- Add functionality to morph a string into an external string

vm/object.cc

Index: vm/object.cc
===================================================================
--- vm/object.cc	(revision 14589)
+++ vm/object.cc	(working copy)
@@ -474,6 +474,46 @@
 }
 
 
+// Make unused space in an object whose type has been transformed safe
+// for traversing during GC.
+// The unused part of the transformed object is marked as an Array
+// object or a regular Object so that it can be traversed during garbage
+// collection.
+void Object::MakeUnusedSpaceTraversable(const Object& obj,
+                                        intptr_t original_size,
+                                        intptr_t used_size) {
+  ASSERT(Isolate::Current()->no_gc_scope_depth() > 0);
+  ASSERT(!obj.IsNull());
+  if (original_size > used_size) {

[Anton Muhin, 2012/11/07 10:05:46]

maybe turn that into an assert?

[siva, 2012/11/07 21:43:27]

I did not want it to be an assert because then the
caller has to do the check. This way the caller just
calls this method and if there is no left over space
nothing is done.

On 2012/11/07 10:05:46, Anton Muhin wrote:

+    intptr_t leftover_size = original_size - used_size;
+
+    uword addr = RawObject::ToAddr(obj.raw()) + used_size;
+    if (leftover_size >= Array::InstanceSize(0)) {
+      // As we have enough space to use an array object, update the leftover
+      // space as an Array object.
+      RawArray* raw = reinterpret_cast<RawArray*>(RawObject::FromAddr(addr));
+      uword tags = 0;
+      tags = RawObject::SizeTag::update(leftover_size, tags);

[Anton Muhin, 2012/11/07 10:05:46]

may there be any issues with alignment?  maybe add an assert that all sizes are
properly aligned if it's indeed the case?

[siva, 2012/11/07 21:43:27]

Objects are always aligned, we have asserts for this
in RawObject::SizeToTagValue(...).

On 2012/11/07 10:05:46, Anton Muhin wrote:

+      tags = RawObject::ClassIdTag::update(kArrayCid, tags);
+      raw->ptr()->tags_ = tags;
+      intptr_t leftover_len =
+          ((leftover_size - Array::InstanceSize(0)) / kWordSize);
+      raw->ptr()->tags_ = tags;
+      raw->ptr()->length_ = Smi::New(leftover_len);
+    } else {
+      // Update the leftover space as a basic object.
+      ASSERT(leftover_size == Object::InstanceSize());
+      RawObject* raw =
+          reinterpret_cast<RawObject*>(RawObject::FromAddr(addr));
+      uword tags = 0;
+      tags = RawObject::SizeTag::update(leftover_size, tags);
+      tags = RawObject::ClassIdTag::update(kInstanceCid, tags);
+      raw->ptr()->tags_ = tags;
+    }
+  }
+}
+
+
 RawClass* Object::CreateAndRegisterInterface(const char* cname,
                                              const Script& script,
                                              const Library& lib) {
@@ -10294,6 +10334,87 @@
 }
 
 
+static void AddFinalizer(const Object& referent,
+                         void* peer,
+                         Dart_WeakPersistentHandleFinalizer callback) {
+  ASSERT(callback != NULL);
+  ApiState* state = Isolate::Current()->api_state();
+  ASSERT(state != NULL);
+  FinalizablePersistentHandle* weak_ref =
+      state->weak_persistent_handles().AllocateHandle();
+  weak_ref->set_raw(referent);
+  weak_ref->set_peer(peer);
+  weak_ref->set_callback(callback);
+}
+
+
+RawString* String::MakeExternal(void* array,
+                                intptr_t length,
+                                void* peer,
+                                Dart_PeerFinalizer cback) const {
+  ASSERT(array != NULL);
+  intptr_t str_length = this->Length();
+  ASSERT(length >= (str_length * this->CharSize()));
+  intptr_t class_id = raw()->GetClassId();
+  intptr_t used_size = 0;
+  intptr_t original_size = 0;
+  uword tags = 0;
+  NoGCScope no_gc;
+
+  if (class_id == kOneByteStringCid) {
+    used_size = ExternalOneByteString::InstanceSize();
+    original_size = OneByteString::InstanceSize(str_length);
+    ASSERT(original_size >= used_size);

[Anton Muhin, 2012/11/07 10:05:46]

is it always true?  something like zero length string on x64 platform?  sorry, I
don't know layout of Dart VM objects so it might be a trivial question.

[siva, 2012/11/07 21:43:27]

Yes this is always true as we always align objects and
the external data representation is just one word.

On 2012/11/07 10:05:46, Anton Muhin wrote:

[Anton Muhin, 2012/11/08 08:05:29]

Okay.  In v8 zero length string would be just a header and externalized string
had no room to install a pointer into (because externalized string requires a
header too).

Glad to know it's not a restriction in VM.  Might be worth adding to a test in
case this would change later.

On 2012/11/07 21:43:27, siva wrote:

+
+    // Copy the data into the external array.
+    memmove(array, OneByteString::CharAddr(*this, 0), str_length);

[Anton Muhin, 2012/11/07 10:05:46]

that might be very unpleasant thing when one attempts to externalize a long
string.  Cannot we ask an embedder to provide data?

[siva, 2012/11/07 21:43:27]

I am not sure I understand your suggestion here. The
embedder would have to extract the data out in someway
to provide it right.

Are you suggesting that we should copy the data using a loop
outside the NoGCScope?

On 2012/11/07 10:05:46, Anton Muhin wrote:

[Anton Muhin, 2012/11/08 08:05:29]

Yes, the embedder has to do that, but he hasn't to do that twice if I got the
idea of API correctly.

Imagine the case when embedder cannot adopt data passed into
Dart_MakeExternalString into his string.  By adopt I mean bypass copying.

In this case what I can do (only accoutnting for utf16 case):

EmbedderString s = EmbedderString::createUninitializedUtf16(len); // Just
allocates memory for the string, doesn't fill it
Dart_StringToUtf16(s, s.data(), len); // Copying data from VM heap into
embedder's string.
int16_t* buffer = new int16_t[len]; // or somewhat sketchier, buffer = s.data()
Dart_MakeExternalString(s, buffer, length); // OOPS, another copy.

That's actually how v8 bindings work:
http://trac.webkit.org/browser/trunk/Source/WebCore/bindings/v8/V8StringResou...

Now in v8 you can just tell VM, here comes utf16 data and it's length, please,
use it.

On 2012/11/07 21:43:27, siva wrote:

+
+    // Update the class information of the object.
+    const intptr_t class_id = kExternalOneByteStringCid;
+    tags = RawObject::SizeTag::update(used_size, tags);
+    tags = RawObject::ClassIdTag::update(class_id, tags);
+    raw_ptr()->tags_ = tags;
+    const String& result = String::Handle(this->raw());
+    ExternalStringData<uint8_t>* ext_data = new ExternalStringData<uint8_t>(
+        reinterpret_cast<const uint8_t*>(array), peer, cback);
+    result.SetLength(str_length);
+    result.SetHash(0);
+    ExternalOneByteString::SetExternalData(result, ext_data);
+    AddFinalizer(result, ext_data, ExternalOneByteString::Finalize);
+  } else {
+    ASSERT(class_id == kTwoByteStringCid);
+    used_size = ExternalTwoByteString::InstanceSize();
+    original_size = TwoByteString::InstanceSize(str_length);
+    ASSERT(original_size >= used_size);
+
+    // Copy the data into the external array.
+    memmove(array,
+            TwoByteString::CharAddr(*this, 0),
+            (str_length * kTwoByteChar));
+
+    // Update the class information of the object.
+    const intptr_t class_id = kExternalTwoByteStringCid;
+    tags = RawObject::SizeTag::update(used_size, tags);
+    tags = RawObject::ClassIdTag::update(class_id, tags);
+    raw_ptr()->tags_ = tags;
+    const String& result = String::Handle(this->raw());
+    ExternalStringData<uint16_t>* ext_data = new ExternalStringData<uint16_t>(
+        reinterpret_cast<const uint16_t*>(array), peer, cback);
+    result.SetLength(str_length);
+    result.SetHash(0);
+    ExternalTwoByteString::SetExternalData(result, ext_data);
+    AddFinalizer(result, ext_data, ExternalTwoByteString::Finalize);
+  }
+
+  // If there is any left over space fill it with either an Array object or
+  // just a plain object (depending on the amount of left over space) so
+  // that it can be traversed over successfully during garbage collection.
+  Object::MakeUnusedSpaceTraversable(*this, original_size, used_size);
+
+  return this->raw();
+}
+
+
 RawString* String::Transform(int32_t (*mapping)(int32_t ch),
                              const String& str,
                              Heap::Space space) {
@@ -10684,20 +10805,6 @@
 }
 
 
-static void AddFinalizer(const Object& referent,
-                         void* peer,
-                         Dart_WeakPersistentHandleFinalizer callback) {
-  ASSERT(callback != NULL);
-  ApiState* state = Isolate::Current()->api_state();
-  ASSERT(state != NULL);
-  FinalizablePersistentHandle* weak_ref =
-      state->weak_persistent_handles().AllocateHandle();
-  weak_ref->set_raw(referent);
-  weak_ref->set_peer(peer);
-  weak_ref->set_callback(callback);
-}
-
-
 RawExternalOneByteString* ExternalOneByteString::New(
     const uint8_t* data,
     intptr_t len,
@@ -10927,33 +11034,8 @@
   // If there is any left over space fill it with either an Array object or
   // just a plain object (depending on the amount of left over space) so
   // that it can be traversed over successfully during garbage collection.
-  if (capacity_size != used_size) {
-    ASSERT(capacity_len > used_len);
-    intptr_t leftover_size = capacity_size - used_size;
+  Object::MakeUnusedSpaceTraversable(array, capacity_size, used_size);
 
-    uword addr = RawObject::ToAddr(array.raw()) + used_size;
-    if (leftover_size >= Array::InstanceSize(0)) {
-      // As we have enough space to use an array object, update the leftover
-      // space as an Array object.
-      RawArray* raw = reinterpret_cast<RawArray*>(RawObject::FromAddr(addr));
-      tags = 0;
-      tags = RawObject::SizeTag::update(leftover_size, tags);
-      tags = RawObject::ClassIdTag::update(kArrayCid, tags);
-      raw->ptr()->tags_ = tags;
-      intptr_t leftover_len =
-          ((leftover_size - Array::InstanceSize(0)) / kWordSize);
-      raw->ptr()->tags_ = tags;
-      raw->ptr()->length_ = Smi::New(leftover_len);
-    } else {
-      // Update the leftover space as a basic object.
-      ASSERT(leftover_size == Object::InstanceSize());
-      RawObject* raw = reinterpret_cast<RawObject*>(RawObject::FromAddr(addr));
-      tags = 0;
-      tags = RawObject::SizeTag::update(leftover_size, tags);
-      tags = RawObject::ClassIdTag::update(kInstanceCid, tags);
-      raw->ptr()->tags_ = tags;
-    }
-  }
   return array.raw();
 }