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

src/wasm/wasm-module-builder.cc

Index: src/wasm/wasm-module-builder.cc
diff --git a/src/wasm/wasm-module-builder.cc b/src/wasm/wasm-module-builder.cc
index 3efcb090aa62796aafb82473775f4a4aa4e08b74..ab76efe5f8b8837811558657c1dab64556497f78 100644
--- a/src/wasm/wasm-module-builder.cc
+++ b/src/wasm/wasm-module-builder.cc
@@ -59,7 +59,8 @@ WasmFunctionBuilder::WasmFunctionBuilder(WasmModuleBuilder* builder)
       i64_temps_(builder->zone()),
       f32_temps_(builder->zone()),
       f64_temps_(builder->zone()),
-      direct_calls_(builder->zone()) {}
+      direct_calls_(builder->zone()),
+      asm_offsets_(builder->zone(), 8) {}
 
 void WasmFunctionBuilder::EmitVarInt(uint32_t val) {
   byte buffer[8];
@@ -153,6 +154,20 @@ void WasmFunctionBuilder::SetName(Vector<const char> name) {
   memcpy(name_.data(), name.start(), name.length());
 }
 
+void WasmFunctionBuilder::AddAsmWasmOffset(int asm_position) {
+  // We only want to emit one mapping per byte offset:
+  DCHECK(asm_offsets_.size() == 0 || body_.size() > last_asm_byte_offset_);
+
+  DCHECK_LE(body_.size(), kMaxUInt32);
+  uint32_t byte_offset = static_cast<uint32_t>(body_.size());
+  asm_offsets_.write_u32v(byte_offset - last_asm_byte_offset_);
+  last_asm_byte_offset_ = byte_offset;
+
+  DCHECK_GE(asm_position, 0);
+  asm_offsets_.write_i32v(asm_position - last_asm_source_position_);
+  last_asm_source_position_ = asm_position;
+}
+
 void WasmFunctionBuilder::WriteSignature(ZoneBuffer& buffer) const {
   buffer.write_u32v(signature_index_);
 }
@@ -188,6 +203,18 @@ void WasmFunctionBuilder::WriteBody(ZoneBuffer& buffer) const {
   }
 }
 
+void WasmFunctionBuilder::WriteAsmWasmOffsetTable(ZoneBuffer& buffer) const {
+  if (asm_offsets_.size() == 0) {
+    buffer.write_size(0);
+    return;
+  }
+  buffer.write_size(asm_offsets_.size() + kInt32Size);
+  // Offset of the recorded byte offsets.
+  DCHECK_GE(kMaxUInt32, locals_.Size());
+  buffer.write_u32(static_cast<uint32_t>(locals_.Size()));
+  buffer.write(asm_offsets_.begin(), asm_offsets_.size());
+}
+
 WasmModuleBuilder::WasmModuleBuilder(Zone* zone)
     : zone_(zone),
       signatures_(zone),
@@ -309,6 +336,7 @@ void WasmModuleBuilder::WriteTo(ZoneBuffer& buffer) const {
 
   // == Emit function signatures ===============================================
   bool has_names = false;
+  bool has_asm_offsets = false;
   if (functions_.size() > 0) {
     size_t start = EmitSection(kFunctionSectionCode, buffer);
     buffer.write_size(functions_.size());
@@ -316,6 +344,7 @@ void WasmModuleBuilder::WriteTo(ZoneBuffer& buffer) const {
       function->WriteSignature(buffer);
       if (function->exported()) exports++;
       if (function->name_.size() > 0) has_names = true;
+      if (function->asm_offsets_.size() > 0) has_asm_offsets = true;
     }
     FixupSection(buffer, start);
   }
@@ -493,6 +522,25 @@ void WasmModuleBuilder::WriteTo(ZoneBuffer& buffer) const {
     }
     FixupSection(buffer, start);
   }
+
+  // == Emit asm.js offset table ===============================================
+  if (has_asm_offsets) {
+    // Emit the section code.
+    buffer.write_u8(kUnknownSectionCode);
+    // Emit a placeholder for the length.
+    size_t start = buffer.reserve_u32v();
+    // Emit the section string.
+    const char* asm_offsets_name = "asm_offsets";
+    buffer.write_size(strlen(asm_offsets_name));
+    buffer.write(reinterpret_cast<const byte*>(asm_offsets_name),
+                 strlen(asm_offsets_name));
+    buffer.write_size(functions_.size());
+    // Emit the offset table per function.
+    for (auto function : functions_) {
+      function->WriteAsmWasmOffsetTable(buffer);
+    }
+    FixupSection(buffer, start);
+  }
 }
 }  // namespace wasm
 }  // namespace internal