Optimize JS compiler caching for standard builds#27026
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brendandahl
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May 28, 2026
| # Sadly we have to skip the caching whenever we have user JS libraries. This is because | ||
| # these libraries can import arbitrary other JS files (either vis node's `import` or via #include) | ||
| if DEBUG or settings.BOOTSTRAPPING_STRUCT_INFO or config.FROZEN_CACHE or settings.JS_LIBRARIES: | ||
| has_user_libs = any(not lib.startswith(utils.path_from_root('src/')) for lib in settings.JS_LIBRARIES) |
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Alternatively, we could stick system libs in a separate setting, but that seems like a bigger change.
sbc100
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May 28, 2026
sbc100
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May 28, 2026
Linker system stubs, such as libpthread_stub.js, are dynamically appended to settings.JS_LIBRARIES for all standard C/C++ builds. Previously, the JS output cache check did not distinguish between these system stubs and user-defined JS libraries (passed via --js-library). As a result, standard builds always bypassed the compilation cache, triggering a Node.js compilation run on every invocation. Restrict the cache bypass condition to check only for user libraries (libraries located outside the Emscripten source tree). Standard compilations now successfully hit the JS output cache, decreasing baseline compilation wall clock time of a hello world C program by 52% (from 472 ms down to 282 ms). Add an integration test to validate JavaScript compilation caching (covering hits, misses, user library bypasses, and option-based cache entries).
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Linker system stubs, such as libpthread_stub.js, are dynamically appended to settings.JS_LIBRARIES for all standard C/C++ builds. Previously, the JS output cache check did not distinguish between these system stubs and user-defined JS libraries (passed via --js-library). As a result, standard builds always bypassed the compilation cache, triggering a Node.js compilation run on every invocation.
Restrict the cache bypass condition to check only for user libraries (libraries located outside the Emscripten source tree). Standard compilations now successfully hit the JS output cache, decreasing baseline compilation wall clock time of a hello world C program by 52% (from 472 ms down to 282 ms).
Add an integration test to validate JavaScript compilation caching (covering hits, misses, user library bypasses, and option-based cache entries).