[gc] Only track potentially live buffers in ABT processing step.

For kUpdateEntry and kRemoveEntry, the ArrayBuffer is no longer present
on the current page. These are the two most common cases; kKeepEntry is
only used for aborted old-page evacuation candidates. Currently we pay
the cost of removing the entry from the array_buffers_ map, even though
the page itself will be cleared (for new space) or only aborted
evacuation candidates will be kept on the page (for old space).

Change-Id: Ib442109d444973a72e378d9072206f404d1c5183
Reviewed-on: https://chromium-review.googlesource.com/1102332Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Peter Marshall <petermarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53757}

src/heap/array-buffer-tracker.cc

@@ -21,17 +21,18 @@ LocalArrayBufferTracker::~LocalArrayBufferTracker() {
 template <typename Callback>
 void LocalArrayBufferTracker::Process(Callback callback) {
   std::vector<JSArrayBuffer::Allocation> backing_stores_to_free;
+  TrackingData kept_array_buffers;
 
   JSArrayBuffer* new_buffer = nullptr;
   JSArrayBuffer* old_buffer = nullptr;
   size_t freed_memory = 0;
   size_t moved_memory = 0;
   for (TrackingData::iterator it = array_buffers_.begin();
-       it != array_buffers_.end();) {
+       it != array_buffers_.end(); ++it) {
     old_buffer = reinterpret_cast<JSArrayBuffer*>(it->first);
     const CallbackResult result = callback(old_buffer, &new_buffer);
     if (result == kKeepEntry) {
-      ++it;
+      kept_array_buffers.insert(*it);
     } else if (result == kUpdateEntry) {
       DCHECK_NOT_NULL(new_buffer);
       Page* target_page = Page::FromAddress(new_buffer->address());
@@ -47,7 +48,6 @@ void LocalArrayBufferTracker::Process(Callback callback) {
         tracker->Add(new_buffer, size);
       }
       moved_memory += it->second;
-      it = array_buffers_.erase(it);
     } else if (result == kRemoveEntry) {
       freed_memory += it->second;
       // We pass backing_store() and stored length to the collector for freeing
@@ -56,7 +56,6 @@ void LocalArrayBufferTracker::Process(Callback callback) {
       backing_stores_to_free.emplace_back(
           old_buffer->backing_store(), it->second, old_buffer->backing_store(),
           old_buffer->allocation_mode(), old_buffer->is_wasm_memory());
-      it = array_buffers_.erase(it);
     } else {
       UNREACHABLE();
     }
@@ -70,6 +69,8 @@ void LocalArrayBufferTracker::Process(Callback callback) {
         static_cast<intptr_t>(freed_memory));
   }
 
+  array_buffers_.swap(kept_array_buffers);
+
   // Pass the backing stores that need to be freed to the main thread for later
   // distribution.
   space_->heap()->array_buffer_collector()->AddGarbageAllocations(