Expose an API on ArrayBufferView to copy out content w/o changing the buffer

BUG=v8:3996
LOG=y
R=dslomov@chromium.org,kbr@chromium.org,hpayer@chromium.org

Review URL: https://codereview.chromium.org/1041403003

Cr-Commit-Position: refs/heads/master@{#27580}

include/v8.h

@@ -3482,6 +3482,17 @@ class V8_EXPORT ArrayBufferView : public Object {
    */
   size_t ByteLength();
 
+  /**
+   * Copy the contents of the ArrayBufferView's buffer to an embedder defined
+   * memory without additional overhead that calling ArrayBufferView::Buffer
+   * might incur.
+   *
+   * Will write at most min(|byte_length|, ByteLength) bytes starting at
+   * ByteOffset of the underling buffer to the memory starting at |dest|.
+   * Returns the number of bytes actually written.
+   */
+  size_t CopyContents(void* dest, size_t byte_length);
+
   V8_INLINE static ArrayBufferView* Cast(Value* obj);
 
   static const int kInternalFieldCount =

src/api.cc

@@ -6425,6 +6425,39 @@ Local<ArrayBuffer> v8::ArrayBufferView::Buffer() {
 }
 
 
+size_t v8::ArrayBufferView::CopyContents(void* dest, size_t byte_length) {
+  i::Handle<i::JSArrayBufferView> obj = Utils::OpenHandle(this);
+  i::Isolate* isolate = obj->GetIsolate();
+  size_t byte_offset = i::NumberToSize(isolate, obj->byte_offset());
+  size_t bytes_to_copy =
+      i::Min(byte_length, i::NumberToSize(isolate, obj->byte_length()));
+  if (bytes_to_copy) {
+    i::DisallowHeapAllocation no_gc;
+    const char* source = nullptr;
+    if (obj->IsJSDataView()) {
+      i::Handle<i::JSDataView> data_view(i::JSDataView::cast(*obj));
+      i::Handle<i::JSArrayBuffer> buffer(
+          i::JSArrayBuffer::cast(data_view->buffer()));
+      source = reinterpret_cast<char*>(buffer->backing_store());
+    } else {
+      DCHECK(obj->IsJSTypedArray());
+      i::Handle<i::JSTypedArray> typed_array(i::JSTypedArray::cast(*obj));
+      if (typed_array->buffer()->IsSmi()) {
+        i::Handle<i::FixedTypedArrayBase> fixed_array(
+            i::FixedTypedArrayBase::cast(typed_array->elements()));
+        source = reinterpret_cast<char*>(fixed_array->DataPtr());
+      } else {
+        i::Handle<i::JSArrayBuffer> buffer(
+            i::JSArrayBuffer::cast(typed_array->buffer()));
+        source = reinterpret_cast<char*>(buffer->backing_store());
+      }
+    }
+    memcpy(dest, source + byte_offset, bytes_to_copy);
+  }
+  return bytes_to_copy;
+}
+
+
 size_t v8::ArrayBufferView::ByteOffset() {
   i::Handle<i::JSArrayBufferView> obj = Utils::OpenHandle(this);
   return static_cast<size_t>(obj->byte_offset()->Number());

test/cctest/cctest.gyp

@@ -158,6 +158,7 @@
         'test-thread-termination.cc',
         'test-threads.cc',
         'test-transitions.cc',
+        'test-typedarrays.cc',
         'test-types.cc',
         'test-unbound-queue.cc',
         'test-unboxed-doubles.cc',

test/cctest/test-typedarrays.cc

+// Copyright 2015 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include <stdlib.h>
+
+#include "src/v8.h"
+#include "test/cctest/cctest.h"
+
+#include "src/api.h"
+#include "src/heap/heap.h"
+#include "src/objects.h"
+
+using namespace v8::internal;
+
+void TestArrayBufferViewContents(LocalContext& env, bool should_use_buffer) {
+  v8::Local<v8::Object> obj_a =
+      v8::Local<v8::Object>::Cast(env->Global()->Get(v8_str("a")));
+  CHECK(obj_a->IsArrayBufferView());
+  v8::Local<v8::ArrayBufferView> array_buffer_view =
+      v8::Local<v8::ArrayBufferView>::Cast(obj_a);
+  Handle<JSArrayBufferView> internal_view(
+      v8::Utils::OpenHandle(*array_buffer_view));
+  bool has_buffer = true;
+  if (internal_view->IsJSTypedArray()) {
+    Handle<JSTypedArray> typed_array(JSTypedArray::cast(*internal_view));
+    has_buffer = !typed_array->buffer()->IsSmi();
+  }
+  CHECK_EQ(has_buffer, should_use_buffer);
+  unsigned char contents[4] = {23, 23, 23, 23};
+  CHECK_EQ(sizeof(contents),
+           array_buffer_view->CopyContents(contents, sizeof(contents)));
+  if (!has_buffer) {
+    CHECK(internal_view->IsJSTypedArray());
+    Handle<JSTypedArray> typed_array(JSTypedArray::cast(*internal_view));
+    CHECK(typed_array->buffer()->IsSmi());
+  }
+  for (size_t i = 0; i < sizeof(contents); ++i) {
+    CHECK_EQ(i, contents[i]);
+  }
+}
+
+
+TEST(CopyContentsTypedArray) {
+  LocalContext env;
+  v8::HandleScope scope(env->GetIsolate());
+  CompileRun(
+      "var a = new Uint8Array(4);"
+      "a[0] = 0;"
+      "a[1] = 1;"
+      "a[2] = 2;"
+      "a[3] = 3;");
+  TestArrayBufferViewContents(env, false);
+}
+
+
+TEST(CopyContentsArray) {
+  LocalContext env;
+  v8::HandleScope scope(env->GetIsolate());
+  CompileRun("var a = new Uint8Array([0, 1, 2, 3]);");
+  TestArrayBufferViewContents(env, true);
+}
+
+
+TEST(CopyContentsView) {
+  LocalContext env;
+  v8::HandleScope scope(env->GetIsolate());
+  CompileRun(
+      "var b = new ArrayBuffer(6);"
+      "var c = new Uint8Array(b);"
+      "c[0] = -1;"
+      "c[1] = -1;"
+      "c[2] = 0;"
+      "c[3] = 1;"
+      "c[4] = 2;"
+      "c[5] = 3;"
+      "var a = new DataView(b, 2);");
+  TestArrayBufferViewContents(env, true);
+}