[wasm] Support undefined indirect table entries, behind a flag.

test/mjsunit/wasm/wasm-module-builder.js

 // Copyright 2016 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 class Binary extends Array {
   emit_u8(val) {
     this.push(val);
   }
 
   emit_u16(val) {
@@ skipping 91 matching lines @@
 
 class WasmModuleBuilder {
   constructor() {
     this.types = [];
     this.imports = [];
     this.functions = [];
     this.exports = [];
     this.table = [];
     this.segments = [];
     this.explicit = [];
+    this.pad = null;
+    return this;
   }
 
   addStart(start_index) {
     this.start_index = start_index;
   }
 
   addMemory(min, max, exp) {
     this.memory = {min: min, max: max, exp: exp};
     return this;
   }
 
+  addPadFunctionTable(size) {
+    this.pad = size;
+    return this;
+  }
+
   addExplicitSection(bytes) {
     this.explicit.push(bytes);
     return this;
   }
 
   addType(type) {
     // TODO: canonicalize types?
     this.types.push(type);
     return this.types.length - 1;
   }
@@ skipping 191 matching lines @@
       binary.emit_section(kDeclNames, section => {
         section.emit_varint(wasm.functions.length);
         for (let func of wasm.functions) {
           var name = func.name == undefined ? "" : func.name;
           section.emit_string(name);
           section.emit_u8(0);  // local names count == 0
         }
       });
     }
 
+    // Add an indirect function table pad section.
+    if (wasm.pad !== null) {
+      if (debug)
+        print("emitting indirect function table pad @ " + binary.length);
+      binary.emit_section(kDeclFunctionTablePad, section => {
+        section.emit_varint(wasm.pad);
+      });
+    }
+
+    // End the module.
+    if (debug) print("emitting end @ " + binary.length);
+    binary.emit_section(kDeclEnd, section => {});
+
     return binary;
   }
 
   toBuffer(debug) {
     let bytes = this.toArray(debug);
     let buffer = new ArrayBuffer(bytes.length);
     let view = new Uint8Array(buffer);
     for (let i = 0; i < bytes.length; i++) {
       let val = bytes[i];
       if ((typeof val) == "string") val = val.charCodeAt(0);
       view[i] = val | 0;
     }
     return buffer;
   }
 
   instantiate(...args) {
     let module = new WebAssembly.Module(this.toBuffer());
     let instance = new WebAssembly.Instance(module, ...args);
     return instance;
   }
 }