Replace std::map with base::SmallMap in ui::LatencyInfo

The typical number of components per LatencyInfo instance is quite small. Avoid
unnecessary heap allocation by using a SmallMap to store component entries.
This reduces the average browser send time cost per touch or scroll event by
~15us on a Nexus 4, and ~22us on a Galaxy Nexus.

BUG=341613

Review URL: https://codereview.chromium.org/157003005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@257740 0039d316-1c4b-4281-b951-d872f2087c98

1 files changed, 36 insertions, 0 deletions

diff --git a/ipc/ipc_message_utils.h b/ipc/ipc_message_utils.h
index b05f76d..102992d 100644
--- a/ipc/ipc_message_utils.h
+++ b/ipc/ipc_message_utils.h
@@ -11,6 +11,7 @@
 #include <string>
 #include <vector>
 
+#include "base/containers/small_map.h"
 #include "base/files/file.h"
 #include "base/format_macros.h"
 #include "base/memory/scoped_vector.h"
@@ -670,6 +671,41 @@ struct ParamTraits<ScopedVector<P> > {
   }
 };
 
+template <typename NormalMap,
+          int kArraySize,
+          typename EqualKey,
+          typename MapInit>
+struct ParamTraits<base::SmallMap<NormalMap, kArraySize, EqualKey, MapInit> > {
+  typedef base::SmallMap<NormalMap, kArraySize, EqualKey, MapInit> param_type;
+  typedef typename param_type::key_type K;
+  typedef typename param_type::data_type V;
+  static void Write(Message* m, const param_type& p) {
+    WriteParam(m, static_cast<int>(p.size()));
+    typename param_type::const_iterator iter;
+    for (iter = p.begin(); iter != p.end(); ++iter) {
+      WriteParam(m, iter->first);
+      WriteParam(m, iter->second);
+    }
+  }
+  static bool Read(const Message* m, PickleIterator* iter, param_type* r) {
+    int size;
+    if (!m->ReadLength(iter, &size))
+      return false;
+    for (int i = 0; i < size; ++i) {
+      K key;
+      if (!ReadParam(m, iter, &key))
+        return false;
+      V& value = (*r)[key];
+      if (!ReadParam(m, iter, &value))
+        return false;
+    }
+    return true;
+  }
+  static void Log(const param_type& p, std::string* l) {
+    l->append("<base::SmallMap>");
+  }
+};
+
 // IPC types ParamTraits -------------------------------------------------------
 
 // A ChannelHandle is basically a platform-inspecific wrapper around the