Replace __init__ function in asm-wasm-builder with the start function section

src/wasm/encoder.cc

Index: src/wasm/encoder.cc
diff --git a/src/wasm/encoder.cc b/src/wasm/encoder.cc
index a6b2f43983e9d1952f0274a7aa5f55e6d244f435..99227c70b0a1b87c3c98cfb784bbbaa0d5c8e613 100644
--- a/src/wasm/encoder.cc
+++ b/src/wasm/encoder.cc
@@ -333,7 +333,6 @@ void WasmDataSegmentEncoder::Serialize(byte* buffer, byte** header,
   (*body) += data_.size();
 }
 
-
 WasmModuleBuilder::WasmModuleBuilder(Zone* zone)
     : zone_(zone),
       signatures_(zone),
@@ -341,8 +340,9 @@ WasmModuleBuilder::WasmModuleBuilder(Zone* zone)
       data_segments_(zone),
       indirect_functions_(zone),
       globals_(zone),
-      signature_map_(zone) {}
-
+      signature_map_(zone),
+      has_start_function_(false),
+      start_function_index_(0) {}
 
 uint16_t WasmModuleBuilder::AddFunction() {
   functions_.push_back(new (zone_) WasmFunctionBuilder(zone_));
@@ -399,6 +399,10 @@ void WasmModuleBuilder::AddIndirectFunction(uint16_t index) {
   indirect_functions_.push_back(index);
 }
 
+void WasmModuleBuilder::MarkStartFunction(uint16_t index) {
+  has_start_function_ = true;
+  start_function_index_ = index;
+}
 
 WasmModuleWriter* WasmModuleBuilder::Build(Zone* zone) {
   WasmModuleWriter* writer = new (zone) WasmModuleWriter(zone);
@@ -417,6 +421,8 @@ WasmModuleWriter* WasmModuleBuilder::Build(Zone* zone) {
   for (auto global : globals_) {
     writer->globals_.push_back(global);
   }
+  writer->has_start_function_ = has_start_function_;
+  writer->start_function_index_ = start_function_index_;
   return writer;
 }
 
@@ -482,6 +488,15 @@ WasmModuleIndex* WasmModuleWriter::WriteTo(Zone* zone) const {
               function->NameSize());
   }
 
+  if (has_start_function_) {
+    sizes.Add(1, 0);
+    uint16_t size = start_function_index_;
+    do {

[titzer, 2016/03/01 02:09:33]

Let's lift out this varint encoding into a helper and use it elsewhere (we
currently create temporary std::vectors for LEB encodings!)

[aseemgarg, 2016/03/01 02:37:34]

This is enclosed in AddSection for other. Just for this one since the section
might be created even with a 0 value is it separate. This is smaller than adding
a flag to the existing method.

[titzer, 2016/03/01 02:43:58]

At least lift it out to a helper that computes the size of the varint for
documentation.

[aseemgarg, 2016/03/01 03:00:24]

Done.

+      sizes.Add(1, 0);
+      size = size >> 7;
+    } while (size > 0);
+  }
+
   sizes.AddSection(data_segments_.size());
   for (auto segment : data_segments_) {
     sizes.Add(segment->HeaderSize(), segment->BodySize());
@@ -547,6 +562,12 @@ WasmModuleIndex* WasmModuleWriter::WriteTo(Zone* zone) const {
     }
   }
 
+  // -- emit start function index ----------------------------------------------
+  if (has_start_function_) {
+    EmitUint8(&header, kDeclStartFunction);
+    EmitVarInt(&header, start_function_index_);
+  }
+
   // -- emit data segments -----------------------------------------------------
   if (data_segments_.size() > 0) {
     EmitUint8(&header, kDeclDataSegments);