[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

diff --git a/include/llvm/ADT/DenseMap.h b/include/llvm/ADT/DenseMap.h
index b311e69ec9d378c21cc538cf4c4d08d2ec746488..84ebae08634ad23fc8bafcb5ed48e00a142e4aaf 100644 (file)
--- a/include/llvm/ADT/DenseMap.h
+++ b/include/llvm/ADT/DenseMap.h
@@ -107,17 +107,23 @@ public:
     }
 
     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);
   }