Special case Recycler::clear(BumpPtrAllocator).

A BumpPtrAllocator has an empty Deallocate() method, but
Recycler::clear() would still call it for every single object ever
allocated, bringing all those objects into cache. As a bonus,
iplist::remove() will also write to the Prev/Next pointers on all the
objects, so all those cache lines have to be written back to RAM before
the pages are given back to the OS.

Stop wasting time and memory bandwith by using the new
clearAndLeakUnsafely() function to jettison all the recycled objects.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@171541 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Support/Recycler.h

@@ -22,6 +22,8 @@
 
 namespace llvm {
 
+class BumpPtrAllocator;
+
 /// PrintRecyclingAllocatorStats - Helper for RecyclingAllocator for
 /// printing statistics.
 ///
@@ -87,6 +89,15 @@ public:
     }
   }
 
+  /// Special case for BumpPtrAllocator which has an empty Deallocate()
+  /// function.
+  ///
+  /// There is no need to traverse the free list, pulling all the objects into
+  /// cache.
+  void clear(BumpPtrAllocator&) {
+    FreeList.clearAndLeakNodesUnsafely();
+  }
+
   template<class SubClass, class AllocatorType>
   SubClass *Allocate(AllocatorType &Allocator) {
     assert(sizeof(SubClass) <= Size &&