[wasm] Provide better stack traces for asm.js code

src/wasm/wasm-module-builder.h

Index: src/wasm/wasm-module-builder.h
diff --git a/src/wasm/wasm-module-builder.h b/src/wasm/wasm-module-builder.h
index 4f463b01df7c55d8ec8be840af2d72931bb320b8..d35313ef47edb364ef0676d6a0bc4a9097f0398d 100644
--- a/src/wasm/wasm-module-builder.h
+++ b/src/wasm/wasm-module-builder.h
@@ -88,10 +88,10 @@ class ZoneBuffer : public ZoneObject {
     }
   }
 
-  size_t offset() { return static_cast<size_t>(pos_ - buffer_); }
-  size_t size() { return static_cast<size_t>(pos_ - buffer_); }
-  const byte* begin() { return buffer_; }
-  const byte* end() { return pos_; }
+  size_t offset() const { return static_cast<size_t>(pos_ - buffer_); }
+  size_t size() const { return static_cast<size_t>(pos_ - buffer_); }
+  const byte* begin() const { return buffer_; }
+  const byte* end() const { return pos_; }
 
   void EnsureSpace(size_t size) {
     if ((pos_ + size) > end_) {
@@ -135,10 +135,12 @@ class V8_EXPORT_PRIVATE WasmFunctionBuilder : public ZoneObject {
   void Export();
   void ExportAs(Vector<const char> name);
   void SetName(Vector<const char> name);
+  void AddAsmWasmOffset(int asm_position);
 
   void WriteSignature(ZoneBuffer& buffer) const;
   void WriteExport(ZoneBuffer& buffer) const;
   void WriteBody(ZoneBuffer& buffer) const;
+  void WriteAsmWasmOffsetTable(ZoneBuffer& buffer) const;
 
   bool exported() { return exported_; }
   uint32_t func_index() { return func_index_; }
@@ -167,6 +169,11 @@ class V8_EXPORT_PRIVATE WasmFunctionBuilder : public ZoneObject {
   ZoneVector<uint32_t> f32_temps_;
   ZoneVector<uint32_t> f64_temps_;
   ZoneVector<DirectCallIndex> direct_calls_;
+
+  // Delta-encoded mapping from wasm bytes to asm.js source positions.
+  ZoneBuffer asm_offsets_;
+  uint32_t last_asm_byte_offset_ = 0;
+  uint32_t last_asm_source_position_ = 0;
 };
 
 class WasmTemporary {
@@ -228,6 +235,7 @@ class V8_EXPORT_PRIVATE WasmModuleBuilder : public ZoneObject {
 
   // Writing methods.
   void WriteTo(ZoneBuffer& buffer) const;
+  void WriteAsmJsOffsetTable(ZoneBuffer& buffer) const;
 
   // TODO(titzer): use SignatureMap from signature-map.h here.
   // This signature map is zone-allocated, but the other is heap allocated.