[wasm] Add support for a start function.

Add a section identifier for declaring a start function as an index into
the function table. (This could also be done as a decl flag on the
function, but don't feel strongly here, since we probably want to redo
this when adding an import/export section.)

The start function must accept no parameters. Its return value is
currently ignored.

R=binji@chromium.org,bradnelson@chromium.org
BUG=chromium:575167
LOG=Y

Review URL: https://codereview.chromium.org/1692173002

Cr-Commit-Position: refs/heads/master@{#33978}

src/wasm/module-decoder.cc

@@ -180,6 +180,29 @@ class ModuleDecoder : public Decoder {
           }
           break;
         }
+        case kDeclStartFunction: {
+          // Declares a start function for a module.
+          CheckForPreviousSection(sections, kDeclFunctions, true);
+          if (module->start_function_index >= 0) {
+            error("start function already declared");
+            break;
+          }
+          int length;
+          const byte* before = pc_;
+          uint32_t index = consume_u32v(&length, "start function index");
+          if (index >= module->functions->size()) {
+            error(before, "invalid start function index");
+            break;
+          }
+          module->start_function_index = static_cast<int>(index);
+          FunctionSig* sig =
+              module->signatures->at(module->functions->at(index).sig_index);
+          if (sig->parameter_count() > 0) {
+            error(before, "invalid start function: non-zero parameter count");
+            break;
+          }
+          break;
+        }
         case kDeclWLL: {
           // Reserved for experimentation by the Web Low-level Language project
           // which is augmenting the binary encoding with source code meta

src/wasm/wasm-module.cc

@@ -256,15 +256,21 @@ bool AllocateGlobals(ErrorThrower* thrower, Isolate* isolate,
 }
 }  // namespace
 
-
 WasmModule::WasmModule()
-    : globals(nullptr),
+    : shared_isolate(nullptr),
+      module_start(nullptr),
+      module_end(nullptr),
+      min_mem_size_log2(0),
+      max_mem_size_log2(0),
+      mem_export(false),
+      mem_external(false),
+      start_function_index(-1),
+      globals(nullptr),
       signatures(nullptr),
       functions(nullptr),
       data_segments(nullptr),
       function_table(nullptr) {}
 
-
 WasmModule::~WasmModule() {
   if (globals) delete globals;
   if (signatures) delete signatures;
@@ -403,6 +409,25 @@ MaybeHandle<JSObject> WasmModule::Instantiate(Isolate* isolate,
   linker.Link(instance.function_table, this->function_table);
   instance.js_object->SetInternalField(kWasmModuleFunctionTable,
                                        Smi::FromInt(0));
+
+  // Run the start function if one was specified.
+  if (this->start_function_index >= 0) {
+    HandleScope scope(isolate);
+    uint32_t index = static_cast<uint32_t>(this->start_function_index);
+    Handle<String> name = isolate->factory()->NewStringFromStaticChars("start");
+    Handle<Code> code = linker.GetFunctionCode(index);
+    Handle<JSFunction> jsfunc = compiler::CompileJSToWasmWrapper(
+        isolate, &module_env, name, code, instance.js_object, index);
+
+    // Call the JS function.
+    Handle<Object> undefined(isolate->heap()->undefined_value(), isolate);
+    MaybeHandle<Object> retval =
+        Execution::Call(isolate, jsfunc, undefined, 0, nullptr);
+
+    if (retval.is_null()) {
+      thrower.Error("WASM.instantiateModule(): start function failed");
+    }
+  }
   return instance.js_object;
 }
 

src/wasm/wasm-module.h

@@ -32,6 +32,7 @@ enum WasmSectionDeclCode {
   kDeclFunctionTable = 0x05,
   kDeclWLL = 0x11,
   kDeclEnd = 0x06,
+  kDeclStartFunction = 0x07,
 };
 
 static const int kMaxModuleSectionCode = 6;
@@ -93,6 +94,7 @@ struct WasmModule {
   uint8_t max_mem_size_log2;  // maximum size of the memory (log base 2).
   bool mem_export;            // true if the memory is exported.
   bool mem_external;          // true if the memory is external.
+  int start_function_index;   // start function, if any.
 
   std::vector<WasmGlobal>* globals;             // globals in this module.
   std::vector<FunctionSig*>* signatures;        // signatures in this module.

test/mjsunit/wasm/start-function.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.
+
+// Flags: --expose-wasm
+
+load("test/mjsunit/wasm/wasm-constants.js");
+
+function instantiate(sig, body) {
+  var module = new Array();
+  module = module.concat([
+    // -- signatures
+    kDeclSignatures, 1,
+  ]);
+  module = module.concat(sig);
+  module = module.concat([
+    // -- functions
+    kDeclFunctions, 1,
+    0,                 // decl flags
+    0, 0,              // signature
+    body.length, 0,    // body size
+  ]);
+  module = module.concat(body);
+  module = module.concat([
+    // -- declare start function
+    kDeclStartFunction,
+    0
+  ]);
+
+  var data = bytes.apply(this, module);
+  print(module);
+  print(data instanceof ArrayBuffer);
+  print(data.byteLength);
+  return _WASMEXP_.instantiateModule(data);
+}
+
+function assertFails(sig, body) {
+  try {
+    var module = instantiate(sig, body);
+    print("expected failure, but passes");
+    assertFalse(true);
+  } catch (expected) {
+    print("ok: " + expected);
+  }
+}
+
+function assertVerifies(sig, body) {
+  var module = instantiate(sig, body);
+  assertFalse(module === undefined);
+  assertFalse(module === null);
+  assertFalse(module === 0);
+  assertEquals("object", typeof module);
+  return module;
+}
+
+assertVerifies([0, kAstStmt], [kExprNop]);
+assertVerifies([0, kAstI32], [kExprI8Const, 0]);
+
+// Arguments aren't allow to start functions.
+assertFails([1, kAstI32, kAstI32], [kExprGetLocal, 0]);
+assertFails([2, kAstI32, kAstI32, kAstF32], [kExprGetLocal, 0]);
+assertFails([3, kAstI32, kAstI32, kAstF32, kAstF64], [kExprGetLocal, 0]);
+
+(function testInvalidIndex() {
+  var kBodySize = 1;
+  var data = bytes(
+    // -- signatures
+    kDeclSignatures, 1,
+    0, kAstStmt,
+    // -- functions
+    kDeclFunctions, 1,
+    0,                 // decl flags
+    0, 0,              // signature
+    kBodySize, 0,      // body size
+    kExprNop,          // body
+    // -- declare start function
+    kDeclStartFunction,
+    1
+  );
+
+  assertThrows(function() { _WASMEXP_.instantiateModule(data); });
+})();
+
+
+(function testTwoStartFuncs() {
+  var kBodySize = 1;
+  var data = bytes(
+    // -- signatures
+    kDeclSignatures, 1,
+    0, kAstStmt,
+    // -- functions
+    kDeclFunctions, 1,
+    0,                 // decl flags
+    0, 0,              // signature
+    kBodySize, 0,      // body size
+    kExprNop,          // body
+    // -- declare start function
+    kDeclStartFunction,
+    0,
+    // -- declare start function
+    kDeclStartFunction,
+    0
+  );
+
+  assertThrows(function() { _WASMEXP_.instantiateModule(data); });
+})();
+
+
+(function testRun() {
+  var kBodySize = 6;
+
+  var data = bytes(
+    kDeclMemory,
+    12, 12, 1,                  // memory
+    // -- signatures
+    kDeclSignatures, 1,
+    0, kAstStmt,
+    // -- start function
+    kDeclFunctions, 1,
+    0,                          // decl flags
+    0, 0,                       // signature
+    kBodySize, 0,               // code size
+    // -- start body
+    kExprI32StoreMem, 0, kExprI8Const, 0, kExprI8Const, 77,
+    // -- declare start function
+    kDeclStartFunction,
+    0
+  );
+
+  var module = _WASMEXP_.instantiateModule(data);
+  var memory = module.memory;
+  var view = new Int8Array(memory);
+  assertEquals(77, view[0]);
+})();
+
+(function testStartFFI() {
+  var kBodySize = 2;
+  var kNameOffset = 4 + 9 + 7 + 3;
+
+  var data = bytes(
+    // -- signatures
+    kDeclSignatures, 1,
+    0, kAstStmt,
+    // -- imported function
+    kDeclFunctions, 2,
+    kDeclFunctionImport | kDeclFunctionName,     // decl flags
+    0, 0,                       // signature
+    kNameOffset, 0, 0, 0,
+    // -- start function
+    0,                          // decl flags
+    0, 0,                       // signature
+    kBodySize, 0,               // code size
+    // -- start body
+    kExprCallFunction, 0,
+    // -- declare start function
+    kDeclStartFunction,
+    1,
+    kDeclEnd,
+    'f', 'o', 'o', 0
+  );
+
+  var ranned = false;
+  var ffi = new Object();
+  ffi.foo = function() {
+    print("we ranned at stert!");
+    ranned = true;
+  }
+  var module = _WASMEXP_.instantiateModule(data, ffi);
+  var memory = module.memory;
+  var view = new Int8Array(memory);
+  assertTrue(ranned);
+})();

test/mjsunit/wasm/wasm-constants.js

@@ -22,6 +22,7 @@ var kDeclFunctions = 0x02;
 var kDeclGlobals = 0x03;
 var kDeclDataSegments = 0x04;
 var kDeclFunctionTable = 0x05;
+var kDeclStartFunction = 0x07;
 var kDeclEnd = 0x06;
 
 // Function declaration flags