Optimize JS compiler caching for standard builds

test/test_sanity.py

@@ -870,3 +870,45 @@ def test_emcc_with_relative_python_path(self):
     output = self.do([EMCC, test_file('hello_world.c')], env=env)
     self.assertNotContained('error', output)
     self.assertExists('a.out.js')
+
+  def test_emcc_javascript_compilation_caching(self):
+    restore_and_set_up()
+
+    # Create a separate temporary cache folder to avoid dirtying or reading from the default cache.
+    test_cache_dir = self.in_dir('test_cache')
+    js_output_cache_dir = os.path.join(test_cache_dir, 'js_output')
+
+    def js_cache_files():
+      if not os.path.exists(js_output_cache_dir):
+        return []
+      return sorted([f for f in os.listdir(js_output_cache_dir) if f.endswith('.js')])
+
+    def js_cache_size():
+      return len(js_cache_files())
+
+    with env_modify({'EM_CACHE': test_cache_dir}):
+      # 1. First compile. Cache-miss: should compile and populate the cache.
+      self.run_process([EMCC, test_file('hello_world.c'), '-O2', '-o', 'out.js'])
+      self.assertExists(js_output_cache_dir)
+
+      self.assertEqual(js_cache_size(), 1, f'Expected 1 cached JS file, found: {js_cache_files()}')
+
+      cached_file_path = os.path.join(js_output_cache_dir, js_cache_files()[0])
+      initial_mtime = os.path.getmtime(cached_file_path)
+
+      # 2. Second compile. Cache-hit: mtime of cache file should remain strictly identical (not overwritten).
+      self.run_process([EMCC, test_file('hello_world.c'), '-O2', '-o', 'out.js'])
+
+      self.assertEqual(js_cache_size(), 1)
+      self.assertEqual(os.path.getmtime(cached_file_path), initial_mtime, 'Cache was overwritten on second compile (expected a cache hit)')
+
+      # 3. Third compile with custom user library. Cache-bypass: should not add any cache entries.
+      create_file('my_lib.js', 'addToLibrary({ my_custom_symbol: () => {} });')
+      self.run_process([EMCC, test_file('hello_world.c'), '-O2', '--js-library=my_lib.js', '-o', 'out.js'])
+
+      self.assertEqual(js_cache_size(), 1, 'Cache entry was incorrectly created for custom JS library compile')
+
+      # 4. Fourth compile with a changed compiler option. Distinct Cache Entry: should generate a second cache entry.
+      self.run_process([EMCC, test_file('hello_world.c'), '-O2', '-sASSERTIONS=1', '-o', 'out.js'])
+
+      self.assertEqual(js_cache_size(), 2, f'Expected 2 cached JS files after compiling with different options, found: {js_cache_files()}')

tools/emscripten.py

@@ -346,7 +346,8 @@ def compile_javascript_cached():
   # this step is performed while the cache is locked.
   # 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)
+  if DEBUG or settings.BOOTSTRAPPING_STRUCT_INFO or config.FROZEN_CACHE or has_user_libs:
     return compile_javascript()
 
   content_hash = generate_js_compiler_input_hash()