the buffer, with only the set_len needing to use the actual length, so that it's always just one store rather than needing a variable number of stores depending on the data width.

Reserving 4 bytes and doing a single store makes little difference other than getting rid of the unsafe, but reserving 4 bytes and doing the same writes makes the non-reallocating path 20% instructions shorter. However, it may cause the string to take up extra space: say, an ASCII char is pushed to a 63-byte string, making it allocate 128 bytes. Is this acceptable?

It's a good question. I started a zulip thread: https://rust-lang.zulipchat.com/#narrow/stream/219381-t-libs/topic/String.3A.3Apush.20capacity.20guarantees/near/438525052

Worth noting here that this effectively just means that you'll have up to three extra bytes reserved always, since reserve takes into account existing capacity. It does slow down the case of, say, adding a newline to an existing string, but it would speed up repeated insertions, which are probably the bigger performance hit.

library/core/src/char/methods.rs

scottmcm

I had a variety of thoughts; let me know what you think.

Also, is there anything here for which it would make sense to have a codegen test to confirm what's happening? Or some other test to help confirm it's better?

lincot · 2024-05-13T20:52:24Z

A codegen check for the absence of memcpy would be nice, since the original String::push has one.

rust-timer · 2024-06-12T19:58:18Z

Insufficient permissions to issue commands to rust-timer.

bors · 2024-06-12T19:58:18Z

@lincot: 🔑 Insufficient privileges: not in try users

rust-timer · 2024-06-12T19:58:51Z

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bors · 2024-06-22T19:02:36Z

☔ The latest upstream changes (presumably #116113) made this pull request unmergeable. Please resolve the merge conflicts.

rustbot · 2024-07-10T18:13:27Z

There are merge commits (commits with multiple parents) in your changes. We have a no merge policy so these commits will need to be removed for this pull request to be merged.

You can start a rebase with the following commands:

$ # rebase
$ git pull --rebase https://github.com/rust-lang/rust.git master
$ git push --force-with-lease

The following commits are merge commits:

9511918

bors · 2024-07-17T06:13:44Z

☔ The latest upstream changes (presumably #127840) made this pull request unmergeable. Please resolve the merge conflicts.

bors · 2024-09-19T07:21:04Z

☔ The latest upstream changes (presumably #130511) made this pull request unmergeable. Please resolve the merge conflicts.

rustbot assigned scottmcm May 6, 2024

scottmcm reviewed May 13, 2024

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scottmcm requested changes May 13, 2024

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rustbot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels May 13, 2024

cuviper mentioned this pull request May 14, 2024

Remove the branches from len_utf8 #125129

Closed

rustbot added the has-merge-commits PR has merge commits, merge with caution. label Jul 10, 2024

lincot force-pushed the speed-up-string-push-and-string-insert branch from 9511918 to 89fa55e Compare July 10, 2024 19:08

rustbot removed the has-merge-commits PR has merge commits, merge with caution. label Jul 10, 2024

lincot added 3 commits August 6, 2024 21:58

speed up String::push and String::insert

d90e0f3

clarify a safety comment

7df40e7

add codegen check for absence of memcpy in String::push

2cb20b3

lincot force-pushed the speed-up-string-push-and-string-insert branch from 89fa55e to 2cb20b3 Compare August 6, 2024 19:00

Dylan-DPC added the S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. label Aug 15, 2024

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speed up `String::push` and `String::insert` #124810

speed up `String::push` and `String::insert` #124810

lincot commented May 6, 2024

rustbot commented May 6, 2024

scottmcm May 13, 2024

lincot May 13, 2024

scottmcm May 13, 2024

lincot May 13, 2024 •

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speed up String::push and String::insert #124810

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speed up String::push and String::insert #124810

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speed up `String::push` and `String::insert` #124810

speed up `String::push` and `String::insert` #124810

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