[wasm] Patch trapping position into stack trace

src/runtime/runtime-internal.cc

Index: src/runtime/runtime-internal.cc
diff --git a/src/runtime/runtime-internal.cc b/src/runtime/runtime-internal.cc
index 0aefae9bf4eb69483fec2b58a71854d8658ae7fb..4a736ff84db80948b20d1ec8aadc805cd0a6805f 100644
--- a/src/runtime/runtime-internal.cc
+++ b/src/runtime/runtime-internal.cc
@@ -101,10 +101,47 @@ RUNTIME_FUNCTION(Runtime_ThrowWasmError) {
   DCHECK_EQ(2, args.length());
   CONVERT_SMI_ARG_CHECKED(message_id, 0);
   CONVERT_SMI_ARG_CHECKED(byte_offset, 1);
-  USE(byte_offset);  // TODO(clemensh): patch the stack trace with this offset
-  Handle<Object> error = isolate->factory()->NewError(
+  Handle<Object> error_obj = isolate->factory()->NewError(
       static_cast<MessageTemplate::Template>(message_id));
-  return isolate->Throw(*error);
+
+  // Now patch the given byte offset into the top-most frame (which must be a

[titzer, 2016/04/28 12:07:59]

Can you adjust the comments a bit here to explain why we do this?

I.e. it's because the above stack trace is inaccurate in the top frame due to
the shared trap code, so we adjust it here.

[Clemens Hammacher, 2016/04/28 12:32:12]

Done.

+  // wasm frame).
+  Handle<JSObject> error = Handle<JSObject>::cast(error_obj);
+  Handle<Object> stack_trace_obj = JSReceiver::GetDataProperty(
+      error, isolate->factory()->stack_trace_symbol());
+  // Patch the stack trace (array of <receiver, function, code, position>).
+  if (stack_trace_obj->IsJSArray()) {
+    Handle<FixedArray> stack_elements(
+        FixedArray::cast(JSArray::cast(*stack_trace_obj)->elements()));
+    DCHECK_EQ(1, stack_elements->length() % 4);
+    DCHECK(Code::cast(stack_elements->get(3))->kind() == Code::WASM_FUNCTION);
+    DCHECK(stack_elements->get(4)->IsSmi() &&
+           Smi::cast(stack_elements->get(4))->value() >= 0);
+    stack_elements->set(4, Smi::FromInt(-1 - byte_offset));
+  }
+  Handle<Object> detailed_stack_trace_obj = JSReceiver::GetDataProperty(
+      error, isolate->factory()->detailed_stack_trace_symbol());
+  // Patch the detailed stack trace (array of JSObjects with various
+  // properties).
+  if (detailed_stack_trace_obj->IsJSArray()) {
+    Handle<FixedArray> stack_elements(
+        FixedArray::cast(JSArray::cast(*detailed_stack_trace_obj)->elements()));
+    DCHECK_GE(stack_elements->length(), 1);
+    Handle<JSObject> top_frame(JSObject::cast(stack_elements->get(0)));
+    Handle<String> wasm_offset_key =
+        isolate->factory()->InternalizeOneByteString(
+            STATIC_CHAR_VECTOR("wasmByteOffset"));
+    LookupIterator it(top_frame, wasm_offset_key, top_frame,
+                      LookupIterator::PROTOTYPE_CHAIN_SKIP_INTERCEPTOR);
+    if (it.IsFound()) {
+      DCHECK(JSReceiver::GetDataProperty(&it)->IsSmi());
+      Maybe<bool> data_set = JSReceiver::SetDataProperty(
+          &it, handle(Smi::FromInt(byte_offset), isolate));
+      DCHECK(data_set.IsJust() && data_set.FromJust() == true);
+    }
+  }
+
+  return isolate->Throw(*error_obj);
 }
 
 RUNTIME_FUNCTION(Runtime_UnwindAndFindExceptionHandler) {