Add offset support to BitstreamBuffer.

content/common/gpu/media/shared_memory_region.cc

+// Copyright (c) 2015 The Chromium 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 "base/sys_info.h"
+#include "content/common/gpu/media/shared_memory_region.h"
+
+namespace content {
+
+SharedMemoryRegion::SharedMemoryRegion(const base::SharedMemoryHandle& handle,
+                                       off_t offset,
+                                       size_t size,
+                                       bool read_only)
+    : shm_(handle, read_only), offset_(offset), size_(size) {}
+
+SharedMemoryRegion::SharedMemoryRegion(
+    const media::BitstreamBuffer& bitstream_buffer,
+    bool read_only)
+    : SharedMemoryRegion(bitstream_buffer.handle(),
+                         bitstream_buffer.offset(),
+                         bitstream_buffer.size(),
+                         read_only) {}
+
+bool SharedMemoryRegion::Map() {
+  off_t mask = base::SysInfo::VMAllocationGranularity() - 1;

[Pawel Osciak, 2015/12/31 02:05:53]

This assumes granularity is a power of 2 (which normally it always should be,
but technically we don't guarantee that from the documentation of the method).
Perhaps something like this instead:

https://code.google.com/p/chromium/codesearch#chromium/src/mojo/edk/embedder/...

[Owen Lin, 2016/01/04 08:54:17]

Done.

+  return shm_.MapAt((offset_ & ~mask), size_ + (offset_ & mask));

[Pawel Osciak, 2015/12/31 02:05:53]

Perhaps we could calculate new offset here (or even in constructor to drop
offset_) and keep only the calculated offset, for use in memory()?

[Owen Lin, 2016/01/04 08:54:17]

Done.

+}
+
+void* SharedMemoryRegion::memory() {
+  off_t mask = base::SysInfo::VMAllocationGranularity() - 1;
+  int8_t* addr = reinterpret_cast<int8_t*>(shm_.memory());
+  return addr ? addr + (offset_ & mask) : nullptr;
+}
+
+}  // namespace content