commit 20fc102287f59de23e40d0f64d62608451c9a3d3
author Martin Kustermann <kustermann@google.com> Thu Feb 27 00:33:34 2025 -0800
committer Commit Queue <dart-scoped@luci-project-accounts.iam.gserviceaccount.com> Thu Feb 27 00:33:34 2025 -0800
tree 4432709ceecb5731d0b5ef6bd0ef6ab25098ee67
parent 092728e8b5d5a42c77839be01173ac57b9c314be

[dart2wasm] Switch to only using JS strings

Currently we have 3 different string types (JS Strings, OneByteString
and TwoByteString)s. There's some advantages to this, mainly that
if strings are used purely inside Dart we have more control over
optimizing them. But it does come with some issues

* Operations on mixture of strings are slow
* We get JS strings from outside (in DevTools e.g. websocket messages)
* Any kind of DOM interaction requires copying strings
* Regular expression matches can result in O(N*N) instead of O(N)
* Encoding of string literals/constants is terrible, high size overhead
* ...

Now that there's a standardized way to access JS strings (via
the `js-string` builtin spec) and this standard is finalized and
enabled in Chrome & Firefox it makes sense for us to switch to it.

It reduces app size:

* Smaller size: hello world -25%, flute -5.5%
* Faster startup

The performance changes are nuanced, some workloads will improve
significantly, some workloads will regress.

Improvements will come especially in cases where
strings are concatenated (due to JS not actually allocating new
strings in this case). That impacts e.g. string interpolations,
string buffer, json-to-string encoding, ...

Regressions will come especially for cases where we have to
construct strings from bytes (e.g. in utf8 decoder, utf8+json
decoder) - mainly due to having to go through an intermediary
`WasmArray<WasmI16>` to allocate strings. Also in cases where we
access individual char codes from the strings.

There's some follow-up improvements we can do, but it's better to
not iterate on this CL even longer but get it landed.

This CL will make the benchmarking system use
`--require-js-string-builtin` as well as most of test CI
(in `pkg/dart2wasm/tool/compile_benchmark`)

Though we run some configurations via overriding with
`--no-require-js-string-builtin`
(in `tools/bots/test_matrix.json`)

Issue https://github.com/flutter/flutter/issues/159400#issuecomment-2538593980
Issue https://github.com/dart-lang/sdk/issues/59699

TEST=ci

Change-Id: I238ac65efe092de569da870f23134f889ac929f9
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/392903
Reviewed-by: Slava Egorov <vegorov@google.com>
Reviewed-by: Lasse Nielsen <lrn@google.com>
Commit-Queue: Martin Kustermann <kustermann@google.com>