Add a rollup plugin for tree shaking Emscripten output.

Rollup has built in tree shaking which can inform the plugin which exports
are no longer used. If those exports correspond to wasm exports they can
potentially be removed from wasm file too.

Overview:
While the modules are parsed, we collect info about the exports before any
tree shaking occurs to build a map from ESM export name to Wasm export
name and info about the local function name within the JS e.g. _foo.

During bundle generation rollup provides a list of exports that are
removed. We then try to match the removed exports to Wasm exports. However,
they all cannot simply be removed though since they may be used internally
in the file still. A new list of the used wasm exports is generated and
passed to wasm-metadce to remove any unused exports.

Note: there are a lot more things we could do there, but this seemed like
a good stopping point. It also shows that even for specific output, tree
shaking is hard to do soundly for dynamically loaded wasm.

Author: brendandahl

.gitignore

@@ -25,6 +25,8 @@ tools/coverage/
 htmlcov/
 coverage.xml
 
+/tools/rollup-plugin-emscripten/node_modules
+
 .DS_Store
 
 # Test output

test/rollup_plugin/index.mjs

@@ -0,0 +1,6 @@
+import init, { _used_externally } from './library.mjs';
+
+await init();
+
+console.log(_used_externally());
+console.log('done');

test/rollup_plugin/library.c

@@ -0,0 +1,16 @@
+#include <emscripten.h>
+
+// Should NOT be removed (used by index.mjs).
+EMSCRIPTEN_KEEPALIVE int used_externally() {
+  return 42;
+}
+
+// Should NOT be removed (used by library.js).
+EMSCRIPTEN_KEEPALIVE int used_internally() {
+  return 99;
+}
+
+// Should be removed.
+EMSCRIPTEN_KEEPALIVE int unused() {
+  return 0xDEAD;
+}

test/rollup_plugin/library.js

@@ -0,0 +1,9 @@
+
+addToLibrary({
+  $placeHolder__deps: ['used_internally'],
+  $placeHolder: function() {
+    _used_internally();
+  }
+});
+
+extraLibraryFuncs.push('$placeHolder');

test/rollup_plugin/package.json

@@ -0,0 +1,4 @@
+{
+  "name": "test-rollup-plugin",
+  "type": "module"
+}

test/rollup_plugin/rollup.config.mjs

@@ -0,0 +1,14 @@
+import emscriptenPlugin from 'rollup-plugin-emscripten';
+
+export default {
+  input: 'index.mjs',
+  output: {
+    dir: 'dist',
+    format: 'es'
+  },
+  plugins: [
+    emscriptenPlugin({
+      'input': 'library.mjs'
+    })
+  ]
+};

test/test_other.py

@@ -15804,6 +15804,33 @@ def test_rollup(self):
     self.run_process(shared.get_npm_cmd('rollup') + ['--config'])
     self.assertContained('hello, world!', self.run_js('bundle.mjs'))
 
+  @parameterized({
+    '': ('',),
+    'O1': ('-O1',),
+    'O2': ('-O2',),
+    'O3': ('-O3',),
+  })
+  def test_rollup_plugin(self, opt):
+    def get_exports(path):
+      with webassembly.Module(path) as module:
+        return [export.name for export in module.get_exports()]
+
+    copytree(test_file('rollup_plugin'), '.')
+    self.run_process([EMCC, 'library.c', opt, '--no-entry', '-sMODULARIZE=instance', '-sENVIRONMENT=node', '--js-library', 'library.js', '-o', 'library.mjs'])
+    self.run_process(['npm', 'install', path_from_root('tools/rollup-plugin-emscripten')])
+    self.run_process(shared.get_npm_cmd('rollup') + ['--config'])
+    exports_pre_rollup = get_exports('library.wasm')
+    self.assertContained('42\ndone\n', self.run_js('dist/index.js'))
+    exports = get_exports('dist/library.wasm')
+    self.assertTrue(len(exports) < len(exports_pre_rollup))
+    if opt != '-O3':
+      # In O3 wasm export names are minified.
+      self.assertTrue('used_externally' in exports)
+      self.assertTrue('used_internally' in exports)
+      self.assertTrue('unused' not in exports)
+
+
+
   def test_rlimit(self):
     self.do_other_test('test_rlimit.c', emcc_args=['-O1'])
 

tools/rollup-plugin-emscripten/package-lock.json

@@ -0,0 +1,10 @@
+{
+  "name": "rollup-plugin-emscripten",
+  "version": "0.0.1",
+  "description": "Rollup plugin for Emscripten output with tree shaking.",
+  "main": "src/index.mjs",
+  "dependencies": {
+    "tmp": "^0.2.3",
+    "which": "^5.0.0"
+  }
+}