| commit | 3b750c5545f880506418453542a49e08a77dcbae | [log] [tgz] |
|---|---|---|
| author | Martin Kustermann <kustermann@google.com> | Mon Feb 10 06:17:49 2025 -0800 |
| committer | Commit Queue <dart-scoped@luci-project-accounts.iam.gserviceaccount.com> | Mon Feb 10 06:17:49 2025 -0800 |
| tree | b54552e9d1bd2e68e60036108f73d8effceedf37 | |
| parent | 0246cc613f0fdff8ab64aa3a9514e187783231bf [diff] |
[dart2wasm] Generate `*.support.js` feature detection files
The file will contain a JS expression that evaluates to a boolean.
If it (at runtime) evalutes to
* `true` it means that all required features are supported by the JS
environment and the dart2wasm-compiled app can be used
* `false` it means some features were not present in the JS
environment and the dart2wasm-compiled app shouldn't be used,
instead a dart2js fallback may be used
We introduce this mechanism to allow users, at compilation time, to tell
dart2wasm to take advantage of new spec features and allow the runtime
to self-detect whether they are available and fallback to dart2js if
not.
The first feature we introduce (already in this PR) is
`--require-js-string-builtin` that will tell dart2wasm it can assume the
`js-string` builtin is available (and emit corresponding `*.support.js`
code to detect it).
If the flag was passed, we take advantage of the `js-string` import
mechanism for string constants that doesn't require emitting them in the
mjs file (which significantly reduces code size and improves startup
time - compared with emitting JS strings in the mjs file).
We enable `--require-js-string-builtin` on one CI configuration for
testing that if we don't use any polyfill, the imports of the builtin
functions as well as magical utf8-encoded wasm imports work.
We also use a template mechanism to generate `*.mjs` as the code
becomes more readable (e.g. to conditionally include the js string
polyfill)
Issue https://github.com/dart-lang/sdk/issues/59951
Change-Id: Ic7e7818a2d5269095935022941352beeb9fed731
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/408781
Reviewed-by: Ömer Ağacan <omersa@google.com>
Commit-Queue: Martin Kustermann <kustermann@google.com>
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