Add async trace events to trace progress of IPC messages

ipc/ipc_message.cc

Index: ipc/ipc_message.cc
diff --git a/ipc/ipc_message.cc b/ipc/ipc_message.cc
index 9908bc7a001bd2ce3770ec1e789d8fbb4c133b20..5cb12ad733398499f47887dcb04937a33c146ba1 100644
--- a/ipc/ipc_message.cc
+++ b/ipc/ipc_message.cc
@@ -4,6 +4,7 @@
 
 #include "ipc/ipc_message.h"
 
+#include "base/atomicops.h"
 #include "base/logging.h"
 #include "build/build_config.h"
 
@@ -11,6 +12,22 @@
 #include "ipc/file_descriptor_set_posix.h"
 #endif
 
+namespace {
+
+base::subtle::Atomic32 g_ref_num = 0;
+
+inline uint32 GetRefNumUpper24() {

[jam, 2012/08/31 23:07:55]

nit: add a comment

[jbates, 2012/08/31 23:27:49]

Done.

+  int32 pid = base::debug::TraceLog::GetInstance()->process_id();
+  int32 count = base::subtle::NoBarrier_AtomicIncrement(&g_ref_num, 1);
+  // The 24 bit hash is composed of 14 bits of the count and 10 bits of the
+  // Process ID. With the current trace event buffer cap, the 14-bit count did
+  // not appear to wrap during a trace. Note that it is not a big deal if
+  // collisions occur, as this is only used for debugging and trace analysis.
+  return ((pid << 14) | (count & 0x3fff)) << 8;
+}
+
+}  // namespace
+
 namespace IPC {
 
 //------------------------------------------------------------------------------
@@ -20,7 +37,8 @@ Message::~Message() {
 
 Message::Message()
     : Pickle(sizeof(Header)) {
-  header()->routing = header()->type = header()->flags = 0;
+  header()->routing = header()->type = 0;
+  header()->flags = GetRefNumUpper24();

[jam, 2012/08/31 23:07:55]

Are you sure this works with the reference builds? Current performance_ui_tests
binaries talk to old chrome binaries through IPC. So the question is whether the
old version is checking the flags to make sure it's within known values?

[jbates, 2012/08/31 23:27:49]

The code that accesses flags in this file just checks for the existing flags in
the first 8 bits. The ipc_tests all pass. Should I look anywhere else for
possible DCHECKs on flags values?

[jam, 2012/09/02 00:35:18]

you can try running tests which use the reference builds locally, but if by
inspection it looks fine to you, you can just commit and see if the perf bots
blow up

 #if defined(OS_POSIX)
   header()->num_fds = 0;
   header()->pad = 0;
@@ -32,7 +50,8 @@ Message::Message(int32 routing_id, uint32 type, PriorityValue priority)
     : Pickle(sizeof(Header)) {
   header()->routing = routing_id;
   header()->type = type;
-  header()->flags = priority;
+  DCHECK((priority & 0xffffff00) == 0);
+  header()->flags = priority | GetRefNumUpper24();
 #if defined(OS_POSIX)
   header()->num_fds = 0;
   header()->pad = 0;