[ADT] Add a much simpler loop to DenseMap::clear when the types are

POD-like and we can just splat the empty key across memory.

Sadly we can't optimize the normal loop well enough because we can't
turn the conditional store into an unconditional store according to the
memory model.

This loop actually showed up in a profile of code that was calling clear
as a serious source of time. =[

llvm-svn: 310189

llvm/include/llvm/ADT/DenseMap.h

@@ -107,6 +107,11 @@ public:
     }
 
     const KeyT EmptyKey = getEmptyKey(), TombstoneKey = getTombstoneKey();
+    if (isPodLike<KeyT>::value && isPodLike<ValueT>::value) {
+      // Use a simpler loop when these are trivial types.
+      for (BucketT *P = getBuckets(), *E = getBucketsEnd(); P != E; ++P)
+        P->getFirst() = EmptyKey;
+    } else {
       unsigned NumEntries = getNumEntries();
       for (BucketT *P = getBuckets(), *E = getBucketsEnd(); P != E; ++P) {
         if (!KeyInfoT::isEqual(P->getFirst(), EmptyKey)) {
@@ -118,6 +123,7 @@ public:
         }
       }
       assert(NumEntries == 0 && "Node count imbalance!");
+    }
     setNumEntries(0);
     setNumTombstones(0);
   }