Reduce the number of flushes while using glQueries.

gpu/command_buffer/client/query_tracker.cc

Index: gpu/command_buffer/client/query_tracker.cc
diff --git a/gpu/command_buffer/client/query_tracker.cc b/gpu/command_buffer/client/query_tracker.cc
index 5ac89e07e0f5ca472d767f99b86815ef2984be5f..2595e7a57a0f475a2ed29152857aa708938a5bc1 100644
--- a/gpu/command_buffer/client/query_tracker.cc
+++ b/gpu/command_buffer/client/query_tracker.cc
@@ -93,7 +93,7 @@ QueryTracker::Query::Query(GLuint id, GLenum target,
       state_(kUninitialized),
       submit_count_(0),
       token_(0),
-      flushed_(false),
+      flush_count_(0),
       client_begin_time_us_(0),
       result_(0) {
     }
@@ -140,6 +140,7 @@ void QueryTracker::Query::End(GLES2Implementation* gl) {
       }
     }
   }
+  flush_count_ = gl->helper()->flush_generation();
   gl->helper()->EndQueryEXT(target(), submit_count());
   MarkAsPending(gl->helper()->InsertToken());
 }
@@ -168,12 +169,7 @@ bool QueryTracker::Query::CheckResultsAvailable(
       }
       state_ = kComplete;
     } else {
-      if (!flushed_) {
-        // TODO(gman): We could reduce the number of flushes by having a
-        // flush count, recording that count at the time we insert the
-        // EndQuery command and then only flushing here if we've have not
-        // passed that count yet.
-        flushed_ = true;
+      if (!(helper->flush_generation()>flush_count_)) {

[piman, 2014/04/08 22:55:40]

nit: spaces around '>'. Also, if (!(a>b)) can be written more clearly if (a <=
b)

[piman, 2014/04/08 22:55:40]

Could we handle wraparound of the generation here? Assuming we don't have 2^31
flushes outstanding (which is not possible), this should work:

if ((helper->flush_generation() - flush_count_ - 1) >= 0x80000000) {
  Flush();
} else {
  Noop();
}

[rptr, 2014/04/09 10:42:04]

Done.

[rptr, 2014/04/09 10:42:04]

nit: sorry for that, forgot to run cl format before upload.

And regarding !(a>b) i thought this way its more clear while reading code as
flush_gen is never going to be less than flush_count recorded during EndQuery. 
Anyways new wraparound change takes care of this.

         helper->Flush();
       } else {
         // Insert no-ops so that eventually the GPU process will see more work.