2 * Copyright 2015 Facebook, Inc.
4 * Licensed under the Apache License, Version 2.0 (the "License");
5 * you may not use this file except in compliance with the License.
6 * You may obtain a copy of the License at
8 * http://www.apache.org/licenses/LICENSE-2.0
10 * Unless required by applicable law or agreed to in writing, software
11 * distributed under the License is distributed on an "AS IS" BASIS,
12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13 * See the License for the specific language governing permissions and
14 * limitations under the License.
18 * Various low-level, bit-manipulation routines.
20 * findFirstSet(x) [constexpr]
21 * find first (least significant) bit set in a value of an integral type,
22 * 1-based (like ffs()). 0 = no bits are set (x == 0)
24 * findLastSet(x) [constexpr]
25 * find last (most significant) bit set in a value of an integral type,
26 * 1-based. 0 = no bits are set (x == 0)
27 * for x != 0, findLastSet(x) == 1 + floor(log2(x))
29 * nextPowTwo(x) [constexpr]
30 * Finds the next power of two >= x.
32 * isPowTwo(x) [constexpr]
33 * return true iff x is a power of two
36 * return the number of 1 bits in x
39 * convert between native, big, and little endian representation
40 * Endian::big(x) big <-> native
41 * Endian::little(x) little <-> native
42 * Endian::swap(x) big <-> little
45 * Wrapper around an iterator over an integral type that iterates
46 * over its underlying bits in MSb to LSb order
48 * findFirstSet(BitIterator begin, BitIterator end)
49 * return a BitIterator pointing to the first 1 bit in [begin, end), or
50 * end if all bits in [begin, end) are 0
52 * @author Tudor Bosman (tudorb@fb.com)
58 #if !defined(__clang__) && !(defined(_MSC_VER) && (_MSC_VER < 1900))
59 #define FOLLY_INTRINSIC_CONSTEXPR constexpr
61 // GCC and MSVC 2015+ are the only compilers with
62 // intrinsics constexpr.
63 #define FOLLY_INTRINSIC_CONSTEXPR const
66 #include <folly/Portability.h>
68 #include <folly/detail/BitsDetail.h>
69 #include <folly/detail/BitIteratorDetail.h>
70 #include <folly/Likely.h>
72 #if FOLLY_HAVE_BYTESWAP_H
73 # include <byteswap.h>
80 #include <type_traits>
81 #include <boost/iterator/iterator_adaptor.hpp>
86 // Generate overloads for findFirstSet as wrappers around
87 // appropriate ffs, ffsl, ffsll gcc builtins
89 inline FOLLY_INTRINSIC_CONSTEXPR
90 typename std::enable_if<
91 (std::is_integral<T>::value &&
92 std::is_unsigned<T>::value &&
93 sizeof(T) <= sizeof(unsigned int)),
96 return __builtin_ffs(x);
100 inline FOLLY_INTRINSIC_CONSTEXPR
101 typename std::enable_if<
102 (std::is_integral<T>::value &&
103 std::is_unsigned<T>::value &&
104 sizeof(T) > sizeof(unsigned int) &&
105 sizeof(T) <= sizeof(unsigned long)),
108 return __builtin_ffsl(x);
112 inline FOLLY_INTRINSIC_CONSTEXPR
113 typename std::enable_if<
114 (std::is_integral<T>::value &&
115 std::is_unsigned<T>::value &&
116 sizeof(T) > sizeof(unsigned long) &&
117 sizeof(T) <= sizeof(unsigned long long)),
120 return __builtin_ffsll(x);
124 inline FOLLY_INTRINSIC_CONSTEXPR
125 typename std::enable_if<
126 (std::is_integral<T>::value && std::is_signed<T>::value),
129 // Note that conversion from a signed type to the corresponding unsigned
130 // type is technically implementation-defined, but will likely work
131 // on any impementation that uses two's complement.
132 return findFirstSet(static_cast<typename std::make_unsigned<T>::type>(x));
135 // findLastSet: return the 1-based index of the highest bit set
136 // for x > 0, findLastSet(x) == 1 + floor(log2(x))
138 inline FOLLY_INTRINSIC_CONSTEXPR
139 typename std::enable_if<
140 (std::is_integral<T>::value &&
141 std::is_unsigned<T>::value &&
142 sizeof(T) <= sizeof(unsigned int)),
145 return x ? 8 * sizeof(unsigned int) - __builtin_clz(x) : 0;
149 inline FOLLY_INTRINSIC_CONSTEXPR
150 typename std::enable_if<
151 (std::is_integral<T>::value &&
152 std::is_unsigned<T>::value &&
153 sizeof(T) > sizeof(unsigned int) &&
154 sizeof(T) <= sizeof(unsigned long)),
157 return x ? 8 * sizeof(unsigned long) - __builtin_clzl(x) : 0;
161 inline FOLLY_INTRINSIC_CONSTEXPR
162 typename std::enable_if<
163 (std::is_integral<T>::value &&
164 std::is_unsigned<T>::value &&
165 sizeof(T) > sizeof(unsigned long) &&
166 sizeof(T) <= sizeof(unsigned long long)),
169 return x ? 8 * sizeof(unsigned long long) - __builtin_clzll(x) : 0;
173 inline FOLLY_INTRINSIC_CONSTEXPR
174 typename std::enable_if<
175 (std::is_integral<T>::value &&
176 std::is_signed<T>::value),
179 return findLastSet(static_cast<typename std::make_unsigned<T>::type>(x));
183 inline FOLLY_INTRINSIC_CONSTEXPR
184 typename std::enable_if<
185 std::is_integral<T>::value && std::is_unsigned<T>::value,
188 return v ? (1ul << findLastSet(v - 1)) : 1;
193 typename std::enable_if<
194 std::is_integral<T>::value && std::is_unsigned<T>::value,
197 return (v != 0) && !(v & (v - 1));
204 inline typename std::enable_if<
205 (std::is_integral<T>::value &&
206 std::is_unsigned<T>::value &&
207 sizeof(T) <= sizeof(unsigned int)),
210 return detail::popcount(x);
214 inline typename std::enable_if<
215 (std::is_integral<T>::value &&
216 std::is_unsigned<T>::value &&
217 sizeof(T) > sizeof(unsigned int) &&
218 sizeof(T) <= sizeof(unsigned long long)),
221 return detail::popcountll(x);
225 * Endianness detection and manipulation primitives.
230 struct EndianIntBase {
238 * If we have the bswap_16 macro from byteswap.h, use it; otherwise, provide our
242 # define our_bswap16 bswap_16
245 template<class Int16>
246 inline constexpr typename std::enable_if<
249 our_bswap16(Int16 x) {
250 return ((x >> 8) & 0xff) | ((x & 0xff) << 8);
256 #define FB_GEN(t, fn) \
257 template<> inline t EndianIntBase<t>::swap(t x) { return fn(x); }
259 // fn(x) expands to (x) if the second argument is empty, which is exactly
260 // what we want for [u]int8_t. Also, gcc 4.7 on Intel doesn't have
261 // __builtin_bswap16 for some reason, so we have to provide our own.
265 FB_GEN( int64_t, _byteswap_uint64)
266 FB_GEN(uint64_t, _byteswap_uint64)
267 FB_GEN( int32_t, _byteswap_ulong)
268 FB_GEN(uint32_t, _byteswap_ulong)
269 FB_GEN( int16_t, _byteswap_ushort)
270 FB_GEN(uint16_t, _byteswap_ushort)
272 FB_GEN( int64_t, __builtin_bswap64)
273 FB_GEN(uint64_t, __builtin_bswap64)
274 FB_GEN( int32_t, __builtin_bswap32)
275 FB_GEN(uint32_t, __builtin_bswap32)
276 FB_GEN( int16_t, our_bswap16)
277 FB_GEN(uint16_t, our_bswap16)
282 #if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
285 struct EndianInt : public detail::EndianIntBase<T> {
287 static T big(T x) { return EndianInt::swap(x); }
288 static T little(T x) { return x; }
291 #elif __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__
294 struct EndianInt : public detail::EndianIntBase<T> {
296 static T big(T x) { return x; }
297 static T little(T x) { return EndianInt::swap(x); }
301 # error Your machine uses a weird endianness!
302 #endif /* __BYTE_ORDER__ */
304 } // namespace detail
306 // big* convert between native and big-endian representations
307 // little* convert between native and little-endian representations
308 // swap* convert between big-endian and little-endian representations
310 // ntohs, htons == big16
311 // ntohl, htonl == big32
312 #define FB_GEN1(fn, t, sz) \
313 static t fn##sz(t x) { return fn<t>(x); } \
315 #define FB_GEN2(t, sz) \
316 FB_GEN1(swap, t, sz) \
317 FB_GEN1(big, t, sz) \
318 FB_GEN1(little, t, sz)
321 FB_GEN2(uint##sz##_t, sz) \
322 FB_GEN2(int##sz##_t, sz)
326 enum class Order : uint8_t {
331 static constexpr Order order =
332 #if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
334 #elif __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__
337 # error Your machine uses a weird endianness!
338 #endif /* __BYTE_ORDER__ */
340 template <class T> static T swap(T x) {
341 return detail::EndianInt<T>::swap(x);
343 template <class T> static T big(T x) {
344 return detail::EndianInt<T>::big(x);
346 template <class T> static T little(T x) {
347 return detail::EndianInt<T>::little(x);
350 #if !defined(__ANDROID__)
363 * Fast bit iteration facility.
367 template <class BaseIter> class BitIterator;
368 template <class BaseIter>
369 BitIterator<BaseIter> findFirstSet(BitIterator<BaseIter>,
370 BitIterator<BaseIter>);
372 * Wrapper around an iterator over an integer type that iterates
373 * over its underlying bits in LSb to MSb order.
375 * BitIterator models the same iterator concepts as the base iterator.
377 template <class BaseIter>
379 : public bititerator_detail::BitIteratorBase<BaseIter>::type {
382 * Return the number of bits in an element of the underlying iterator.
384 static unsigned int bitsPerBlock() {
385 return std::numeric_limits<
386 typename std::make_unsigned<
387 typename std::iterator_traits<BaseIter>::value_type
393 * Construct a BitIterator that points at a given bit offset (default 0)
396 #pragma GCC diagnostic push // bitOffset shadows a member
397 #pragma GCC diagnostic ignored "-Wshadow"
398 explicit BitIterator(const BaseIter& iter, size_t bitOffset=0)
399 : bititerator_detail::BitIteratorBase<BaseIter>::type(iter),
400 bitOffset_(bitOffset) {
401 assert(bitOffset_ < bitsPerBlock());
403 #pragma GCC diagnostic pop
405 size_t bitOffset() const {
409 void advanceToNextBlock() {
411 ++this->base_reference();
414 BitIterator& operator=(const BaseIter& other) {
415 this->~BitIterator();
416 new (this) BitIterator(other);
421 friend class boost::iterator_core_access;
422 friend BitIterator findFirstSet<>(BitIterator, BitIterator);
424 typedef bititerator_detail::BitReference<
425 typename std::iterator_traits<BaseIter>::reference,
426 typename std::iterator_traits<BaseIter>::value_type
429 void advanceInBlock(size_t n) {
431 assert(bitOffset_ < bitsPerBlock());
434 BitRef dereference() const {
435 return BitRef(*this->base_reference(), bitOffset_);
438 void advance(ssize_t n) {
439 size_t bpb = bitsPerBlock();
440 ssize_t blocks = n / bpb;
441 bitOffset_ += n % bpb;
442 if (bitOffset_ >= bpb) {
446 this->base_reference() += blocks;
450 if (++bitOffset_ == bitsPerBlock()) {
451 advanceToNextBlock();
456 if (bitOffset_-- == 0) {
457 bitOffset_ = bitsPerBlock() - 1;
458 --this->base_reference();
462 bool equal(const BitIterator& other) const {
463 return (bitOffset_ == other.bitOffset_ &&
464 this->base_reference() == other.base_reference());
467 ssize_t distance_to(const BitIterator& other) const {
469 (other.base_reference() - this->base_reference()) * bitsPerBlock() +
470 other.bitOffset_ - bitOffset_;
473 unsigned int bitOffset_;
477 * Helper function, so you can write
478 * auto bi = makeBitIterator(container.begin());
480 template <class BaseIter>
481 BitIterator<BaseIter> makeBitIterator(const BaseIter& iter) {
482 return BitIterator<BaseIter>(iter);
487 * Find first bit set in a range of bit iterators.
488 * 4.5x faster than the obvious std::find(begin, end, true);
490 template <class BaseIter>
491 BitIterator<BaseIter> findFirstSet(BitIterator<BaseIter> begin,
492 BitIterator<BaseIter> end) {
493 // shortcut to avoid ugly static_cast<>
494 static const typename BaseIter::value_type one = 1;
496 while (begin.base() != end.base()) {
497 typename BaseIter::value_type v = *begin.base();
498 // mask out the bits that don't matter (< begin.bitOffset)
499 v &= ~((one << begin.bitOffset()) - 1);
500 size_t firstSet = findFirstSet(v);
502 --firstSet; // now it's 0-based
503 assert(firstSet >= begin.bitOffset());
504 begin.advanceInBlock(firstSet - begin.bitOffset());
507 begin.advanceToNextBlock();
510 // now begin points to the same block as end
511 if (end.bitOffset() != 0) { // assume end is dereferenceable
512 typename BaseIter::value_type v = *begin.base();
513 // mask out the bits that don't matter (< begin.bitOffset)
514 v &= ~((one << begin.bitOffset()) - 1);
515 // mask out the bits that don't matter (>= end.bitOffset)
516 v &= (one << end.bitOffset()) - 1;
517 size_t firstSet = findFirstSet(v);
519 --firstSet; // now it's 0-based
520 assert(firstSet >= begin.bitOffset());
521 begin.advanceInBlock(firstSet - begin.bitOffset());
530 template <class T, class Enable=void> struct Unaligned;
533 * Representation of an unaligned value of a POD type.
539 typename std::enable_if<std::is_pod<T>::value>::type> {
540 Unaligned() = default; // uninitialized
541 /* implicit */ Unaligned(T v) : value(v) { }
547 * Read an unaligned value of type T and return it.
550 inline T loadUnaligned(const void* p) {
551 static_assert(sizeof(Unaligned<T>) == sizeof(T), "Invalid unaligned size");
552 static_assert(alignof(Unaligned<T>) == 1, "Invalid alignment");
553 return static_cast<const Unaligned<T>*>(p)->value;
557 * Write an unaligned value of type T.
560 inline void storeUnaligned(void* p, T value) {
561 static_assert(sizeof(Unaligned<T>) == sizeof(T), "Invalid unaligned size");
562 static_assert(alignof(Unaligned<T>) == 1, "Invalid alignment");
563 new (p) Unaligned<T>(value);
568 #endif /* FOLLY_BITS_H_ */