Bug 1372569 - Skip sweeping empty weak caches r=sfink

js/public/GCHashTable.h

@@ -69,6 +69,10 @@ class GCHashMap : public js::HashMap<Key, Value, HashPolicy, AllocPolicy>
         }
     }
 
+    bool needsSweep() const {
+        return this->initialized() && !this->empty();
+    }
+
     void sweep() {
         if (!this->initialized())
             return;
@@ -246,6 +250,10 @@ class GCHashSet : public js::HashSet<T, HashPolicy, AllocPolicy>
             GCPolicy<T>::trace(trc, &e.mutableFront(), "hashset element");
     }
 
+    bool needsSweep() const {
+        return this->initialized() && !this->empty();
+    }
+
     void sweep() {
         if (!this->initialized())
             return;

js/public/GCVector.h

@@ -135,6 +135,10 @@ class GCVector
             GCPolicy<T>::trace(trc, &elem, "vector element");
     }
 
+    bool needsSweep() const {
+        return !this->empty();
+    }
+
     void sweep() {
         uint32_t src, dst = 0;
         for (src = 0; src < length(); src++) {

js/public/SweepingAPI.h

@@ -38,6 +38,7 @@ class WeakCacheBase : public mozilla::LinkedListElement<WeakCacheBase>
     virtual ~WeakCacheBase() {}
 
     virtual void sweep() = 0;
+    virtual bool needsSweep() = 0;
 };
 } // namespace detail
 
@@ -69,6 +70,10 @@ class WeakCache : protected detail::WeakCacheBase,
     void sweep() override {
         GCPolicy<T>::sweep(&cache);
     }
+
+    bool needsSweep() override {
+        return cache.needsSweep();
+    }
 };
 
 } // namespace JS

js/src/jsgc.cpp

@@ -5233,6 +5233,9 @@ PrepareWeakCacheTasks(JSRuntime* rt)
     // Build a vector of sweep tasks to run on a helper thread.
     WeakCacheTaskVector tasks;
     bool ok = IterateWeakCaches(rt, [&] (JS::detail::WeakCacheBase* cache) {
+        if (!cache->needsSweep())
+            return true;
+
         return tasks.emplaceBack(rt, *cache);
     });
 

js/src/vm/ArrayBufferObject.h

@@ -599,6 +599,10 @@ class InnerViewTable
     void sweep();
     void sweepAfterMinorGC();
 
+    bool needsSweep() const {
+        return map.needsSweep();
+    }
+
     bool needsSweepAfterMinorGC() const {
         return !nurseryKeys.empty() || !nurseryKeysValid;
     }