From 7c5c12bd4d60070c90161df9f6ae078c1f7b1ce5 Mon Sep 17 00:00:00 2001
From: Jakob Stoklund Olesen <stoklund@2pi.dk>
Date: Fri, 4 Jan 2013 22:35:42 +0000
Subject: Add an iplist::clearAndLeakNodesUnsafely() function.

The iplist::clear() function can be quite expensive because it traverses
the entire list, calling deleteNode() and removeNodeFromList() on each
element. If node destruction and deallocation can be handled some other
way, clearAndLeakNodesUnsafely() can be used to jettison all nodes
without bringing them into cache.

The function name is meant to be ominous.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@171540 91177308-0d34-0410-b5e6-96231b3b80d8
---
 include/llvm/ADT/ilist.h    | 11 +++++++++++
 unittests/ADT/ilistTest.cpp | 33 +++++++++++++++++++++++++++++++++
 2 files changed, 44 insertions(+)

diff --git a/include/llvm/ADT/ilist.h b/include/llvm/ADT/ilist.h
index 36650d4375..20cdae3255 100644
--- a/include/llvm/ADT/ilist.h
+++ b/include/llvm/ADT/ilist.h
@@ -465,6 +465,17 @@ public:
     return where;
   }
 
+  /// Remove all nodes from the list like clear(), but do not call
+  /// removeNodeFromList() or deleteNode().
+  ///
+  /// This should only be used immediately before freeing nodes in bulk to
+  /// avoid traversing the list and bringing all the nodes into cache.
+  void clearAndLeakNodesUnsafely() {
+    if (Head) {
+      Head = getTail();
+      this->setPrev(Head, Head);
+    }
+  }
 
 private:
   // transfer - The heart of the splice function.  Move linked list nodes from
diff --git a/unittests/ADT/ilistTest.cpp b/unittests/ADT/ilistTest.cpp
index 07bd5ec601..0c0cd0fd56 100644
--- a/unittests/ADT/ilistTest.cpp
+++ b/unittests/ADT/ilistTest.cpp
@@ -22,6 +22,7 @@ struct Node : ilist_node<Node> {
 
   Node() {}
   Node(int _Value) : Value(_Value) {}
+  ~Node() { Value = -1; }
 };
 
 TEST(ilistTest, Basic) {
@@ -62,4 +63,36 @@ TEST(ilistTest, SpliceOne) {
   EXPECT_EQ(3, List.back().Value);
 }
 
+TEST(ilistTest, UnsafeClear) {
+  ilist<Node> List;
+
+  // Before even allocating a sentinel.
+  List.clearAndLeakNodesUnsafely();
+  EXPECT_EQ(0u, List.size());
+
+  // Empty list with sentinel.
+  ilist<Node>::iterator E = List.end();
+  List.clearAndLeakNodesUnsafely();
+  EXPECT_EQ(0u, List.size());
+  // The sentinel shouldn't change.
+  EXPECT_TRUE(E == List.end());
+
+  // List with contents.
+  List.push_back(1);
+  ASSERT_EQ(1u, List.size());
+  Node *N = List.begin();
+  EXPECT_EQ(1, N->Value);
+  List.clearAndLeakNodesUnsafely();
+  EXPECT_EQ(0u, List.size());
+  ASSERT_EQ(1, N->Value);
+  delete N;
+
+  // List is still functional.
+  List.push_back(5);
+  List.push_back(6);
+  ASSERT_EQ(2u, List.size());
+  EXPECT_EQ(5, List.front().Value);
+  EXPECT_EQ(6, List.back().Value);
+}
+
 }
-- 
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