[wasm] separate snapshot-able stages

This CLprepares the terrain for serialization/deserialization. It sets up
the instantiation stages such that we have a point wereh we can split off
obtaining the code from a snapshot, or snapshot. That point is after we
compile and produce the code table, but before we attach the
deoptimization info we use for stack tracing.

Opportunistically, performed more cleanup to improve maintainability:
- clarified sequential vs parallel compilation stages. FinishCompilation
was somewhat ambiguous in that it performed a few responsibilities:
compiling functions in the sequential case, and then populating the
linker and code tables.
- removed the "results" set, which is unnecessary. The linker simply
shares the function_code vector, and so do the compilation stages.
- populate the code table fixed array separately from compilation. This
falls out of the decisions above.

BUG=

Review-Url: https://codereview.chromium.org/2008043006
Cr-Commit-Position: refs/heads/master@{#36618}

src/compiler/wasm-compiler.cc

@@ -1908,7 +1908,7 @@ Node* WasmGraphBuilder::CallDirect(uint32_t index, Node** args,
   DCHECK_NULL(args[0]);
 
   // Add code object as constant.
-  args[0] = HeapConstant(module_->GetFunctionCode(index));
+  args[0] = HeapConstant(module_->GetCodeOrPlaceholder(index));
   wasm::FunctionSig* sig = module_->GetFunctionSignature(index);
 
   return BuildWasmCall(sig, args, position);
@@ -3292,16 +3292,6 @@ Handle<Code> WasmCompilationUnit::FinishCompilation() {
   }
   Handle<Code> code = info_.code();
   DCHECK(!code.is_null());
-  DCHECK(code->deoptimization_data() == nullptr ||
-         code->deoptimization_data()->length() == 0);
-  Handle<FixedArray> deopt_data =
-      isolate_->factory()->NewFixedArray(2, TENURED);
-  if (!module_env_->instance->js_object.is_null()) {
-    deopt_data->set(0, *module_env_->instance->js_object);
-  }
-  deopt_data->set(1, Smi::FromInt(function_->func_index));
-  deopt_data->set_length(2);
-  code->set_deoptimization_data(*deopt_data);
 
   RecordFunctionCompilation(
       Logger::FUNCTION_TAG, &info_, "WASM_function", function_->func_index,

src/wasm/wasm-module.h

@@ -231,7 +231,11 @@ struct WasmModuleInstance {
   byte* globals_start;  // start of the globals area.
 
   explicit WasmModuleInstance(const WasmModule* m)
-      : module(m), mem_start(nullptr), mem_size(0), globals_start(nullptr) {}
+      : module(m),
+        function_code(m->functions.size()),
+        mem_start(nullptr),
+        mem_size(0),
+        globals_start(nullptr) {}
 };
 
 // forward declaration.
@@ -248,7 +252,7 @@ struct ModuleEnv {
   bool IsValidGlobal(uint32_t index) {
     return module && index < module->globals.size();
   }
-  bool IsValidFunction(uint32_t index) {
+  bool IsValidFunction(uint32_t index) const {
     return module && index < module->functions.size();
   }
   bool IsValidSignature(uint32_t index) {
@@ -279,7 +283,7 @@ struct ModuleEnv {
 
   bool asm_js() { return origin == kAsmJsOrigin; }
 
-  Handle<Code> GetFunctionCode(uint32_t index);
+  Handle<Code> GetCodeOrPlaceholder(uint32_t index) const;
   Handle<Code> GetImportCode(uint32_t index);
   Handle<FixedArray> GetFunctionTable();