In SurfaceFlinger, test fence before merging

If the old fence for a given layer has already expired, we can replace
it with the new fence rather than merging the two.

This saves 300-400us per frame on Android Wear

Change-Id: I5d907c336c7383ae4e2e0f71e42f23749494a359

1 files changed, 12 insertions, 0 deletions

diff --git a/libs/gui/ConsumerBase.cpp b/libs/gui/ConsumerBase.cpp
index e7bc7dc73d..65e4feefde 100644
--- a/libs/gui/ConsumerBase.cpp
+++ b/libs/gui/ConsumerBase.cpp
@@ -314,6 +314,18 @@ status_t ConsumerBase::addReleaseFenceLocked(int slot,
 
     if (!mSlots[slot].mFence.get()) {
         mSlots[slot].mFence = fence;
+        return OK;
+    }
+
+    auto signaled = mSlots[slot].mFence->hasSignaled();
+
+    if (!signaled) {
+        CB_LOGE("fence has invalid state");
+        return BAD_VALUE;
+    }
+
+    if (*signaled) {
+        mSlots[slot].mFence = fence;
     } else {
         char fenceName[32] = {};
         snprintf(fenceName, 32, "%.28s:%d", mName.string(), slot);