[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. =[

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@310189 91177308-0d34-0410-b5e6-96231b3b80d8

Author: chandlerc

include/llvm/ADT/DenseMap.h

@@ -107,17 +107,23 @@ class DenseMapBase : public DebugEpochBase {
     }
 
     const KeyT EmptyKey = getEmptyKey(), TombstoneKey = getTombstoneKey();
-    unsigned NumEntries = getNumEntries();
-    for (BucketT *P = getBuckets(), *E = getBucketsEnd(); P != E; ++P) {
-      if (!KeyInfoT::isEqual(P->getFirst(), EmptyKey)) {
-        if (!KeyInfoT::isEqual(P->getFirst(), TombstoneKey)) {
-          P->getSecond().~ValueT();
-          --NumEntries;
-        }
+    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)) {
+          if (!KeyInfoT::isEqual(P->getFirst(), TombstoneKey)) {
+            P->getSecond().~ValueT();
+            --NumEntries;
+          }
+          P->getFirst() = EmptyKey;
+        }
       }
+      assert(NumEntries == 0 && "Node count imbalance!");
     }
-    assert(NumEntries == 0 && "Node count imbalance!");
     setNumEntries(0);
     setNumTombstones(0);
   }