what

emscripten是一个c/cpp的编译器，能够将c/cpp代码用 LLVM，编译为 WebAssembly。其中emcc 是他的cli，相似于罕用的makefile的作用。

编译产物

可选的比拟多 html js wasm 都能够自由组合

js产物源码解读

应用emcc 对一个c文件进行编译会产出一个js文件和一个wasm文件。js文件中的源码齐全兼容了node环境和web环境，本局部仅探讨web环境。

if (ENVIRONMENT_IS_WEB || ENVIRONMENT_IS_WORKER) {  if (ENVIRONMENT_IS_WORKER) { // Check worker, not web, since window could be polyfilled    scriptDirectory = self.location.href;  } else if (typeof document !== 'undefined' && document.currentScript) { // web    scriptDirectory = document.currentScript.src;  }  // blob urls look like blob:http://site.com/etc/etc and we cannot infer anything from them.  // otherwise, slice off the final part of the url to find the script directory.  // if scriptDirectory does not contain a slash, lastIndexOf will return -1,  // and scriptDirectory will correctly be replaced with an empty string.  if (scriptDirectory.indexOf('blob:') !== 0) {    scriptDirectory = scriptDirectory.substr(0, scriptDirectory.lastIndexOf('/')+1);  } else {    scriptDirectory = '';  }  // Differentiate the Web Worker from the Node Worker case, as reading must  // be done differently.  {// include: web_or_worker_shell_read.js  read_ = function shell_read(url) {      var xhr = new XMLHttpRequest();      xhr.open('GET', url, false);      xhr.send(null);      return xhr.responseText;  };  if (ENVIRONMENT_IS_WORKER) {    readBinary = function readBinary(url) {        var xhr = new XMLHttpRequest();        xhr.open('GET', url, false);        xhr.responseType = 'arraybuffer';        xhr.send(null);        return new Uint8Array(/** @type{!ArrayBuffer} */(xhr.response));    };  }  readAsync = function readAsync(url, onload, onerror) {    var xhr = new XMLHttpRequest();    xhr.open('GET', url, true);    xhr.responseType = 'arraybuffer';    xhr.onload = function xhr_onload() {      if (xhr.status == 200 || (xhr.status == 0 && xhr.response)) { // file URLs can return 0        onload(xhr.response);        return;      }      onerror();    };    xhr.onerror = onerror;    xhr.send(null);  };// end include: web_or_worker_shell_read.js  }  setWindowTitle = function(title) { document.title = title };} else{  throw new Error('environment detection error');}

以上为em 构建的在web中的文件系统

如何革新em输入的模块
em会取全局的Module对象 并且反对Module对象的革新，能够通过这个形式改写em中的

em中的一些罕用函数
js中定义了罕用的工具函数，比方如何通过办法名拜访c模块或者c函数，c与js中数据类型的转换
这部分其实就是https://emscripten.org/docs/a...的内容

em js的生命周期

function preRun() {  if (Module['preRun']) {    if (typeof Module['preRun'] == 'function') Module['preRun'] = [Module['preRun']];    while (Module['preRun'].length) {      addOnPreRun(Module['preRun'].shift());    }  }  callRuntimeCallbacks(__ATPRERUN__);}function initRuntime() {  checkStackCookie();  assert(!runtimeInitialized);  runtimeInitialized = true;  if (!Module["noFSInit"] && !FS.init.initialized) FS.init();TTY.init();  callRuntimeCallbacks(__ATINIT__);}function preMain() {  checkStackCookie();  FS.ignorePermissions = false;  callRuntimeCallbacks(__ATMAIN__);}function exitRuntime() {  checkStackCookie();  runtimeExited = true;}

这里能够减少在asm运行中的钩子函数

外围函数createWasm

function createWasm() {  // prepare imports  var info = {    'env': asmLibraryArg,    'wasi_snapshot_preview1': asmLibraryArg,  };  // Load the wasm module and create an instance of using native support in the JS engine.  // handle a generated wasm instance, receiving its exports and  // performing other necessary setup  /** @param {WebAssembly.Module=} module*/  function receiveInstance(instance, module) {    var exports = instance.exports;    Module['asm'] = exports;    wasmMemory = Module['asm']['memory'];    assert(wasmMemory, "memory not found in wasm exports");    // This assertion doesn't hold when emscripten is run in --post-link    // mode.    // TODO(sbc): Read INITIAL_MEMORY out of the wasm file in post-link mode.    //assert(wasmMemory.buffer.byteLength === 16777216);    updateGlobalBufferAndViews(wasmMemory.buffer);    wasmTable = Module['asm']['__indirect_function_table'];    assert(wasmTable, "table not found in wasm exports");    removeRunDependency('wasm-instantiate');  }  // we can't run yet (except in a pthread, where we have a custom sync instantiator)  addRunDependency('wasm-instantiate');  // Async compilation can be confusing when an error on the page overwrites Module  // (for example, if the order of elements is wrong, and the one defining Module is  // later), so we save Module and check it later.  var trueModule = Module;  function receiveInstantiatedSource(output) {    // 'output' is a WebAssemblyInstantiatedSource object which has both the module and instance.    // receiveInstance() will swap in the exports (to Module.asm) so they can be called    assert(Module === trueModule, 'the Module object should not be replaced during async compilation - perhaps the order of HTML elements is wrong?');    trueModule = null;    // TODO: Due to Closure regression https://github.com/google/closure-compiler/issues/3193, the above line no longer optimizes out down to the following line.    // When the regression is fixed, can restore the above USE_PTHREADS-enabled path.    receiveInstance(output['instance']);  }  function instantiateArrayBuffer(receiver) {    return getBinaryPromise().then(function(binary) {      return WebAssembly.instantiate(binary, info);    }).then(receiver, function(reason) {      err('failed to asynchronously prepare wasm: ' + reason);      abort(reason);    });  }  // Prefer streaming instantiation if available.  function instantiateAsync() {    if (!wasmBinary &&        typeof WebAssembly.instantiateStreaming === 'function' &&        !isDataURI(wasmBinaryFile) &&        // Don't use streaming for file:// delivered objects in a webview, fetch them synchronously.        !isFileURI(wasmBinaryFile) &&        typeof fetch === 'function') {      return fetch(wasmBinaryFile, { credentials: 'same-origin' }).then(function (response) {        var result = WebAssembly.instantiateStreaming(response, info);        return result.then(receiveInstantiatedSource, function(reason) {            // We expect the most common failure cause to be a bad MIME type for the binary,            // in which case falling back to ArrayBuffer instantiation should work.            err('wasm streaming compile failed: ' + reason);            err('falling back to ArrayBuffer instantiation');            return instantiateArrayBuffer(receiveInstantiatedSource);          });      });    } else {      return instantiateArrayBuffer(receiveInstantiatedSource);    }  }  // User shell pages can write their own Module.instantiateWasm = function(imports, successCallback) callback  // to manually instantiate the Wasm module themselves. This allows pages to run the instantiation parallel  // to any other async startup actions they are performing.  if (Module['instantiateWasm']) {    try {      var exports = Module['instantiateWasm'](info, receiveInstance);      return exports;    } catch(e) {      err('Module.instantiateWasm callback failed with error: ' + e);      return false;    }  }  instantiateAsync();  return {}; // no exports yet; we'll fill them in later}

加载wasm文件，这里次要是进行了初始化的申明 蕴含了内存空间的申明和依赖的注入，这里都是由em主动治理的

关注点 wasm的加载 看到源码中wasm文件都是近程加载的 此时咱们须要替换咱们的wasm文件到cdn上，咱们须要减少Module['locateFile'] 保障咱们的wasm 能够指向咱们须要的地址

var wasmBinaryFile = 'xxx.wasm';if (!isDataURI(wasmBinaryFile)) {//dataurl 判断 这里只有不是base64的都会为true  wasmBinaryFile = locateFile(wasmBinaryFile);}// `/` should be present at the end if `scriptDirectory` is not emptyvar scriptDirectory = '';function locateFile(path) {  if (Module['locateFile']) {    return Module['locateFile'](path, scriptDirectory);  }  return scriptDirectory + path;}if (ENVIRONMENT_IS_WORKER) { // Check worker, not web, since window could be polyfilled    scriptDirectory = self.location.href;  } else if (typeof document !== 'undefined' && document.currentScript) { // web    scriptDirectory = document.currentScript.src;  }//scriptDirectory就是脚本的src  不过这个不重要 Module['locateFile']能够从入参中获取

执行完数createWasm 咱们就取得了一个promise

源码中还有要害的函数钩子 Module.instantiateWasm 咱们能够在这里获取wasm的实例，到此 整个wasm的初始化后的输出模块咱们就能够取得了

main函数的运行
这里值得是c中的main函数，如果是模块输入其实不须要main函数
他对照了模块中的run函数 在加载后 默认是会间接运行的 咱们能够通过noInitialRun让主函数不执行，而后利用模块的run函数去运行，反对参数的传入，非常适合wasm模块的初始化场景

工程化设计

  1. 编写定制化的Module对象，必须改写的函数次要是生命周期函数和'locateFile'
  2. 梳理c/cpp模块的边界，哪些是js实现哪些是c实现，确认输入输出，这里能够用组件化的思路解决
  3. 编译c代码，输入为js和wasm即可
  4. 对export进去的模块进行二次包装，次要函数的整顿，入参的类型解决
  5. 实现组件的生命周期的管制
  6. 联调外围性能