Index: lib/esan/esan_circular_buffer.h
===================================================================
--- /dev/null
+++ lib/esan/esan_circular_buffer.h
@@ -0,0 +1,83 @@
+//===-- esan_circular_buffer.h ----------------------------------*- C++ -*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This file is a part of EfficiencySanitizer, a family of performance tuners.
+//
+// Circular buffer data structure.
+//===----------------------------------------------------------------------===//
+
+#include "sanitizer_common/sanitizer_common.h"
+
+namespace __esan {
+
+// A circular buffer for POD data whose memory is allocated using mmap.
+template<typename T>
+class CircularBuffer {
+ public:
+  // To support global instances we cannot initialize any field in the
+  // constructor.
+  explicit CircularBuffer() {}
+  void initialize(uptr BufferCapacity) {
+    Capacity = BufferCapacity;
+    // MmapOrDie rounds up to the page size for us.
+    Data = (T *)MmapOrDie(Capacity * sizeof(T), "CircularBuffer");
+    StartIdx = 0;
+    Count = 0;
+  }
+  void free() {
+    UnmapOrDie(Data, Capacity * sizeof(T));
+  }
+  T &operator[](uptr Idx) {
+    CHECK_LT(Idx, Count);
+    uptr ArrayIdx = (StartIdx + Idx) % Capacity;
+    return Data[ArrayIdx];
+  }
+  const T &operator[](uptr Idx) const {
+    CHECK_LT(Idx, Count);
+    uptr ArrayIdx = (StartIdx + Idx) % Capacity;
+    return Data[ArrayIdx];
+  }
+  void push_back(const T &Item) {
+    CHECK_GT(Capacity, 0);
+    uptr ArrayIdx = (StartIdx + Count) % Capacity;
+    Data[ArrayIdx] = Item;
+    if (Count < Capacity)
+      ++Count;
+    else
+      StartIdx = (StartIdx + 1) % Capacity;
+  }
+  T &back() {
+    CHECK_GT(Count, 0);
+    uptr ArrayIdx = (StartIdx + Count - 1) % Capacity;
+    return Data[ArrayIdx];
+  }
+  void pop_back() {
+    CHECK_GT(Count, 0);
+    --Count;
+  }
+  uptr size() const {
+    return Count;
+  }
+  void clear() {
+    StartIdx = 0;
+    Count = 0;
+  }
+  bool empty() const { return size() == 0; }
+
+ private:
+  CircularBuffer(const CircularBuffer&);
+  void operator=(const CircularBuffer&);
+
+  T *Data;
+  uptr Capacity;
+  uptr StartIdx;
+  uptr Count;
+};
+
+} // namespace __esan
Index: test/esan/Unit/circular_buffer.cpp
===================================================================
--- /dev/null
+++ test/esan/Unit/circular_buffer.cpp
@@ -0,0 +1,44 @@
+// RUN: %clangxx_unit -O0 %s -o %t 2>&1
+// RUN: %run %t 2>&1 | FileCheck %s
+
+#include "esan/esan_circular_buffer.h"
+#include <assert.h>
+#include <stdio.h>
+
+int main()
+{
+  __esan::CircularBuffer<int> Buf;
+  Buf.initialize(4);
+
+  assert(Buf.size() == 0);
+  assert(Buf.empty());
+
+  Buf.push_back(1);
+  assert(Buf.back() == 1);
+  assert(Buf[0] == 1);
+  assert(Buf.size() == 1);
+  assert(!Buf.empty());
+
+  Buf.push_back(2);
+  Buf.push_back(3);
+  Buf.push_back(4);
+  Buf.push_back(5);
+  assert(Buf[0] == 2);
+  assert(Buf.size() == 4);
+
+  Buf.pop_back();
+  assert(Buf[0] == 2);
+  assert(Buf.size() == 3);
+
+  Buf.pop_back();
+  Buf.pop_back();
+  assert(Buf[0] == 2);
+  assert(Buf.size() == 1);
+  assert(!Buf.empty());
+
+  Buf.pop_back();
+  assert(Buf.empty());
+  printf("All checks passed.\n");
+  // CHECK: All checks passed.
+  return 0;
+}
Index: test/esan/lit.cfg
===================================================================
--- test/esan/lit.cfg
+++ test/esan/lit.cfg
@@ -14,6 +14,10 @@
 
 frag_cflags = (["-fsanitize=efficiency-cache-frag"] + base_cflags)
 wset_cflags = (["-fsanitize=efficiency-working-set"] + base_cflags)
+esan_incdir = config.test_source_root + "/../../lib"
+unit_cxxflags = (["-I%s" % esan_incdir, "-std=c++11",
+                  # We need to link with the esan runtime.
+                  "-fsanitize=efficiency-working-set"] + base_cxxflags)
 
 def build_invocation(compile_flags):
   return " " + " ".join([config.clang] + compile_flags) + " "
@@ -22,6 +26,8 @@
                               build_invocation(frag_cflags)) )
 config.substitutions.append( ("%clang_esan_wset ",
                               build_invocation(wset_cflags)) )
+config.substitutions.append( ("%clangxx_unit",
+                              build_invocation(unit_cxxflags)) )
 
 default_esan_opts = ''
 config.substitutions.append(('%env_esan_opts=',