From: Chandler Carruth <chandlerc@gmail.com>
Date: Sat, 5 Aug 2017 22:48:37 +0000 (+0000)
Subject: [ADT] Add a much simpler loop to DenseMap::clear when the types are
X-Git-Url: https://granicus.if.org/sourcecode?a=commitdiff_plain;h=549896a7368869a9f93b59071a5bcf11b2383a88;p=llvm

[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 b311e69ec9d..84ebae08634 100644
--- 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);
   }