Support source-phase imports in the acorn/terser optimizer pipeline#26967
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Could you add -O2 to the test_esm_source_phase_imports test? Maybe using @parameterized? I assume if you do this without this patch the test would crash?
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Sure, I've bumped to |
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lgtm, but maybe we want to wait for the terser changes to land, either in our fork or in upstream. Unless you want to land this first for some reason?
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lgtm! Would you mind updating the README in the terser directory as part of this PR? I noticed it still mentions emscripten_patches_v4.8.0. |
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When -sSOURCE_PHASE_IMPORTS=1, emcc emits ```js import source wasmModule from './foo.wasm'; ``` in its JS runtime. At -O2/-O3/-Os/-Oz the emitted JS is run through tools/acorn-optimizer.mjs which currently fails with a SyntaxError at parse time because acorn 8.x does not yet understand the source-phase imports proposal (https://github.com/tc39/proposal-source-phase-imports). Wire in the acorn-import-phases plugin so acorn can parse the syntax, and pull in the matching terser support (downstream of emscripten-core/terser#1) so the round-trip through from_mozilla_ast / to_mozilla_ast preserves the `phase` keyword on the way back out. Without the terser side, terser would silently drop the keyword and the host would return the module's exports namespace instead of a WebAssembly.Module, changing runtime semantics. * package.json: add acorn-import-phases dependency. * tools/acorn-optimizer.mjs: extend acorn with the plugin and use the extended parser at the parse site. * third_party/terser/terser.js: rebuilt from the emscripten-core/terser branch with source-phase imports support (PR emscripten-core/terser#1, the v5.18.2 downstream port of upstream terser PR terser/terser#1682). * test/js_optimizer/sourcePhaseImports{,-output}.js: new fixture that feeds two `import source` declarations through the JSDCE pass and checks the keyword survives. * test/test_other.py: register the fixture in test_js_optimizer, and parametrize test_esm_source_phase_imports across no-args and -O2 to exercise the optimizer pipeline.
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LGTM! Do you want me to land this once the CI passes? |
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That would be great, didn't hit many other bugs with the Emscripten flow other than this! |
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With
-sSOURCE_PHASE_IMPORTS=1, emcc emitsimport source wasmModule from './foo.wasm'in its JS runtime, but at-O2+ the optimizer crashes because acorn 8.x doesn't yet understand the syntax. Even once acorn parses it, the bundled terser silently drops thesourcekeyword on the mozilla-AST round-trip, which changes runtime semantics (host returns the exports namespace instead of aWebAssembly.Module).This wires in the
acorn-import-phasesplugin so acorn parses the syntax, and pulls in matching terser support so the keyword survives printing. Terser side is emscripten-core/terser#1, which is vendored viathird_party/terser/terser.jsrebuilt from that branch.New fixture
test/js_optimizer/sourcePhaseImports.jsruns twoimport sourcedeclarations through JSDCE and checks the keyword survives; runs via./test/runner other.test_js_optimizer_sourcePhaseImports.That terser PR should ideally land first so this can be rebuilt from a merged branch; happy to rebase the bundle afterwards.