From 35054c8b8eefa0abaded33ff4150d195c9bf0e80 Mon Sep 17 00:00:00 2001
From: Yedidya Feldblum <yfeldblum@fb.com>
Date: Fri, 8 Dec 2017 00:24:44 -0800
Subject: [PATCH] Move folly/Bits.h to folly/lang/

Summary: [Folly] Move `folly/Bits.h` to `folly/lang/`.

Reviewed By: phoad, Orvid

Differential Revision: D6495547

fbshipit-source-id: a93159321df8277f8a4b4f10a5e4e0fc58cb6022
---
 CMakeLists.txt                              |   4 +-
 folly/AtomicHashArray-inl.h                 |   2 +-
 folly/AtomicUnorderedMap.h                  |   2 +-
 folly/BitIterator.h                         |   2 +-
 folly/Bits.h                                | 369 +------------------
 folly/GroupVarint.h                         |   2 +-
 folly/MacAddress.h                          |   2 +-
 folly/Makefile.am                           |   1 +
 folly/compression/Compression.cpp           |   2 +-
 folly/compression/Utils.h                   |   2 +-
 folly/experimental/BitVectorCoding.h        |   2 +-
 folly/experimental/Bits.h                   |   2 +-
 folly/experimental/EliasFanoCoding.h        |   2 +-
 folly/experimental/EventCount.h             |   3 +-
 folly/hash/Hash.h                           |   2 +-
 folly/io/Cursor.h                           |   2 +-
 folly/io/async/AsyncSSLSocket.cpp           |   2 +-
 folly/io/async/AsyncSSLSocket.h             |   2 +-
 folly/io/async/HHWheelTimer.cpp             |   2 +-
 folly/json.cpp                              |   2 +-
 folly/lang/Bits.h                           | 384 ++++++++++++++++++++
 folly/{ => lang}/test/BitsTest.cpp          |   2 +-
 folly/test/BitsBenchmark.cpp                |   2 +-
 folly/test/EndianTest.cpp                   |   2 +-
 folly/test/IPAddressTest.cpp                |   2 +-
 folly/tracing/test/StaticTracepointTest.cpp |   2 +-
 26 files changed, 411 insertions(+), 392 deletions(-)
 create mode 100644 folly/lang/Bits.h
 rename folly/{ => lang}/test/BitsTest.cpp (99%)

diff --git a/CMakeLists.txt b/CMakeLists.txt
index fc0084dc..ae6ab6d0 100755
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -517,6 +517,9 @@ if (BUILD_TESTS)
     DIRECTORY io/async/ssl/test/
       TEST ssl_errors_test SOURCES SSLErrorsTest.cpp
 
+    DIRECTORY lang/test/
+      TEST bits_test SOURCES BitsTest.cpp
+
     DIRECTORY memory/test/
       TEST arena_test SOURCES ArenaTest.cpp
       TEST thread_cached_arena_test SOURCES ThreadCachedArenaTest.cpp
@@ -560,7 +563,6 @@ if (BUILD_TESTS)
       TEST atomic_struct_test SOURCES AtomicStructTest.cpp
       TEST atomic_unordered_map_test SOURCES AtomicUnorderedMapTest.cpp
       TEST bit_iterator_test SOURCES BitIteratorTest.cpp
-      TEST bits_test SOURCES BitsTest.cpp
       TEST cacheline_padded_test SOURCES CachelinePaddedTest.cpp
       TEST clock_gettime_wrappers_test SOURCES ClockGettimeWrappersTest.cpp
       TEST concurrent_skip_list_test SOURCES ConcurrentSkipListTest.cpp
diff --git a/folly/AtomicHashArray-inl.h b/folly/AtomicHashArray-inl.h
index f8251f33..4333e5a0 100644
--- a/folly/AtomicHashArray-inl.h
+++ b/folly/AtomicHashArray-inl.h
@@ -20,8 +20,8 @@
 
 #include <type_traits>
 
-#include <folly/Bits.h>
 #include <folly/detail/AtomicHashUtils.h>
+#include <folly/lang/Bits.h>
 
 namespace folly {
 
diff --git a/folly/AtomicUnorderedMap.h b/folly/AtomicUnorderedMap.h
index ca867fc1..edf3b1da 100644
--- a/folly/AtomicUnorderedMap.h
+++ b/folly/AtomicUnorderedMap.h
@@ -26,11 +26,11 @@
 
 #include <boost/type_traits/has_trivial_destructor.hpp>
 
-#include <folly/Bits.h>
 #include <folly/Conv.h>
 #include <folly/Likely.h>
 #include <folly/Random.h>
 #include <folly/detail/AtomicUnorderedMapUtils.h>
+#include <folly/lang/Bits.h>
 #include <folly/portability/SysMman.h>
 #include <folly/portability/Unistd.h>
 
diff --git a/folly/BitIterator.h b/folly/BitIterator.h
index b7545701..0446bd13 100644
--- a/folly/BitIterator.h
+++ b/folly/BitIterator.h
@@ -38,9 +38,9 @@
 
 #include <boost/iterator/iterator_adaptor.hpp>
 
-#include <folly/Bits.h>
 #include <folly/Portability.h>
 #include <folly/detail/BitIteratorDetail.h>
+#include <folly/lang/Bits.h>
 
 namespace folly {
 
diff --git a/folly/Bits.h b/folly/Bits.h
index 889b3a0b..a3cff3ea 100644
--- a/folly/Bits.h
+++ b/folly/Bits.h
@@ -14,371 +14,4 @@
  * limitations under the License.
  */
 
-/**
- * Various low-level, bit-manipulation routines.
- *
- * findFirstSet(x)  [constexpr]
- *    find first (least significant) bit set in a value of an integral type,
- *    1-based (like ffs()).  0 = no bits are set (x == 0)
- *
- * findLastSet(x)  [constexpr]
- *    find last (most significant) bit set in a value of an integral type,
- *    1-based.  0 = no bits are set (x == 0)
- *    for x != 0, findLastSet(x) == 1 + floor(log2(x))
- *
- * nextPowTwo(x)  [constexpr]
- *    Finds the next power of two >= x.
- *
- * isPowTwo(x)  [constexpr]
- *    return true iff x is a power of two
- *
- * popcount(x)
- *    return the number of 1 bits in x
- *
- * Endian
- *    convert between native, big, and little endian representation
- *    Endian::big(x)      big <-> native
- *    Endian::little(x)   little <-> native
- *    Endian::swap(x)     big <-> little
- *
- * @author Tudor Bosman (tudorb@fb.com)
- */
-
-#pragma once
-
-// MSVC does not support intrinsics constexpr
-#if defined(_MSC_VER)
-#define FOLLY_INTRINSIC_CONSTEXPR const
-#else
-#define FOLLY_INTRINSIC_CONSTEXPR constexpr
-#endif
-
-#include <cassert>
-#include <cinttypes>
-#include <cstdint>
-#include <cstring>
-#include <limits>
-#include <type_traits>
-
-#include <folly/Portability.h>
-#include <folly/lang/Assume.h>
-#include <folly/portability/Builtins.h>
-
-namespace folly {
-
-// Generate overloads for findFirstSet as wrappers around
-// appropriate ffs, ffsl, ffsll gcc builtins
-template <class T>
-inline FOLLY_INTRINSIC_CONSTEXPR
-typename std::enable_if<
-  (std::is_integral<T>::value &&
-   std::is_unsigned<T>::value &&
-   sizeof(T) <= sizeof(unsigned int)),
-  unsigned int>::type
-  findFirstSet(T x) {
-  return static_cast<unsigned int>(__builtin_ffs(static_cast<int>(x)));
-}
-
-template <class T>
-inline FOLLY_INTRINSIC_CONSTEXPR
-typename std::enable_if<
-  (std::is_integral<T>::value &&
-   std::is_unsigned<T>::value &&
-   sizeof(T) > sizeof(unsigned int) &&
-   sizeof(T) <= sizeof(unsigned long)),
-  unsigned int>::type
-  findFirstSet(T x) {
-  return static_cast<unsigned int>(__builtin_ffsl(static_cast<long>(x)));
-}
-
-template <class T>
-inline FOLLY_INTRINSIC_CONSTEXPR
-typename std::enable_if<
-  (std::is_integral<T>::value &&
-   std::is_unsigned<T>::value &&
-   sizeof(T) > sizeof(unsigned long) &&
-   sizeof(T) <= sizeof(unsigned long long)),
-  unsigned int>::type
-  findFirstSet(T x) {
-  return static_cast<unsigned int>(__builtin_ffsll(static_cast<long long>(x)));
-}
-
-template <class T>
-inline FOLLY_INTRINSIC_CONSTEXPR
-typename std::enable_if<
-  (std::is_integral<T>::value && std::is_signed<T>::value),
-  unsigned int>::type
-  findFirstSet(T x) {
-  // Note that conversion from a signed type to the corresponding unsigned
-  // type is technically implementation-defined, but will likely work
-  // on any impementation that uses two's complement.
-  return findFirstSet(static_cast<typename std::make_unsigned<T>::type>(x));
-}
-
-// findLastSet: return the 1-based index of the highest bit set
-// for x > 0, findLastSet(x) == 1 + floor(log2(x))
-template <class T>
-inline FOLLY_INTRINSIC_CONSTEXPR
-typename std::enable_if<
-  (std::is_integral<T>::value &&
-   std::is_unsigned<T>::value &&
-   sizeof(T) <= sizeof(unsigned int)),
-  unsigned int>::type
-  findLastSet(T x) {
-  // If X is a power of two X - Y = ((X - 1) ^ Y) + 1. Doing this transformation
-  // allows GCC to remove its own xor that it adds to implement clz using bsr
-  return x ? ((8 * sizeof(unsigned int) - 1) ^ __builtin_clz(x)) + 1 : 0;
-}
-
-template <class T>
-inline FOLLY_INTRINSIC_CONSTEXPR
-typename std::enable_if<
-  (std::is_integral<T>::value &&
-   std::is_unsigned<T>::value &&
-   sizeof(T) > sizeof(unsigned int) &&
-   sizeof(T) <= sizeof(unsigned long)),
-  unsigned int>::type
-  findLastSet(T x) {
-  return x ? ((8 * sizeof(unsigned long) - 1) ^ __builtin_clzl(x)) + 1 : 0;
-}
-
-template <class T>
-inline FOLLY_INTRINSIC_CONSTEXPR
-typename std::enable_if<
-  (std::is_integral<T>::value &&
-   std::is_unsigned<T>::value &&
-   sizeof(T) > sizeof(unsigned long) &&
-   sizeof(T) <= sizeof(unsigned long long)),
-  unsigned int>::type
-  findLastSet(T x) {
-  return x ? ((8 * sizeof(unsigned long long) - 1) ^ __builtin_clzll(x)) + 1
-           : 0;
-}
-
-template <class T>
-inline FOLLY_INTRINSIC_CONSTEXPR
-typename std::enable_if<
-  (std::is_integral<T>::value &&
-   std::is_signed<T>::value),
-  unsigned int>::type
-  findLastSet(T x) {
-  return findLastSet(static_cast<typename std::make_unsigned<T>::type>(x));
-}
-
-template <class T>
-inline FOLLY_INTRINSIC_CONSTEXPR
-typename std::enable_if<
-  std::is_integral<T>::value && std::is_unsigned<T>::value,
-  T>::type
-nextPowTwo(T v) {
-  return v ? (T(1) << findLastSet(v - 1)) : 1;
-}
-
-template <class T>
-inline FOLLY_INTRINSIC_CONSTEXPR typename std::
-    enable_if<std::is_integral<T>::value && std::is_unsigned<T>::value, T>::type
-    prevPowTwo(T v) {
-  return v ? (T(1) << (findLastSet(v) - 1)) : 0;
-}
-
-template <class T>
-inline constexpr typename std::enable_if<
-    std::is_integral<T>::value && std::is_unsigned<T>::value,
-    bool>::type
-isPowTwo(T v) {
-  return (v != 0) && !(v & (v - 1));
-}
-
-/**
- * Population count
- */
-template <class T>
-inline typename std::enable_if<
-  (std::is_integral<T>::value &&
-   std::is_unsigned<T>::value &&
-   sizeof(T) <= sizeof(unsigned int)),
-  size_t>::type
-  popcount(T x) {
-  return size_t(__builtin_popcount(x));
-}
-
-template <class T>
-inline typename std::enable_if<
-  (std::is_integral<T>::value &&
-   std::is_unsigned<T>::value &&
-   sizeof(T) > sizeof(unsigned int) &&
-   sizeof(T) <= sizeof(unsigned long long)),
-  size_t>::type
-  popcount(T x) {
-  return size_t(__builtin_popcountll(x));
-}
-
-/**
- * Endianness detection and manipulation primitives.
- */
-namespace detail {
-
-template <size_t Size>
-struct uint_types_by_size;
-
-#define FB_GEN(sz, fn)                                      \
-  static inline uint##sz##_t byteswap_gen(uint##sz##_t v) { \
-    return fn(v);                                           \
-  }                                                         \
-  template <>                                               \
-  struct uint_types_by_size<sz / 8> {                       \
-    using type = uint##sz##_t;                              \
-  };
-
-FB_GEN(8, uint8_t)
-#ifdef _MSC_VER
-FB_GEN(64, _byteswap_uint64)
-FB_GEN(32, _byteswap_ulong)
-FB_GEN(16, _byteswap_ushort)
-#else
-FB_GEN(64, __builtin_bswap64)
-FB_GEN(32, __builtin_bswap32)
-FB_GEN(16, __builtin_bswap16)
-#endif
-
-#undef FB_GEN
-
-template <class T>
-struct EndianInt {
-  static_assert(
-      (std::is_integral<T>::value && !std::is_same<T, bool>::value) ||
-          std::is_floating_point<T>::value,
-      "template type parameter must be non-bool integral or floating point");
-  static T swap(T x) {
-    // we implement this with memcpy because that is defined behavior in C++
-    // we rely on compilers to optimize away the memcpy calls
-    constexpr auto s = sizeof(T);
-    using B = typename uint_types_by_size<s>::type;
-    B b;
-    std::memcpy(&b, &x, s);
-    b = byteswap_gen(b);
-    std::memcpy(&x, &b, s);
-    return x;
-  }
-  static T big(T x) {
-    return kIsLittleEndian ? EndianInt::swap(x) : x;
-  }
-  static T little(T x) {
-    return kIsBigEndian ? EndianInt::swap(x) : x;
-  }
-};
-
-} // namespace detail
-
-// big* convert between native and big-endian representations
-// little* convert between native and little-endian representations
-// swap* convert between big-endian and little-endian representations
-//
-// ntohs, htons == big16
-// ntohl, htonl == big32
-#define FB_GEN1(fn, t, sz) \
-  static t fn##sz(t x) { return fn<t>(x); } \
-
-#define FB_GEN2(t, sz) \
-  FB_GEN1(swap, t, sz) \
-  FB_GEN1(big, t, sz) \
-  FB_GEN1(little, t, sz)
-
-#define FB_GEN(sz) \
-  FB_GEN2(uint##sz##_t, sz) \
-  FB_GEN2(int##sz##_t, sz)
-
-class Endian {
- public:
-  enum class Order : uint8_t {
-    LITTLE,
-    BIG
-  };
-
-  static constexpr Order order = kIsLittleEndian ? Order::LITTLE : Order::BIG;
-
-  template <class T> static T swap(T x) {
-    return folly::detail::EndianInt<T>::swap(x);
-  }
-  template <class T> static T big(T x) {
-    return folly::detail::EndianInt<T>::big(x);
-  }
-  template <class T> static T little(T x) {
-    return folly::detail::EndianInt<T>::little(x);
-  }
-
-#if !defined(__ANDROID__)
-  FB_GEN(64)
-  FB_GEN(32)
-  FB_GEN(16)
-  FB_GEN(8)
-#endif
-};
-
-#undef FB_GEN
-#undef FB_GEN2
-#undef FB_GEN1
-
-
-template <class T, class Enable=void> struct Unaligned;
-
-/**
- * Representation of an unaligned value of a POD type.
- */
-FOLLY_PACK_PUSH
-template <class T>
-struct Unaligned<
-    T,
-    typename std::enable_if<std::is_pod<T>::value>::type> {
-  Unaligned() = default;  // uninitialized
-  /* implicit */ Unaligned(T v) : value(v) { }
-  T value;
-} FOLLY_PACK_ATTR;
-FOLLY_PACK_POP
-
-/**
- * Read an unaligned value of type T and return it.
- */
-template <class T>
-inline T loadUnaligned(const void* p) {
-  static_assert(sizeof(Unaligned<T>) == sizeof(T), "Invalid unaligned size");
-  static_assert(alignof(Unaligned<T>) == 1, "Invalid alignment");
-  if (kHasUnalignedAccess) {
-    return static_cast<const Unaligned<T>*>(p)->value;
-  } else {
-    T value;
-    memcpy(&value, p, sizeof(T));
-    return value;
-  }
-}
-
-/**
- * Write an unaligned value of type T.
- */
-template <class T>
-inline void storeUnaligned(void* p, T value) {
-  static_assert(sizeof(Unaligned<T>) == sizeof(T), "Invalid unaligned size");
-  static_assert(alignof(Unaligned<T>) == 1, "Invalid alignment");
-  if (kHasUnalignedAccess) {
-    // Prior to C++14, the spec says that a placement new like this
-    // is required to check that p is not nullptr, and to do nothing
-    // if p is a nullptr. By assuming it's not a nullptr, we get a
-    // nice loud segfault in optimized builds if p is nullptr, rather
-    // than just silently doing nothing.
-    folly::assume(p != nullptr);
-    new (p) Unaligned<T>(value);
-  } else {
-    memcpy(p, &value, sizeof(T));
-  }
-}
-
-template <typename T>
-T bitReverse(T n) {
-  auto m = static_cast<typename std::make_unsigned<T>::type>(n);
-  m = ((m & 0xAAAAAAAAAAAAAAAA) >> 1) | ((m & 0x5555555555555555) << 1);
-  m = ((m & 0xCCCCCCCCCCCCCCCC) >> 2) | ((m & 0x3333333333333333) << 2);
-  m = ((m & 0xF0F0F0F0F0F0F0F0) >> 4) | ((m & 0x0F0F0F0F0F0F0F0F) << 4);
-  return static_cast<T>(Endian::swap(m));
-}
-
-} // namespace folly
+#include <folly/lang/Bits.h> // @shim
diff --git a/folly/GroupVarint.h b/folly/GroupVarint.h
index ed7dae63..ad0bb361 100644
--- a/folly/GroupVarint.h
+++ b/folly/GroupVarint.h
@@ -30,9 +30,9 @@
 #if FOLLY_X64 || defined(__i386__) || FOLLY_PPC64 || FOLLY_AARCH64
 #define HAVE_GROUP_VARINT 1
 
-#include <folly/Bits.h>
 #include <folly/Range.h>
 #include <folly/detail/GroupVarintDetail.h>
+#include <folly/lang/Bits.h>
 #include <folly/portability/Builtins.h>
 
 #if FOLLY_SSE >= 3
diff --git a/folly/MacAddress.h b/folly/MacAddress.h
index ef7c3ac1..2a2c1fc4 100644
--- a/folly/MacAddress.h
+++ b/folly/MacAddress.h
@@ -18,8 +18,8 @@
 
 #include <iosfwd>
 
-#include <folly/Bits.h>
 #include <folly/Range.h>
+#include <folly/lang/Bits.h>
 
 namespace folly {
 
diff --git a/folly/Makefile.am b/folly/Makefile.am
index 121c1eeb..2b077c75 100644
--- a/folly/Makefile.am
+++ b/folly/Makefile.am
@@ -325,6 +325,7 @@ nobase_follyinclude_HEADERS = \
 	io/async/test/Util.h \
 	json.h \
 	lang/Assume.h \
+	lang/Bits.h \
 	lang/ColdClass.h \
 	lang/Launder.h \
 	lang/RValueReferenceWrapper.h \
diff --git a/folly/compression/Compression.cpp b/folly/compression/Compression.cpp
index 6f288b5c..f94a381b 100644
--- a/folly/compression/Compression.cpp
+++ b/folly/compression/Compression.cpp
@@ -48,7 +48,6 @@
 #include <bzlib.h>
 #endif
 
-#include <folly/Bits.h>
 #include <folly/Conv.h>
 #include <folly/Memory.h>
 #include <folly/Portability.h>
@@ -56,6 +55,7 @@
 #include <folly/Varint.h>
 #include <folly/compression/Utils.h>
 #include <folly/io/Cursor.h>
+#include <folly/lang/Bits.h>
 #include <algorithm>
 #include <unordered_set>
 
diff --git a/folly/compression/Utils.h b/folly/compression/Utils.h
index 8d23723f..4804548b 100644
--- a/folly/compression/Utils.h
+++ b/folly/compression/Utils.h
@@ -18,9 +18,9 @@
 
 #include <type_traits>
 
-#include <folly/Bits.h>
 #include <folly/io/Cursor.h>
 #include <folly/io/IOBuf.h>
+#include <folly/lang/Bits.h>
 
 /**
  * Helper functions for compression codecs.
diff --git a/folly/experimental/BitVectorCoding.h b/folly/experimental/BitVectorCoding.h
index 99c9c576..a24b9550 100644
--- a/folly/experimental/BitVectorCoding.h
+++ b/folly/experimental/BitVectorCoding.h
@@ -20,7 +20,6 @@
 #include <limits>
 #include <type_traits>
 
-#include <folly/Bits.h>
 #include <folly/Likely.h>
 #include <folly/Portability.h>
 #include <folly/Range.h>
@@ -28,6 +27,7 @@
 #include <folly/experimental/CodingDetail.h>
 #include <folly/experimental/Instructions.h>
 #include <folly/experimental/Select64.h>
+#include <folly/lang/Bits.h>
 #include <glog/logging.h>
 
 #if !FOLLY_X64
diff --git a/folly/experimental/Bits.h b/folly/experimental/Bits.h
index 0b1960c2..f6ab45d7 100644
--- a/folly/experimental/Bits.h
+++ b/folly/experimental/Bits.h
@@ -22,9 +22,9 @@
 
 #include <glog/logging.h>
 
-#include <folly/Bits.h>
 #include <folly/Portability.h>
 #include <folly/Range.h>
+#include <folly/lang/Bits.h>
 
 namespace folly {
 
diff --git a/folly/experimental/EliasFanoCoding.h b/folly/experimental/EliasFanoCoding.h
index 04a0f158..8936ef0d 100644
--- a/folly/experimental/EliasFanoCoding.h
+++ b/folly/experimental/EliasFanoCoding.h
@@ -28,7 +28,6 @@
 #include <limits>
 #include <type_traits>
 
-#include <folly/Bits.h>
 #include <folly/Likely.h>
 #include <folly/Portability.h>
 #include <folly/Range.h>
@@ -36,6 +35,7 @@
 #include <folly/experimental/Instructions.h>
 #include <folly/experimental/Select64.h>
 #include <folly/lang/Assume.h>
+#include <folly/lang/Bits.h>
 #include <glog/logging.h>
 
 #if !FOLLY_X64
diff --git a/folly/experimental/EventCount.h b/folly/experimental/EventCount.h
index 1496b8b1..95594f38 100644
--- a/folly/experimental/EventCount.h
+++ b/folly/experimental/EventCount.h
@@ -22,13 +22,12 @@
 
 #include <glog/logging.h>
 
-#include <folly/Bits.h>
 #include <folly/Likely.h>
 #include <folly/detail/Futex.h>
+#include <folly/lang/Bits.h>
 #include <folly/portability/SysTime.h>
 #include <folly/portability/Unistd.h>
 
-
 namespace folly {
 
 /**
diff --git a/folly/hash/Hash.h b/folly/hash/Hash.h
index 8f0743de..6b6f137a 100644
--- a/folly/hash/Hash.h
+++ b/folly/hash/Hash.h
@@ -24,10 +24,10 @@
 #include <type_traits>
 #include <utility>
 
-#include <folly/Bits.h>
 #include <folly/functional/ApplyTuple.h>
 #include <folly/hash/SpookyHashV1.h>
 #include <folly/hash/SpookyHashV2.h>
+#include <folly/lang/Bits.h>
 
 /*
  * Various hashing functions.
diff --git a/folly/io/Cursor.h b/folly/io/Cursor.h
index ba0dd36f..c32e3042 100644
--- a/folly/io/Cursor.h
+++ b/folly/io/Cursor.h
@@ -23,13 +23,13 @@
 #include <stdexcept>
 #include <type_traits>
 
-#include <folly/Bits.h>
 #include <folly/Likely.h>
 #include <folly/Memory.h>
 #include <folly/Portability.h>
 #include <folly/Range.h>
 #include <folly/io/IOBuf.h>
 #include <folly/io/IOBufQueue.h>
+#include <folly/lang/Bits.h>
 #include <folly/portability/BitsFunctexcept.h>
 
 /**
diff --git a/folly/io/async/AsyncSSLSocket.cpp b/folly/io/async/AsyncSSLSocket.cpp
index 0cb7c73a..bfcb8b9e 100644
--- a/folly/io/async/AsyncSSLSocket.cpp
+++ b/folly/io/async/AsyncSSLSocket.cpp
@@ -26,12 +26,12 @@
 #include <chrono>
 #include <memory>
 
-#include <folly/Bits.h>
 #include <folly/Format.h>
 #include <folly/SocketAddress.h>
 #include <folly/SpinLock.h>
 #include <folly/io/Cursor.h>
 #include <folly/io/IOBuf.h>
+#include <folly/lang/Bits.h>
 #include <folly/portability/OpenSSL.h>
 
 using folly::SocketAddress;
diff --git a/folly/io/async/AsyncSSLSocket.h b/folly/io/async/AsyncSSLSocket.h
index 1f1d6798..c3e38bbe 100644
--- a/folly/io/async/AsyncSSLSocket.h
+++ b/folly/io/async/AsyncSSLSocket.h
@@ -18,7 +18,6 @@
 
 #include <iomanip>
 
-#include <folly/Bits.h>
 #include <folly/Optional.h>
 #include <folly/String.h>
 #include <folly/io/Cursor.h>
@@ -30,6 +29,7 @@
 #include <folly/io/async/ssl/OpenSSLUtils.h>
 #include <folly/io/async/ssl/SSLErrors.h>
 #include <folly/io/async/ssl/TLSDefinitions.h>
+#include <folly/lang/Bits.h>
 #include <folly/portability/OpenSSL.h>
 #include <folly/portability/Sockets.h>
 #include <folly/ssl/OpenSSLPtrTypes.h>
diff --git a/folly/io/async/HHWheelTimer.cpp b/folly/io/async/HHWheelTimer.cpp
index 57bb1ce9..dd7301f5 100644
--- a/folly/io/async/HHWheelTimer.cpp
+++ b/folly/io/async/HHWheelTimer.cpp
@@ -22,7 +22,7 @@
 #include <folly/Optional.h>
 #include <folly/ScopeGuard.h>
 
-#include <folly/Bits.h>
+#include <folly/lang/Bits.h>
 
 #include <cassert>
 
diff --git a/folly/json.cpp b/folly/json.cpp
index 029278e2..db80b332 100644
--- a/folly/json.cpp
+++ b/folly/json.cpp
@@ -22,8 +22,8 @@
 
 #include <boost/algorithm/string.hpp>
 #include <boost/next_prior.hpp>
-#include <folly/Bits.h>
 #include <folly/Portability.h>
+#include <folly/lang/Bits.h>
 
 #include <folly/Conv.h>
 #include <folly/Range.h>
diff --git a/folly/lang/Bits.h b/folly/lang/Bits.h
new file mode 100644
index 00000000..889b3a0b
--- /dev/null
+++ b/folly/lang/Bits.h
@@ -0,0 +1,384 @@
+/*
+ * Copyright 2017 Facebook, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+/**
+ * Various low-level, bit-manipulation routines.
+ *
+ * findFirstSet(x)  [constexpr]
+ *    find first (least significant) bit set in a value of an integral type,
+ *    1-based (like ffs()).  0 = no bits are set (x == 0)
+ *
+ * findLastSet(x)  [constexpr]
+ *    find last (most significant) bit set in a value of an integral type,
+ *    1-based.  0 = no bits are set (x == 0)
+ *    for x != 0, findLastSet(x) == 1 + floor(log2(x))
+ *
+ * nextPowTwo(x)  [constexpr]
+ *    Finds the next power of two >= x.
+ *
+ * isPowTwo(x)  [constexpr]
+ *    return true iff x is a power of two
+ *
+ * popcount(x)
+ *    return the number of 1 bits in x
+ *
+ * Endian
+ *    convert between native, big, and little endian representation
+ *    Endian::big(x)      big <-> native
+ *    Endian::little(x)   little <-> native
+ *    Endian::swap(x)     big <-> little
+ *
+ * @author Tudor Bosman (tudorb@fb.com)
+ */
+
+#pragma once
+
+// MSVC does not support intrinsics constexpr
+#if defined(_MSC_VER)
+#define FOLLY_INTRINSIC_CONSTEXPR const
+#else
+#define FOLLY_INTRINSIC_CONSTEXPR constexpr
+#endif
+
+#include <cassert>
+#include <cinttypes>
+#include <cstdint>
+#include <cstring>
+#include <limits>
+#include <type_traits>
+
+#include <folly/Portability.h>
+#include <folly/lang/Assume.h>
+#include <folly/portability/Builtins.h>
+
+namespace folly {
+
+// Generate overloads for findFirstSet as wrappers around
+// appropriate ffs, ffsl, ffsll gcc builtins
+template <class T>
+inline FOLLY_INTRINSIC_CONSTEXPR
+typename std::enable_if<
+  (std::is_integral<T>::value &&
+   std::is_unsigned<T>::value &&
+   sizeof(T) <= sizeof(unsigned int)),
+  unsigned int>::type
+  findFirstSet(T x) {
+  return static_cast<unsigned int>(__builtin_ffs(static_cast<int>(x)));
+}
+
+template <class T>
+inline FOLLY_INTRINSIC_CONSTEXPR
+typename std::enable_if<
+  (std::is_integral<T>::value &&
+   std::is_unsigned<T>::value &&
+   sizeof(T) > sizeof(unsigned int) &&
+   sizeof(T) <= sizeof(unsigned long)),
+  unsigned int>::type
+  findFirstSet(T x) {
+  return static_cast<unsigned int>(__builtin_ffsl(static_cast<long>(x)));
+}
+
+template <class T>
+inline FOLLY_INTRINSIC_CONSTEXPR
+typename std::enable_if<
+  (std::is_integral<T>::value &&
+   std::is_unsigned<T>::value &&
+   sizeof(T) > sizeof(unsigned long) &&
+   sizeof(T) <= sizeof(unsigned long long)),
+  unsigned int>::type
+  findFirstSet(T x) {
+  return static_cast<unsigned int>(__builtin_ffsll(static_cast<long long>(x)));
+}
+
+template <class T>
+inline FOLLY_INTRINSIC_CONSTEXPR
+typename std::enable_if<
+  (std::is_integral<T>::value && std::is_signed<T>::value),
+  unsigned int>::type
+  findFirstSet(T x) {
+  // Note that conversion from a signed type to the corresponding unsigned
+  // type is technically implementation-defined, but will likely work
+  // on any impementation that uses two's complement.
+  return findFirstSet(static_cast<typename std::make_unsigned<T>::type>(x));
+}
+
+// findLastSet: return the 1-based index of the highest bit set
+// for x > 0, findLastSet(x) == 1 + floor(log2(x))
+template <class T>
+inline FOLLY_INTRINSIC_CONSTEXPR
+typename std::enable_if<
+  (std::is_integral<T>::value &&
+   std::is_unsigned<T>::value &&
+   sizeof(T) <= sizeof(unsigned int)),
+  unsigned int>::type
+  findLastSet(T x) {
+  // If X is a power of two X - Y = ((X - 1) ^ Y) + 1. Doing this transformation
+  // allows GCC to remove its own xor that it adds to implement clz using bsr
+  return x ? ((8 * sizeof(unsigned int) - 1) ^ __builtin_clz(x)) + 1 : 0;
+}
+
+template <class T>
+inline FOLLY_INTRINSIC_CONSTEXPR
+typename std::enable_if<
+  (std::is_integral<T>::value &&
+   std::is_unsigned<T>::value &&
+   sizeof(T) > sizeof(unsigned int) &&
+   sizeof(T) <= sizeof(unsigned long)),
+  unsigned int>::type
+  findLastSet(T x) {
+  return x ? ((8 * sizeof(unsigned long) - 1) ^ __builtin_clzl(x)) + 1 : 0;
+}
+
+template <class T>
+inline FOLLY_INTRINSIC_CONSTEXPR
+typename std::enable_if<
+  (std::is_integral<T>::value &&
+   std::is_unsigned<T>::value &&
+   sizeof(T) > sizeof(unsigned long) &&
+   sizeof(T) <= sizeof(unsigned long long)),
+  unsigned int>::type
+  findLastSet(T x) {
+  return x ? ((8 * sizeof(unsigned long long) - 1) ^ __builtin_clzll(x)) + 1
+           : 0;
+}
+
+template <class T>
+inline FOLLY_INTRINSIC_CONSTEXPR
+typename std::enable_if<
+  (std::is_integral<T>::value &&
+   std::is_signed<T>::value),
+  unsigned int>::type
+  findLastSet(T x) {
+  return findLastSet(static_cast<typename std::make_unsigned<T>::type>(x));
+}
+
+template <class T>
+inline FOLLY_INTRINSIC_CONSTEXPR
+typename std::enable_if<
+  std::is_integral<T>::value && std::is_unsigned<T>::value,
+  T>::type
+nextPowTwo(T v) {
+  return v ? (T(1) << findLastSet(v - 1)) : 1;
+}
+
+template <class T>
+inline FOLLY_INTRINSIC_CONSTEXPR typename std::
+    enable_if<std::is_integral<T>::value && std::is_unsigned<T>::value, T>::type
+    prevPowTwo(T v) {
+  return v ? (T(1) << (findLastSet(v) - 1)) : 0;
+}
+
+template <class T>
+inline constexpr typename std::enable_if<
+    std::is_integral<T>::value && std::is_unsigned<T>::value,
+    bool>::type
+isPowTwo(T v) {
+  return (v != 0) && !(v & (v - 1));
+}
+
+/**
+ * Population count
+ */
+template <class T>
+inline typename std::enable_if<
+  (std::is_integral<T>::value &&
+   std::is_unsigned<T>::value &&
+   sizeof(T) <= sizeof(unsigned int)),
+  size_t>::type
+  popcount(T x) {
+  return size_t(__builtin_popcount(x));
+}
+
+template <class T>
+inline typename std::enable_if<
+  (std::is_integral<T>::value &&
+   std::is_unsigned<T>::value &&
+   sizeof(T) > sizeof(unsigned int) &&
+   sizeof(T) <= sizeof(unsigned long long)),
+  size_t>::type
+  popcount(T x) {
+  return size_t(__builtin_popcountll(x));
+}
+
+/**
+ * Endianness detection and manipulation primitives.
+ */
+namespace detail {
+
+template <size_t Size>
+struct uint_types_by_size;
+
+#define FB_GEN(sz, fn)                                      \
+  static inline uint##sz##_t byteswap_gen(uint##sz##_t v) { \
+    return fn(v);                                           \
+  }                                                         \
+  template <>                                               \
+  struct uint_types_by_size<sz / 8> {                       \
+    using type = uint##sz##_t;                              \
+  };
+
+FB_GEN(8, uint8_t)
+#ifdef _MSC_VER
+FB_GEN(64, _byteswap_uint64)
+FB_GEN(32, _byteswap_ulong)
+FB_GEN(16, _byteswap_ushort)
+#else
+FB_GEN(64, __builtin_bswap64)
+FB_GEN(32, __builtin_bswap32)
+FB_GEN(16, __builtin_bswap16)
+#endif
+
+#undef FB_GEN
+
+template <class T>
+struct EndianInt {
+  static_assert(
+      (std::is_integral<T>::value && !std::is_same<T, bool>::value) ||
+          std::is_floating_point<T>::value,
+      "template type parameter must be non-bool integral or floating point");
+  static T swap(T x) {
+    // we implement this with memcpy because that is defined behavior in C++
+    // we rely on compilers to optimize away the memcpy calls
+    constexpr auto s = sizeof(T);
+    using B = typename uint_types_by_size<s>::type;
+    B b;
+    std::memcpy(&b, &x, s);
+    b = byteswap_gen(b);
+    std::memcpy(&x, &b, s);
+    return x;
+  }
+  static T big(T x) {
+    return kIsLittleEndian ? EndianInt::swap(x) : x;
+  }
+  static T little(T x) {
+    return kIsBigEndian ? EndianInt::swap(x) : x;
+  }
+};
+
+} // namespace detail
+
+// big* convert between native and big-endian representations
+// little* convert between native and little-endian representations
+// swap* convert between big-endian and little-endian representations
+//
+// ntohs, htons == big16
+// ntohl, htonl == big32
+#define FB_GEN1(fn, t, sz) \
+  static t fn##sz(t x) { return fn<t>(x); } \
+
+#define FB_GEN2(t, sz) \
+  FB_GEN1(swap, t, sz) \
+  FB_GEN1(big, t, sz) \
+  FB_GEN1(little, t, sz)
+
+#define FB_GEN(sz) \
+  FB_GEN2(uint##sz##_t, sz) \
+  FB_GEN2(int##sz##_t, sz)
+
+class Endian {
+ public:
+  enum class Order : uint8_t {
+    LITTLE,
+    BIG
+  };
+
+  static constexpr Order order = kIsLittleEndian ? Order::LITTLE : Order::BIG;
+
+  template <class T> static T swap(T x) {
+    return folly::detail::EndianInt<T>::swap(x);
+  }
+  template <class T> static T big(T x) {
+    return folly::detail::EndianInt<T>::big(x);
+  }
+  template <class T> static T little(T x) {
+    return folly::detail::EndianInt<T>::little(x);
+  }
+
+#if !defined(__ANDROID__)
+  FB_GEN(64)
+  FB_GEN(32)
+  FB_GEN(16)
+  FB_GEN(8)
+#endif
+};
+
+#undef FB_GEN
+#undef FB_GEN2
+#undef FB_GEN1
+
+
+template <class T, class Enable=void> struct Unaligned;
+
+/**
+ * Representation of an unaligned value of a POD type.
+ */
+FOLLY_PACK_PUSH
+template <class T>
+struct Unaligned<
+    T,
+    typename std::enable_if<std::is_pod<T>::value>::type> {
+  Unaligned() = default;  // uninitialized
+  /* implicit */ Unaligned(T v) : value(v) { }
+  T value;
+} FOLLY_PACK_ATTR;
+FOLLY_PACK_POP
+
+/**
+ * Read an unaligned value of type T and return it.
+ */
+template <class T>
+inline T loadUnaligned(const void* p) {
+  static_assert(sizeof(Unaligned<T>) == sizeof(T), "Invalid unaligned size");
+  static_assert(alignof(Unaligned<T>) == 1, "Invalid alignment");
+  if (kHasUnalignedAccess) {
+    return static_cast<const Unaligned<T>*>(p)->value;
+  } else {
+    T value;
+    memcpy(&value, p, sizeof(T));
+    return value;
+  }
+}
+
+/**
+ * Write an unaligned value of type T.
+ */
+template <class T>
+inline void storeUnaligned(void* p, T value) {
+  static_assert(sizeof(Unaligned<T>) == sizeof(T), "Invalid unaligned size");
+  static_assert(alignof(Unaligned<T>) == 1, "Invalid alignment");
+  if (kHasUnalignedAccess) {
+    // Prior to C++14, the spec says that a placement new like this
+    // is required to check that p is not nullptr, and to do nothing
+    // if p is a nullptr. By assuming it's not a nullptr, we get a
+    // nice loud segfault in optimized builds if p is nullptr, rather
+    // than just silently doing nothing.
+    folly::assume(p != nullptr);
+    new (p) Unaligned<T>(value);
+  } else {
+    memcpy(p, &value, sizeof(T));
+  }
+}
+
+template <typename T>
+T bitReverse(T n) {
+  auto m = static_cast<typename std::make_unsigned<T>::type>(n);
+  m = ((m & 0xAAAAAAAAAAAAAAAA) >> 1) | ((m & 0x5555555555555555) << 1);
+  m = ((m & 0xCCCCCCCCCCCCCCCC) >> 2) | ((m & 0x3333333333333333) << 2);
+  m = ((m & 0xF0F0F0F0F0F0F0F0) >> 4) | ((m & 0x0F0F0F0F0F0F0F0F) << 4);
+  return static_cast<T>(Endian::swap(m));
+}
+
+} // namespace folly
diff --git a/folly/test/BitsTest.cpp b/folly/lang/test/BitsTest.cpp
similarity index 99%
rename from folly/test/BitsTest.cpp
rename to folly/lang/test/BitsTest.cpp
index 1d722cb5..e2708f8b 100644
--- a/folly/test/BitsTest.cpp
+++ b/folly/lang/test/BitsTest.cpp
@@ -16,7 +16,7 @@
 
 // @author Tudor Bosman (tudorb@fb.com)
 
-#include <folly/Bits.h>
+#include <folly/lang/Bits.h>
 
 #include <folly/Random.h>
 #include <random>
diff --git a/folly/test/BitsBenchmark.cpp b/folly/test/BitsBenchmark.cpp
index 5d2ba9e1..da0cab43 100644
--- a/folly/test/BitsBenchmark.cpp
+++ b/folly/test/BitsBenchmark.cpp
@@ -16,7 +16,7 @@
 
 // @author Tudor Bosman (tudorb@fb.com)
 
-#include <folly/Bits.h>
+#include <folly/lang/Bits.h>
 
 #include <folly/Benchmark.h>
 
diff --git a/folly/test/EndianTest.cpp b/folly/test/EndianTest.cpp
index ab7d21c2..30809c1e 100644
--- a/folly/test/EndianTest.cpp
+++ b/folly/test/EndianTest.cpp
@@ -14,7 +14,7 @@
  * limitations under the License.
  */
 
-#include <folly/Bits.h>
+#include <folly/lang/Bits.h>
 
 #include <folly/portability/GTest.h>
 
diff --git a/folly/test/IPAddressTest.cpp b/folly/test/IPAddressTest.cpp
index 02f54676..23ab3c01 100644
--- a/folly/test/IPAddressTest.cpp
+++ b/folly/test/IPAddressTest.cpp
@@ -19,12 +19,12 @@
 #include <string>
 
 #include <folly/BitIterator.h>
-#include <folly/Bits.h>
 #include <folly/Format.h>
 #include <folly/IPAddress.h>
 #include <folly/MacAddress.h>
 #include <folly/String.h>
 #include <folly/detail/IPAddressSource.h>
+#include <folly/lang/Bits.h>
 #include <folly/portability/GMock.h>
 #include <folly/portability/GTest.h>
 
diff --git a/folly/tracing/test/StaticTracepointTest.cpp b/folly/tracing/test/StaticTracepointTest.cpp
index d6b76ec8..75a89b7c 100644
--- a/folly/tracing/test/StaticTracepointTest.cpp
+++ b/folly/tracing/test/StaticTracepointTest.cpp
@@ -22,12 +22,12 @@
 #include <vector>
 
 #include <boost/filesystem.hpp>
-#include <folly/Bits.h>
 #include <folly/Conv.h>
 #include <folly/Format.h>
 #include <folly/Random.h>
 #include <folly/String.h>
 #include <folly/Subprocess.h>
+#include <folly/lang/Bits.h>
 #include <folly/portability/GTest.h>
 #include <folly/portability/Unistd.h>
 #include <folly/tracing/StaticTracepoint.h>
-- 
2.34.1