Bug 1113300 - Add a way to use SegmentedVector like a stack. r=froydnj

mfbt/SegmentedVector.h

@@ -92,6 +92,13 @@ class SegmentedVector : private AllocPolicy
       new (elem) T(mozilla::Forward<U>(aU));
     }
 
+    void PopLast()
+    {
+      MOZ_ASSERT(mLength > 0);
+      (*this)[mLength - 1].~T();
+      mLength--;
+    }
+
     uint32_t mLength;
 
     // The union ensures that the elements are appropriately aligned.
@@ -185,6 +192,32 @@ public:
     }
   }
 
+  T& GetLast()
+  {
+    MOZ_ASSERT(!IsEmpty());
+    Segment* last = mSegments.getLast();
+    return (*last)[last->Length() - 1];
+  }
+
+  const T& GetLast() const
+  {
+    MOZ_ASSERT(!IsEmpty());
+    Segment* last = mSegments.getLast();
+    return (*last)[last->Length() - 1];
+  }
+
+  void PopLast()
+  {
+    MOZ_ASSERT(!IsEmpty());
+    Segment* last = mSegments.getLast();
+    last->PopLast();
+    if (!last->Length()) {
+      mSegments.popLast();
+      last->~Segment();
+      this->free_(last);
+    }
+  }
+
   // Use this class to iterate over a SegmentedVector, like so:
   //
   //  for (auto iter = v.Iter(); !iter.Done(); iter.Next()) {

mfbt/tests/TestSegmentedVector.cpp

@@ -80,6 +80,22 @@ void TestBasics()
   }
   MOZ_RELEASE_ASSERT(n == 1000);
 
+  // Pop off all of the elements.
+  MOZ_RELEASE_ASSERT(v.Length() == 1000);
+  for (int len = (int)v.Length(); len > 0; len--) {
+    MOZ_RELEASE_ASSERT(v.GetLast() == len - 1);
+    v.PopLast();
+  }
+  MOZ_RELEASE_ASSERT(v.IsEmpty());
+  MOZ_RELEASE_ASSERT(v.Length() == 0);
+
+  // Fill the vector up again to prepare for the clear.
+  for (i = 0; i < 1000; i++) {
+    v.InfallibleAppend(mozilla::Move(i));
+  }
+  MOZ_RELEASE_ASSERT(!v.IsEmpty());
+  MOZ_RELEASE_ASSERT(v.Length() == 1000);
+
   v.Clear();
   MOZ_RELEASE_ASSERT(v.IsEmpty());
   MOZ_RELEASE_ASSERT(v.Length() == 0);
@@ -115,7 +131,9 @@ void TestConstructorsAndDestructors()
     gDummy = v.Append(mozilla::Move(y));  // move constructor called
     NonPOD z(1);                          // explicit constructor called
     v.InfallibleAppend(mozilla::Move(z)); // move constructor called
-    v.Clear();                            // destructor called 3 times
+    v.PopLast();                          // destructor called 1 time
+    MOZ_RELEASE_ASSERT(gNumDtors == 1);
+    v.Clear();                            // destructor called 2 times
 
     MOZ_RELEASE_ASSERT(gNumDefaultCtors  == 0);
     MOZ_RELEASE_ASSERT(gNumExplicitCtors == 3);