[llvm] r310189 - [ADT] Add a much simpler loop to DenseMap::clear when the types are

[Chandler Carruth via llvm-commits]

Author: chandlerc
Date: Sat Aug  5 15:48:37 2017
New Revision: 310189

URL: http://llvm.org/viewvc/llvm-project?rev=310189&view=rev
Log:
[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. =[

Modified:
    llvm/trunk/include/llvm/ADT/DenseMap.h

Modified: llvm/trunk/include/llvm/ADT/DenseMap.h
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/ADT/DenseMap.h?rev=310189&r1=310188&r2=310189&view=diff
==============================================================================
--- llvm/trunk/include/llvm/ADT/DenseMap.h (original)
+++ llvm/trunk/include/llvm/ADT/DenseMap.h Sat Aug  5 15:48:37 2017
@@ -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);
   }