kj::Arena for fast arena-style allocation.

c++/Makefile.am

@@ -107,6 +107,7 @@ includekj_HEADERS =                                            \
   src/kj/string.h                                              \
   src/kj/exception.h                                           \
   src/kj/debug.h                                               \
+  src/kj/arena.h                                               \
   src/kj/io.h                                                  \
   src/kj/tuple.h
 
@@ -142,6 +143,7 @@ libcapnp_la_SOURCES=                                           \
   src/kj/string.c++                                            \
   src/kj/exception.c++                                         \
   src/kj/debug.c++                                             \
+  src/kj/arena.c++                                             \
   src/kj/io.c++                                                \
   src/kj/parse/char.c++                                        \
   src/capnp/blob.c++                                           \
@@ -189,6 +191,7 @@ capnp_test_SOURCES =                                           \
   src/kj/string-test.c++                                       \
   src/kj/exception-test.c++                                    \
   src/kj/debug-test.c++                                        \
+  src/kj/arena-test.c++                                        \
   src/kj/units-test.c++                                        \
   src/kj/tuple-test.c++                                        \
   src/kj/parse/common-test.c++                                 \

c++/src/kj/arena-test.c++

+// Copyright (c) 2013, Kenton Varda <temporal@gmail.com>
+// All rights reserved.
+//
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are met:
+//
+// 1. Redistributions of source code must retain the above copyright notice, this
+//    list of conditions and the following disclaimer.
+// 2. Redistributions in binary form must reproduce the above copyright notice,
+//    this list of conditions and the following disclaimer in the documentation
+//    and/or other materials provided with the distribution.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND
+// ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
+// WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+// DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR
+// ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
+// (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
+// LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
+// ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
+// SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+#include "arena.h"
+#include "debug.h"
+#include <gtest/gtest.h>
+#include <stdint.h>
+
+namespace kj {
+namespace {
+
+struct TestObject {
+  TestObject() {
+    index = count;
+    KJ_ASSERT(index != throwAt);
+    ++count;
+  }
+  TestObject(const TestObject& other) {
+    KJ_ASSERT(other.index != throwAt);
+    index = -1;
+    copiedCount++;
+  }
+  ~TestObject() noexcept(false) {
+    if (index == -1) {
+      --copiedCount;
+    } else {
+      --count;
+      EXPECT_EQ(index, count);
+      KJ_ASSERT(count != throwAt);
+    }
+  }
+
+  int index;
+
+  static int count;
+  static int copiedCount;
+  static int throwAt;
+};
+
+int TestObject::count = 0;
+int TestObject::copiedCount = 0;
+int TestObject::throwAt = -1;
+
+TEST(Arena, Object) {
+  TestObject::count = 0;
+  TestObject::throwAt = -1;
+
+  {
+    Arena arena;
+
+    TestObject& obj1 = arena.allocate<TestObject>();
+    TestObject& obj2 = arena.allocate<TestObject>();
+
+    EXPECT_LT(&obj1, &obj2);
+
+    EXPECT_EQ(2, TestObject::count);
+  }
+
+  EXPECT_EQ(0, TestObject::count);
+}
+
+TEST(Arena, TrivialObject) {
+  Arena arena;
+
+  int& i1 = arena.allocate<int>();
+  int& i2 = arena.allocate<int>();
+
+  // Trivial objects should be tightly-packed.
+  EXPECT_EQ(&i1 + 1, &i2);
+}
+
+TEST(Arena, OwnObject) {
+  TestObject::count = 0;
+  TestObject::throwAt = -1;
+
+  Arena arena;
+
+  {
+    Own<TestObject> obj1 = arena.allocateOwn<TestObject>();
+    Own<TestObject> obj2 = arena.allocateOwn<TestObject>();
+    EXPECT_LT(obj1.get(), obj2.get());
+
+    EXPECT_EQ(2, TestObject::count);
+  }
+
+  EXPECT_EQ(0, TestObject::count);
+}
+
+TEST(Arena, Array) {
+  TestObject::count = 0;
+  TestObject::throwAt = -1;
+
+  {
+    Arena arena;
+    ArrayPtr<TestObject> arr1 = arena.allocateArray<TestObject>(4);
+    ArrayPtr<TestObject> arr2 = arena.allocateArray<TestObject>(2);
+    EXPECT_EQ(4, arr1.size());
+    EXPECT_EQ(2, arr2.size());
+    EXPECT_LE(arr1.end(), arr2.begin());
+    EXPECT_EQ(6, TestObject::count);
+  }
+
+  EXPECT_EQ(0, TestObject::count);
+}
+
+TEST(Arena, TrivialArray) {
+  Arena arena;
+  ArrayPtr<int> arr1 = arena.allocateArray<int>(16);
+  ArrayPtr<int> arr2 = arena.allocateArray<int>(8);
+
+  // Trivial arrays should be tightly-packed.
+  EXPECT_EQ(arr1.end(), arr2.begin());
+}
+
+TEST(Arena, OwnArray) {
+  TestObject::count = 0;
+  TestObject::throwAt = -1;
+
+  Arena arena;
+
+  {
+    Array<TestObject> arr1 = arena.allocateOwnArray<TestObject>(4);
+    Array<TestObject> arr2 = arena.allocateOwnArray<TestObject>(2);
+    EXPECT_EQ(4, arr1.size());
+    EXPECT_EQ(2, arr2.size());
+    EXPECT_LE(arr1.end(), arr2.begin());
+    EXPECT_EQ(6, TestObject::count);
+  }
+
+  EXPECT_EQ(0, TestObject::count);
+}
+
+#ifndef KJ_NO_EXCEPTIONS
+
+TEST(Arena, ObjectThrow) {
+  TestObject::count = 0;
+  TestObject::throwAt = 1;
+
+  {
+    Arena arena;
+
+    arena.allocate<TestObject>();
+    EXPECT_ANY_THROW(arena.allocate<TestObject>());
+    EXPECT_EQ(1, TestObject::count);
+  }
+
+  EXPECT_EQ(0, TestObject::count);
+}
+
+TEST(Arena, ArrayThrow) {
+  TestObject::count = 0;
+  TestObject::throwAt = 2;
+
+  {
+    Arena arena;
+    EXPECT_ANY_THROW(arena.allocateArray<TestObject>(4));
+    EXPECT_EQ(2, TestObject::count);
+  }
+
+  EXPECT_EQ(0, TestObject::count);
+}
+
+#endif
+
+TEST(Arena, Alignment) {
+  Arena arena;
+
+  char& c = arena.allocate<char>();
+  long& l = arena.allocate<long>();
+  char& c2 = arena.allocate<char>();
+  ArrayPtr<char> arr = arena.allocateArray<char>(8);
+
+  EXPECT_EQ(alignof(long) + sizeof(long), &c2 - &c);
+  EXPECT_EQ(alignof(long), reinterpret_cast<char*>(&l) - &c);
+  EXPECT_EQ(sizeof(char), arr.begin() - &c2);
+}
+
+TEST(Arena, EndOfChunk) {
+  union {
+    byte scratch[16];
+    uint64_t align;
+  };
+  Arena arena(arrayPtr(scratch, sizeof(scratch)));
+
+  uint64_t& i = arena.allocate<uint64_t>();
+  EXPECT_EQ(scratch, reinterpret_cast<byte*>(&i));
+
+  uint64_t& i2 = arena.allocate<uint64_t>();
+  EXPECT_EQ(scratch + 8, reinterpret_cast<byte*>(&i2));
+
+  uint64_t& i3 = arena.allocate<uint64_t>();
+  EXPECT_NE(scratch + 16, reinterpret_cast<byte*>(&i3));
+}
+
+TEST(Arena, EndOfChunkAlignment) {
+  union {
+    byte scratch[10];
+    uint64_t align;
+  };
+  Arena arena(arrayPtr(scratch, sizeof(scratch)));
+
+  uint16_t& i = arena.allocate<uint16_t>();
+  EXPECT_EQ(scratch, reinterpret_cast<byte*>(&i));
+
+  // Although there is technically enough space in the scratch array, it is not aligned correctly.
+  uint64_t& i2 = arena.allocate<uint64_t>();
+  EXPECT_TRUE(reinterpret_cast<byte*>(&i2) < scratch ||
+              reinterpret_cast<byte*>(&i2) > scratch + sizeof(scratch));
+}
+
+TEST(Arena, TooBig) {
+  Arena arena(1024);
+
+  byte& b1 = arena.allocate<byte>();
+
+  ArrayPtr<byte> arr = arena.allocateArray<byte>(1024);
+
+  byte& b2 = arena.allocate<byte>();
+
+  EXPECT_EQ(&b1 + 1, &b2);
+
+  // Write to the array to make sure it's valid.
+  memset(arr.begin(), 0xbe, arr.size());
+}
+
+TEST(Arena, MultiSegment) {
+  Arena arena(24);
+
+  uint64_t& i1 = arena.allocate<uint64_t>();
+  uint64_t& i2 = arena.allocate<uint64_t>();
+  uint64_t& i3 = arena.allocate<uint64_t>();
+
+  EXPECT_EQ(&i1 + 1, &i2);
+  EXPECT_NE(&i2 + 1, &i3);
+
+  i1 = 1234;
+  i2 = 5678;
+  i3 = 9012;
+}
+
+TEST(Arena, Constructor) {
+  Arena arena;
+
+  EXPECT_EQ(123, arena.allocate<uint64_t>(123));
+  EXPECT_EQ("foo", arena.allocate<StringPtr>("foo", 3));
+}
+
+TEST(Arena, Strings) {
+  Arena arena;
+
+  StringPtr foo = arena.copyString("foo");
+  StringPtr bar = arena.copyString("bar");
+  StringPtr quux = arena.copyString("quux");
+  StringPtr corge = arena.copyString("corge");
+
+  EXPECT_EQ("foo", foo);
+  EXPECT_EQ("bar", bar);
+  EXPECT_EQ("quux", quux);
+  EXPECT_EQ("corge", corge);
+
+  EXPECT_EQ(foo.end() + 1, bar.begin());
+  EXPECT_EQ(bar.end() + 1, quux.begin());
+  EXPECT_EQ(quux.end() + 1, corge.begin());
+}
+
+}  // namespace
+}  // namespace kj

c++/src/kj/arena.c++

+// Copyright (c) 2013, Kenton Varda <temporal@gmail.com>
+// All rights reserved.
+//
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are met:
+//
+// 1. Redistributions of source code must retain the above copyright notice, this
+//    list of conditions and the following disclaimer.
+// 2. Redistributions in binary form must reproduce the above copyright notice,
+//    this list of conditions and the following disclaimer in the documentation
+//    and/or other materials provided with the distribution.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND
+// ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
+// WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+// DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR
+// ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
+// (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
+// LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
+// ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
+// SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+#include "arena.h"
+#include "debug.h"
+#include <stdint.h>
+
+namespace kj {
+
+const Arena::ArrayDisposerImpl Arena::ArrayDisposerImpl::instance = Arena::ArrayDisposerImpl();
+
+Arena::Arena(size_t chunkSize): state(chunkSize) {}
+
+Arena::Arena(ArrayPtr<byte> scratch, size_t chunkSize): state(chunkSize) {
+  state.pos = scratch.begin();
+  state.chunkEnd = scratch.end();
+}
+
+Arena::~Arena() noexcept(false) {
+  // Run cleanup explicitly.  It will be executed again implicitly when state's destructor is
+  // called.  This ensures that if the first pass throws an exception, remaining objects are still
+  // destroyed.  If the second pass throws, the program terminates, but any destructors that could
+  // throw should be using UnwindDetector to avoid this.
+  state.cleanup();
+}
+
+void Arena::State::cleanup() {
+  while (objectList != nullptr) {
+    void* ptr = objectList + 1;
+    auto destructor = objectList->destructor;
+    objectList = objectList->next;
+    destructor(ptr);
+  }
+
+  while (chunkList != nullptr) {
+    void* ptr = chunkList;
+    chunkList = chunkList->next;
+    operator delete(ptr);
+  }
+}
+
+namespace {
+
+constexpr bool isPowerOfTwo(size_t value) {
+  return (value & value - 1) == 0;
+}
+
+inline byte* alignTo(byte* p, uint alignment) {
+  // Round the pointer up to the next aligned value.
+
+  KJ_DASSERT(isPowerOfTwo(alignment), alignment);
+  uintptr_t mask = alignment - 1;
+  uintptr_t i = reinterpret_cast<uintptr_t>(p);
+  return reinterpret_cast<byte*>((i + mask) & ~mask);
+}
+
+}  // namespace
+
+void* Arena::allocateBytes(size_t amount, uint alignment, bool hasDisposer) {
+  // Code below depends on power-of-two alignment and header sizes.
+  static_assert(isPowerOfTwo(sizeof(ChunkHeader)), "sizeof(ChunkHeader) is not a power of 2.");
+  static_assert(isPowerOfTwo(sizeof(ObjectHeader)), "sizeof(ObjectHeader) is not a power of 2.");
+  KJ_DASSERT(isPowerOfTwo(alignment), alignment);
+
+  // Offset we must apply if the allocated space is being prefixed with these headers.
+  uint chunkHeaderSize = kj::max(alignment, sizeof(ChunkHeader));
+  uint objectHeaderSize = kj::max(alignment, sizeof(ObjectHeader));
+
+  if (hasDisposer) {
+    amount += objectHeaderSize;
+  }
+
+  void* result;
+  byte* alignedPos = alignTo(state.pos, alignment);
+  byte* endPos = alignedPos + amount;
+  if (endPos <= state.chunkEnd) {
+    // There's enough space in the current chunk.
+    result = alignedPos;
+    state.pos = alignedPos + amount;
+  } else if (amount + chunkHeaderSize > state.chunkSize ||
+             state.chunkEnd - state.pos > state.chunkSize / 4) {
+    // This object is too big to fit in one chunk, or we'd waste more than a quarter of the chunk
+    // by starting a new one now.  Instead, allocate the object in its own independent chunk.
+    ChunkHeader* newChunk = reinterpret_cast<ChunkHeader*>(operator new(chunkHeaderSize + amount));
+    result = reinterpret_cast<byte*>(newChunk) + chunkHeaderSize;
+    newChunk->next = state.chunkList;
+    state.chunkList = newChunk;
+    // We don't update state.pos and state.chunkEnd because the new chunk has no extra space but
+    // the old chunk might.
+  } else {
+    // Allocate a new chunk.
+    ChunkHeader* newChunk = reinterpret_cast<ChunkHeader*>(operator new(state.chunkSize));
+    result = reinterpret_cast<byte*>(newChunk) + chunkHeaderSize;
+    newChunk->next = state.chunkList;
+    state.chunkList = newChunk;
+    state.pos = reinterpret_cast<byte*>(result) + amount;
+    state.chunkEnd = reinterpret_cast<byte*>(newChunk) + state.chunkSize;
+  }
+
+  if (hasDisposer) {
+    // Reserve space for the ObjectHeader, but don't add it to the object list yet.
+    result = reinterpret_cast<byte*>(result) + objectHeaderSize;
+  }
+  return result;
+}
+
+StringPtr Arena::copyString(StringPtr content) {
+  char* data = reinterpret_cast<char*>(allocateBytes(content.size() + 1, 1, false));
+  memcpy(data, content.cStr(), content.size() + 1);
+  return StringPtr(data, content.size());
+}
+
+void Arena::setDestructor(void* ptr, void (*destructor)(void*)) {
+  ObjectHeader* header = reinterpret_cast<ObjectHeader*>(ptr) - 1;
+  header->destructor = destructor;
+  header->next = state.objectList;
+  state.objectList = header;
+}
+
+void Arena::ArrayDisposerImpl::disposeImpl(
+    void* firstElement, size_t elementSize, size_t elementCount,
+    size_t capacity, void (*destroyElement)(void*)) const {
+  if (destroyElement != nullptr) {
+    ExceptionSafeArrayUtil guard(firstElement, elementSize, elementCount, destroyElement);
+    guard.destroyAll();
+  }
+}
+
+}  // namespace kj

c++/src/kj/arena.h

+// Copyright (c) 2013, Kenton Varda <temporal@gmail.com>
+// All rights reserved.
+//
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are met:
+//
+// 1. Redistributions of source code must retain the above copyright notice, this
+//    list of conditions and the following disclaimer.
+// 2. Redistributions in binary form must reproduce the above copyright notice,
+//    this list of conditions and the following disclaimer in the documentation
+//    and/or other materials provided with the distribution.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND
+// ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
+// WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+// DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR
+// ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
+// (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
+// LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
+// ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
+// SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+#ifndef KJ_ARENA_H_
+#define KJ_ARENA_H_
+
+#include "memory.h"
+#include "array.h"
+#include "string.h"
+
+namespace kj {
+
+class Arena {
+  // A class which allows several objects to be allocated in contiguous chunks of memory, then
+  // frees them all at once.
+
+public:
+  Arena(size_t chunkSize = 1024);
+  // Create an Arena that tries to allocate in chunks of the given size.  It may deviate from the
+  // size if an object is too large to fit or to avoid excessive fragmentation.  Each chunk has
+  // a one-word header which is included in the chunk size.
+
+  Arena(ArrayPtr<byte> scratch, size_t chunkSize = 1024);
+  // Allocates from the given scratch space first, only resorting to the heap when it runs out.
+
+  KJ_DISALLOW_COPY(Arena);
+  ~Arena() noexcept(false);
+
+  template <typename T, typename... Params>
+  T& allocate(Params&&... params);
+  template <typename T>
+  ArrayPtr<T> allocateArray(size_t size);
+  // Allocate an object or array of type T.  If T has a non-trivial destructor, that destructor
+  // will be run during the Arena's destructor.  Such destructors are run in opposite order of
+  // allocation.  Note that these methods must maintain a list of destructors to call, which has
+  // overhead, but this overhead only applies if T has a non-trivial destructor.
+
+  template <typename T, typename... Params>
+  Own<T> allocateOwn(Params&&... params);
+  template <typename T>
+  Array<T> allocateOwnArray(size_t size);
+  template <typename T>
+  ArrayBuilder<T> allocateOwnArrayBuilder(size_t capacity);
+  // Allocate an object or array of type T.  Destructors are executed when the returned Own<T>
+  // or Array<T> goes out-of-scope, which must happen before the Arena is destroyed.  This variant
+  // is useful when you need to control when the destructor is called.  This variant also avoids
+  // the need for the Arena itself to keep track of destructors to call later, which may make it
+  // slightly more efficient.
+
+  StringPtr copyString(StringPtr content);
+  // Make a copy of the given string inside the arena, and return a pointer to the copy.
+
+private:
+  struct ChunkHeader {
+    ChunkHeader* next;
+  };
+  struct ObjectHeader {
+    void (*destructor)(void*);
+    ObjectHeader* next;
+  };
+
+  struct State {
+    size_t chunkSize;
+    ChunkHeader* chunkList;
+    ObjectHeader* objectList;
+    byte* pos;
+    byte* chunkEnd;
+
+    inline State(size_t chunkSize)
+        : chunkSize(chunkSize), chunkList(nullptr), objectList(nullptr),
+          pos(nullptr), chunkEnd(nullptr) {}
+    inline ~State() noexcept(false) { cleanup(); }
+
+    void cleanup();
+    // Run all destructors, leaving the above pointers null.  If a destructor throws, the State is
+    // left in a consistent state, such that if cleanup() is called again, it will pick up where
+    // it left off.
+  };
+  State state;
+
+  void* allocateBytes(size_t amount, uint alignment, bool hasDisposer);
+  // Allocate the given number of bytes.  `hasDisposer` must be true if `setDisposer()` may be
+  // called on this pointer later.
+
+  void setDestructor(void* ptr, void (*destructor)(void*));
+  // Schedule the given destructor to be executed when the Arena is destroyed.  `ptr` must be a
+  // pointer previously returned by an `allocateBytes()` call for which `hasDisposer` was true.
+
+  template <typename T>
+  class DisposerImpl: public Disposer {
+  public:
+    static const DisposerImpl instance;
+
+    void disposeImpl(void* pointer) const override {
+      reinterpret_cast<T*>(pointer)->~T();
+    }
+  };
+
+  class ArrayDisposerImpl: public ArrayDisposer {
+  public:
+    static const ArrayDisposerImpl instance;
+
+    void disposeImpl(void* firstElement, size_t elementSize, size_t elementCount,
+                     size_t capacity, void (*destroyElement)(void*)) const override;
+  };
+
+  template <typename T>
+  static void destroyArray(void* pointer) {
+    size_t elementCount = *reinterpret_cast<size_t*>(pointer);
+    constexpr size_t prefixSize = kj::max(alignof(T), sizeof(size_t));
+    ArrayDisposerImpl::instance.disposeImpl(
+        reinterpret_cast<byte*>(pointer) + prefixSize,
+        sizeof(T), elementCount, elementCount, &destroyObject<T>);
+  }
+
+  template <typename T>
+  static void destroyObject(void* pointer) {
+    dtor(*reinterpret_cast<T*>(pointer));
+  }
+};
+
+// =======================================================================================
+// Inline implementation details
+
+template <typename T, typename... Params>
+T& Arena::allocate(Params&&... params) {
+  T& result = *reinterpret_cast<T*>(allocateBytes(
+      sizeof(T), alignof(T), !__has_trivial_destructor(T)));
+  if (!__has_trivial_constructor(T) || sizeof...(Params) > 0) {
+    ctor(result, kj::fwd<Params>(params)...);
+  }
+  if (!__has_trivial_destructor(T)) {
+    setDestructor(&result, &destroyObject<T>);
+  }
+  return result;
+}
+
+template <typename T>
+ArrayPtr<T> Arena::allocateArray(size_t size) {
+  if (__has_trivial_destructor(T)) {
+    ArrayPtr<T> result =
+        arrayPtr(reinterpret_cast<T*>(allocateBytes(
+            sizeof(T) * size, alignof(T), false)), size);
+    if (!__has_trivial_constructor(T)) {
+      for (size_t i = 0; i < size; i++) {
+        ctor(result[i]);
+      }
+    }
+    return result;
+  } else {
+    // Allocate with a 64-bit prefix in which we store the array size.
+    constexpr size_t prefixSize = kj::max(alignof(T), sizeof(size_t));
+    void* base = allocateBytes(sizeof(T) * size + prefixSize, alignof(T), true);
+    size_t& tag = *reinterpret_cast<size_t*>(base);
+    ArrayPtr<T> result =
+        arrayPtr(reinterpret_cast<T*>(reinterpret_cast<byte*>(base) + prefixSize), size);
+    setDestructor(base, &destroyArray<T>);
+
+    if (__has_trivial_constructor(T)) {
+      tag = size;
+    } else {
+      // In case of constructor exceptions, we need the tag to end up storing the number of objects
+      // that were successfully constructed, so that they'll be properly destroyed.
+      tag = 0;
+      for (size_t i = 0; i < size; i++) {
+        ctor(result[i]);
+        tag = i + 1;
+      }
+    }
+    return result;
+  }
+}
+
+template <typename T, typename... Params>
+Own<T> Arena::allocateOwn(Params&&... params) {
+  T& result = *reinterpret_cast<T*>(allocateBytes(sizeof(T), alignof(T), false));
+  if (!__has_trivial_constructor(T) || sizeof...(Params) > 0) {
+    ctor(result, kj::fwd<Params>(params)...);
+  }
+  return Own<T>(&result, DisposerImpl<T>::instance);
+}
+
+template <typename T>
+Array<T> Arena::allocateOwnArray(size_t size) {
+  ArrayBuilder<T> result = allocateOwnArrayBuilder<T>(size);
+  for (size_t i = 0; i < size; i++) {
+    result.add();
+  }
+  return result.finish();
+}
+
+template <typename T>
+ArrayBuilder<T> Arena::allocateOwnArrayBuilder(size_t capacity) {
+  return ArrayBuilder<T>(
+      reinterpret_cast<T*>(allocateBytes(sizeof(T) * capacity, alignof(T), false)),
+      capacity, ArrayDisposerImpl::instance);
+}
+
+template <typename T>
+const Arena::DisposerImpl<T> Arena::DisposerImpl<T>::instance = Arena::DisposerImpl<T>();
+
+}  // namespace kj
+
+#endif  // KJ_ARENA_H_

c++/src/kj/array.c++

@@ -24,36 +24,25 @@
 #include "array.h"
 
 namespace kj {
-namespace _ {  // private
-
-struct HeapArrayDisposer::ExceptionGuard {
-  byte* pos;
-  size_t elementSize;
-  size_t elementCount;
-  size_t constructedCount;
-  void (*destroyElement)(void*);
 
-  ExceptionGuard(void* ptr, size_t elementSize, size_t elementCount,
-                 void (*destroyElement)(void*))
-      : pos(reinterpret_cast<byte*>(ptr) + elementSize * elementCount),
-        elementSize(elementSize), elementCount(elementCount),
-        destroyElement(destroyElement) {}
-
-  ~ExceptionGuard() noexcept(false) {
-    if (pos != nullptr) {
-      destroyAll();
-      operator delete(pos);
-    }
+void ExceptionSafeArrayUtil::construct(size_t count, void (*constructElement)(void*)) {
+  while (count > 0) {
+    constructElement(pos);
+    pos += elementSize;
+    ++constructedElementCount;
+    --count;
   }
+}
 
-  void destroyAll() {
-    while (elementCount > 0) {
+void ExceptionSafeArrayUtil::destroyAll() {
+  while (constructedElementCount > 0) {
     pos -= elementSize;
-      --elementCount;
+    --constructedElementCount;
     destroyElement(pos);
   }
-  }
-};
+}
+
+namespace _ {  // private
 
 void* HeapArrayDisposer::allocateImpl(size_t elementSize, size_t elementCount, size_t capacity,
                                       void (*constructElement)(void*),
@@ -70,13 +59,9 @@ void* HeapArrayDisposer::allocateImpl(size_t elementSize, size_t elementCount, s
       --elementCount;
     }
   } else {
-    ExceptionGuard guard(result, elementSize, 0, destroyElement);
-    while (guard.elementCount < elementCount) {
-      constructElement(guard.pos);
-      guard.pos += elementSize;
-      ++guard.elementCount;
-    }
-    guard.pos = nullptr;
+    ExceptionSafeArrayUtil guard(result, elementSize, 0, destroyElement);
+    guard.construct(elementCount, constructElement);
+    guard.release();
   }
 
   return result;
@@ -90,11 +75,9 @@ void HeapArrayDisposer::disposeImpl(
   if (destroyElement == nullptr) {
     operator delete(firstElement);
   } else {
-    ExceptionGuard guard(firstElement, elementSize, elementCount, destroyElement);
+    KJ_DEFER(operator delete(firstElement));
+    ExceptionSafeArrayUtil guard(firstElement, elementSize, elementCount, destroyElement);
     guard.destroyAll();
-
-    // If an exception is thrown, we'll continue the destruction process in ExceptionGuard's
-    // destructor.  If _that_ throws an exception, the program terminates according to C++ rules.
   }
 }
 

c++/src/kj/array.h

@@ -61,6 +61,45 @@ private:
   struct Dispose_;
 };
 
+class ExceptionSafeArrayUtil {
+  // Utility class that assists in constructing or destroying elements of an array, where the
+  // constructor or destructor could throw exceptions.  In case of an exception,
+  // ExceptionSafeArrayUtil's destructor will call destructors on all elements that have been
+  // constructed but not destroyed.  Remember that destructors that throw exceptions are required
+  // to use UnwindDetector to detect unwind and avoid exceptions in this case.  Therefore, no more
+  // than one exception will be thrown (and the program will not terminate).
+
+public:
+  inline ExceptionSafeArrayUtil(void* ptr, size_t elementSize, size_t constructedElementCount,
+                                void (*destroyElement)(void*))
+      : pos(reinterpret_cast<byte*>(ptr) + elementSize * constructedElementCount),
+        elementSize(elementSize), constructedElementCount(constructedElementCount),
+        destroyElement(destroyElement) {}
+  KJ_DISALLOW_COPY(ExceptionSafeArrayUtil);
+
+  inline ~ExceptionSafeArrayUtil() noexcept(false) {
+    if (constructedElementCount > 0) destroyAll();
+  }
+
+  void construct(size_t count, void (*constructElement)(void*));
+  // Construct the given number of elements.
+
+  void destroyAll();
+  // Destroy all elements.  Call this immediately before ExceptionSafeArrayUtil goes out-of-scope
+  // to ensure that one element throwing an exception does not prevent the others from being
+  // destroyed.
+
+  void release() { constructedElementCount = 0; }
+  // Prevent ExceptionSafeArrayUtil's destructor from destroying the constructed elements.
+  // Call this after you've successfully finished constructing.
+
+private:
+  byte* pos;
+  size_t elementSize;
+  size_t constructedElementCount;
+  void (*destroyElement)(void*);
+};
+
 // =======================================================================================
 // Array
 
@@ -185,8 +224,6 @@ private:
   template <typename T, bool hasTrivialConstructor = __has_trivial_constructor(T),
                         bool hasNothrowConstructor = __has_nothrow_constructor(T)>
   struct Allocate_;
-
-  struct ExceptionGuard;
 };
 
 }  // namespace _ (private)
@@ -295,14 +332,14 @@ public:
   void addAll(Iterator start, Iterator end);
 
   Array<T> finish() {
-    // We could safely remove this check as long as HeapArrayDisposer relies on operator delete
-    // (which doesn't need to know the original capacity) or if we created a custom disposer for
-    // ArrayBuilder which stores the capacity in a prefix.  But that would mean we can't allow
-    // arbitrary disposers with ArrayBuilder in the future, and anyway this check might catch bugs.
-    // Probably we should just create a new Vector-like data structure if we want to allow building
-    // of arrays without knowing the final size in advance.
+    // We could safely remove this check if we assume that the disposer implementation doesn't
+    // need to know the original capacity, as is thes case with HeapArrayDisposer since it uses
+    // operator new() or if we created a custom disposer for ArrayBuilder which stores the capacity
+    // in a prefix.  But that would make it hard to write cleverer heap allocators, and anyway this
+    // check might catch bugs.  Probably people should use Vector if they want to build arrays
+    // without knowing the final size in advance.
     KJ_IREQUIRE(pos == endPtr, "ArrayBuilder::finish() called prematurely.");
-    Array<T> result(reinterpret_cast<T*>(ptr), pos - ptr, _::HeapArrayDisposer::instance);
+    Array<T> result(reinterpret_cast<T*>(ptr), pos - ptr, *disposer);
     ptr = nullptr;
     pos = nullptr;
     endPtr = nullptr;

c++/src/kj/common-test.c++

@@ -246,5 +246,23 @@ TEST(Common, MinMax) {
   EXPECT_EQ(1234567890123456789ll, kj::max('a', 1234567890123456789ll));
 }
 
+TEST(Common, Defer) {
+  uint i = 0;
+  uint j = 1;
+  bool k = false;
+
+  {
+    KJ_DEFER(++i);
+    KJ_DEFER(j += 3; k = true);
+    EXPECT_EQ(0, i);
+    EXPECT_EQ(1, j);
+    EXPECT_FALSE(k);
+  }
+
+  EXPECT_EQ(1, i);
+  EXPECT_EQ(4, j);
+  EXPECT_TRUE(k);
+}
+
 }  // namespace
 }  // namespace kj

c++/src/kj/common.h

@@ -176,6 +176,12 @@ void inlineRequireFailure(
 
 #define KJ_ARRAY_SIZE(arr) (sizeof(arr) / sizeof(arr[0]))
 
+#define KJ_CONCAT_(x, y) x##y
+#define KJ_CONCAT(x, y) KJ_CONCAT_(x, y)
+#define KJ_UNIQUE_NAME(prefix) KJ_CONCAT(prefix, __LINE__)
+// Create a unique identifier name.  We use concatenate __LINE__ rather than __COUNTER__ so that
+// the name can be used multiple times in the same macro.
+
 // =======================================================================================
 // Template metaprogramming helpers.
 
@@ -283,9 +289,9 @@ template<typename T> constexpr T&& mv(T& t) noexcept { return static_cast<T&&>(t
 template<typename T> constexpr T&& fwd(NoInfer<T>& t) noexcept { return static_cast<T&&>(t); }
 
 template <typename T, typename U>
-auto min(T&& a, U&& b) -> decltype(a < b ? a : b) { return a < b ? a : b; }
+inline constexpr auto min(T&& a, U&& b) -> decltype(a < b ? a : b) { return a < b ? a : b; }
 template <typename T, typename U>
-auto max(T&& a, U&& b) -> decltype(a > b ? a : b) { return a > b ? a : b; }
+inline constexpr auto max(T&& a, U&& b) -> decltype(a > b ? a : b) { return a > b ? a : b; }
 
 // =======================================================================================
 // Manually invoking constructors and destructors
@@ -710,6 +716,29 @@ To& downcast(From& from) {
   return static_cast<To&>(from);
 }
 
+// =======================================================================================
+// Defer
+
+namespace _ {  // private
+
+template <typename Func>
+class Deferred {
+public:
+  inline Deferred(Func func): func(func) {}
+  inline ~Deferred() { func(); }
+private:
+  Func func;
+};
+
+template <typename Func>
+Deferred<Decay<Func>> defer(Func&& func) {
+  return Deferred<Decay<Func>>(kj::fwd<Func>(func));
+}
+
+}  // namespace _ (private)
+
+#define KJ_DEFER(code) auto KJ_UNIQUE_NAME(_kjDefer) = ::kj::_::defer([&](){code;})
+
 }  // namespace kj
 
 #endif  // KJ_COMMON_H_

c++/src/kj/debug.h

@@ -140,11 +140,12 @@ namespace kj {
            errorNumber, code, #__VA_ARGS__, ##__VA_ARGS__);; f.fatal())
 
 #define KJ_CONTEXT(...) \
-  auto _kjContextFunc = [&]() -> ::kj::_::Debug::Context::Value { \
+  auto KJ_UNIQUE_NAME(_kjContextFunc) = [&]() -> ::kj::_::Debug::Context::Value { \
         return ::kj::_::Debug::Context::Value(__FILE__, __LINE__, \
             ::kj::_::Debug::makeContextDescription(#__VA_ARGS__, ##__VA_ARGS__)); \
       }; \
-  ::kj::_::Debug::ContextImpl<decltype(_kjContextFunc)> _kjContext(_kjContextFunc)
+  ::kj::_::Debug::ContextImpl<decltype(KJ_UNIQUE_NAME(_kjContextFunc))> \
+      KJ_UNIQUE_NAME(_kjContext)(KJ_UNIQUE_NAME(_kjContextFunc))
 
 #ifdef NDEBUG
 #define KJ_DLOG(...) do {} while (false)