Reduce heap allocations in BLASTBufferQueue.

* Replace std::unordered_map members with ftl::SmallMap with empirically
  determined sizes.
* Eliminate creating an unnecessary std::vector

Test: unit, manual
Bug: 285377983
Flag: EXEMPT refactoring
Change-Id: I20402bf58d6619573cb1ff85be5092447f841b8a

Author: Anton Ivanov

include/ftl/small_map.h

@@ -234,6 +234,12 @@ class SmallMap final {
   //
   bool erase(const key_type& key) { return erase(key, begin()); }
 
+  // Removes a mapping.
+  //
+  // The last() and end() iterators, as well as those to the erased mapping, are invalidated.
+  //
+  void erase(iterator it) { map_.unstable_erase(it); }
+
   // Removes all mappings.
   //
   // All iterators are invalidated.

libs/gui/BLASTBufferQueue.cpp

@@ -415,14 +415,12 @@ void BLASTBufferQueue::transactionCallback(nsecs_t /*latchTime*/, const sp<Fence
                                                     stat.frameEventStats.dequeueReadyTime);
                 }
                 auto currFrameNumber = stat.frameEventStats.frameNumber;
-                std::vector<ReleaseCallbackId> staleReleases;
-                for (const auto& [key, value]: mSubmitted) {
-                    if (currFrameNumber > key.framenumber) {
-                        staleReleases.push_back(key);
+                // Release stale buffers.
+                for (const auto& [key, _] : mSubmitted) {
+                    if (currFrameNumber <= key.framenumber) {
+                        continue; // not stale.
                     }
-                }
-                for (const auto& staleRelease : staleReleases) {
-                    releaseBufferCallbackLocked(staleRelease,
+                    releaseBufferCallbackLocked(key,
                                                 stat.previousReleaseFence
                                                         ? stat.previousReleaseFence
                                                         : Fence::NO_FENCE,
@@ -618,7 +616,7 @@ status_t BLASTBufferQueue::acquireNextBufferLocked(
     mNumAcquired++;
     mLastAcquiredFrameNumber = bufferItem.mFrameNumber;
     ReleaseCallbackId releaseCallbackId(buffer->getId(), mLastAcquiredFrameNumber);
-    mSubmitted[releaseCallbackId] = bufferItem;
+    mSubmitted.emplace_or_replace(releaseCallbackId, bufferItem);
 
     bool needsDisconnect = false;
     mBufferItemConsumer->getConnectionEvents(bufferItem.mFrameNumber, &needsDisconnect);
@@ -851,7 +849,7 @@ void BLASTBufferQueue::onFrameReplaced(const BufferItem& item) {
 
 void BLASTBufferQueue::onFrameDequeued(const uint64_t bufferId) {
     std::lock_guard _lock{mTimestampMutex};
-    mDequeueTimestamps[bufferId] = systemTime();
+    mDequeueTimestamps.emplace_or_replace(bufferId, systemTime());
 };
 
 void BLASTBufferQueue::onFrameCancelled(const uint64_t bufferId) {

libs/gui/include/gui/BLASTBufferQueue.h

@@ -20,6 +20,7 @@
 #include <optional>
 #include <queue>
 
+#include <ftl/small_map.h>
 #include <gui/BufferItem.h>
 #include <gui/BufferItemConsumer.h>
 #include <gui/IGraphicBufferConsumer.h>
@@ -36,6 +37,10 @@
 
 namespace android {
 
+// Sizes determined empirically to avoid allocations during common activity.
+constexpr size_t kSubmittedBuffersMapSizeHint = 8;
+constexpr size_t kDequeueTimestampsMapSizeHint = 32;
+
 class BLASTBufferQueue;
 class BufferItemConsumer;
 
@@ -206,7 +211,7 @@ class BLASTBufferQueue : public ConsumerBase::FrameAvailableListener {
 
     // Keep a reference to the submitted buffers so we can release when surfaceflinger drops the
     // buffer or the buffer has been presented and a new buffer is ready to be presented.
-    std::unordered_map<ReleaseCallbackId, BufferItem, ReleaseBufferCallbackIdHash> mSubmitted
+    ftl::SmallMap<ReleaseCallbackId, BufferItem, kSubmittedBuffersMapSizeHint> mSubmitted
             GUARDED_BY(mMutex);
 
     // Keep a queue of the released buffers instead of immediately releasing
@@ -291,8 +296,8 @@ class BLASTBufferQueue : public ConsumerBase::FrameAvailableListener {
     std::mutex mTimestampMutex;
     // Tracks buffer dequeue times by the client. This info is sent to SurfaceFlinger which uses
     // it for debugging purposes.
-    std::unordered_map<uint64_t /* bufferId */, nsecs_t> mDequeueTimestamps
-            GUARDED_BY(mTimestampMutex);
+    ftl::SmallMap<uint64_t /* bufferId */, nsecs_t, kDequeueTimestampsMapSizeHint>
+            mDequeueTimestamps GUARDED_BY(mTimestampMutex);
 
     // Keep track of SurfaceControls that have submitted a transaction and BBQ is waiting on a
     // callback for them.