Use WebAssembly.Function constructor if available (#9908)

With the type reflection proposal, we will get a way more efficient way
to convert a JS function into a wasm exported function. Use that if
available.

AUTHORS

@@ -441,3 +441,4 @@ a license to everyone to use it as detailed in LICENSE.)
 * Gergely Nagy <[email protected]>
 * Jan Habermann <[email protected]>
 * John Granström <[email protected]>
+* Clemens Backes <[email protected]> (copyright owned by Google, Inc.)

src/support.js

@@ -661,14 +661,32 @@ var functionPointers = new Array({{{ RESERVED_FUNCTION_POINTERS }}});
 
 #if WASM
 // Wraps a JS function as a wasm function with a given signature.
-// In the future, we may get a WebAssembly.Function constructor. Until then,
-// we create a wasm module that takes the JS function as an import with a given
-// signature, and re-exports that as a wasm function.
 function convertJsFunctionToWasm(func, sig) {
 #if WASM2JS
   return func;
 #else // WASM2JS
 
+  // If the type reflection proposal is available, use the new
+  // "WebAssembly.Function" constructor.
+  // Otherwise, construct a minimal wasm module importing the JS function and
+  // re-exporting it.
+  if (typeof WebAssembly.Function === "function") {
+    var typeNames = {
+      'i': 'i32',
+      'j': 'i64',
+      'f': 'f32',
+      'd': 'f64'
+    };
+    var type = {
+      parameters: [],
+      results: sig[0] == 'v' ? [] : [typeNames[sig[0]]]
+    };
+    for (var i = 1; i < sig.length; ++i) {
+      type.parameters.push(typeNames[sig[i]]);
+    }
+    return new WebAssembly.Function(type, func);
+  }
+
   // The module is static, with the exception of the type section, which is
   // generated based on the signature passed in.
   var typeSection = [