1 //===-- llvm/ADT/Hashing.h - Utilities for hashing --------------*- C++ -*-===//
2 //
3 //                     The LLVM Compiler Infrastructure
4 //
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
7 //
8 //===----------------------------------------------------------------------===//
9 //
10 // This file implements the newly proposed standard C++ interfaces for hashing
11 // arbitrary data and building hash functions for user-defined types. This
12 // interface was originally proposed in N3333[1] and is currently under review
13 // for inclusion in a future TR and/or standard.
14 //
15 // The primary interfaces provide are comprised of one type and three functions:
16 //
17 //  -- 'hash_code' class is an opaque type representing the hash code for some
18 //     data. It is the intended product of hashing, and can be used to implement
19 //     hash tables, checksumming, and other common uses of hashes. It is not an
20 //     integer type (although it can be converted to one) because it is risky
21 //     to assume much about the internals of a hash_code. In particular, each
22 //     execution of the program has a high probability of producing a different
23 //     hash_code for a given input. Thus their values are not stable to save or
24 //     persist, and should only be used during the execution for the
25 //     construction of hashing datastructures.
26 //
27 //  -- 'hash_value' is a function designed to be overloaded for each
28 //     user-defined type which wishes to be used within a hashing context. It
29 //     should be overloaded within the user-defined type's namespace and found
30 //     via ADL. Overloads for primitive types are provided by this library.
31 //
32 //  -- 'hash_combine' and 'hash_combine_range' are functions designed to aid
33 //      programmers in easily and intuitively combining a set of data into
34 //      a single hash_code for their object. They should only logically be used
35 //      within the implementation of a 'hash_value' routine or similar context.
36 //
37 // Note that 'hash_combine_range' contains very special logic for hashing
38 // a contiguous array of integers or pointers. This logic is *extremely* fast,
39 // on a modern Intel "Gainestown" Xeon (Nehalem uarch) @2.2 GHz, these were
40 // benchmarked at over 6.5 GiB/s for large keys, and <20 cycles/hash for keys
41 // under 32-bytes.
42 //
43 //===----------------------------------------------------------------------===//
44 
45 #ifndef LLVM_ADT_HASHING_H
46 #define LLVM_ADT_HASHING_H
47 
48 #include "llvm/Support/DataTypes.h"
49 #include "llvm/Support/Host.h"
50 #include "llvm/Support/SwapByteOrder.h"
51 #include "llvm/Support/type_traits.h"
52 #include <algorithm>
53 #include <cassert>
54 #include <cstring>
55 #include <iterator>
56 #include <string>
57 #include <utility>
58 
59 namespace llvm {
60 
61 /// \brief An opaque object representing a hash code.
62 ///
63 /// This object represents the result of hashing some entity. It is intended to
64 /// be used to implement hashtables or other hashing-based data structures.
65 /// While it wraps and exposes a numeric value, this value should not be
66 /// trusted to be stable or predictable across processes or executions.
67 ///
68 /// In order to obtain the hash_code for an object 'x':
69 /// \code
70 ///   using llvm::hash_value;
71 ///   llvm::hash_code code = hash_value(x);
72 /// \endcode
73 class hash_code {
74   size_t value;
75 
76 public:
77   /// \brief Default construct a hash_code.
78   /// Note that this leaves the value uninitialized.
79   hash_code() = default;
80 
81   /// \brief Form a hash code directly from a numerical value.
hash_code(size_t value)82   hash_code(size_t value) : value(value) {}
83 
84   /// \brief Convert the hash code to its numerical value for use.
size_t()85   /*explicit*/ operator size_t() const { return value; }
86 
87   friend bool operator==(const hash_code &lhs, const hash_code &rhs) {
88     return lhs.value == rhs.value;
89   }
90   friend bool operator!=(const hash_code &lhs, const hash_code &rhs) {
91     return lhs.value != rhs.value;
92   }
93 
94   /// \brief Allow a hash_code to be directly run through hash_value.
hash_value(const hash_code & code)95   friend size_t hash_value(const hash_code &code) { return code.value; }
96 };
97 
98 /// \brief Compute a hash_code for any integer value.
99 ///
100 /// Note that this function is intended to compute the same hash_code for
101 /// a particular value without regard to the pre-promotion type. This is in
102 /// contrast to hash_combine which may produce different hash_codes for
103 /// differing argument types even if they would implicit promote to a common
104 /// type without changing the value.
105 template <typename T>
106 typename std::enable_if<is_integral_or_enum<T>::value, hash_code>::type
107 hash_value(T value);
108 
109 /// \brief Compute a hash_code for a pointer's address.
110 ///
111 /// N.B.: This hashes the *address*. Not the value and not the type.
112 template <typename T> hash_code hash_value(const T *ptr);
113 
114 /// \brief Compute a hash_code for a pair of objects.
115 template <typename T, typename U>
116 hash_code hash_value(const std::pair<T, U> &arg);
117 
118 /// \brief Compute a hash_code for a standard string.
119 template <typename T>
120 hash_code hash_value(const std::basic_string<T> &arg);
121 
122 
123 /// \brief Override the execution seed with a fixed value.
124 ///
125 /// This hashing library uses a per-execution seed designed to change on each
126 /// run with high probability in order to ensure that the hash codes are not
127 /// attackable and to ensure that output which is intended to be stable does
128 /// not rely on the particulars of the hash codes produced.
129 ///
130 /// That said, there are use cases where it is important to be able to
131 /// reproduce *exactly* a specific behavior. To that end, we provide a function
132 /// which will forcibly set the seed to a fixed value. This must be done at the
133 /// start of the program, before any hashes are computed. Also, it cannot be
134 /// undone. This makes it thread-hostile and very hard to use outside of
135 /// immediately on start of a simple program designed for reproducible
136 /// behavior.
137 void set_fixed_execution_hash_seed(size_t fixed_value);
138 
139 
140 // All of the implementation details of actually computing the various hash
141 // code values are held within this namespace. These routines are included in
142 // the header file mainly to allow inlining and constant propagation.
143 namespace hashing {
144 namespace detail {
145 
fetch64(const char * p)146 inline uint64_t fetch64(const char *p) {
147   uint64_t result;
148   memcpy(&result, p, sizeof(result));
149   if (sys::IsBigEndianHost)
150     sys::swapByteOrder(result);
151   return result;
152 }
153 
fetch32(const char * p)154 inline uint32_t fetch32(const char *p) {
155   uint32_t result;
156   memcpy(&result, p, sizeof(result));
157   if (sys::IsBigEndianHost)
158     sys::swapByteOrder(result);
159   return result;
160 }
161 
162 /// Some primes between 2^63 and 2^64 for various uses.
163 static const uint64_t k0 = 0xc3a5c85c97cb3127ULL;
164 static const uint64_t k1 = 0xb492b66fbe98f273ULL;
165 static const uint64_t k2 = 0x9ae16a3b2f90404fULL;
166 static const uint64_t k3 = 0xc949d7c7509e6557ULL;
167 
168 /// \brief Bitwise right rotate.
169 /// Normally this will compile to a single instruction, especially if the
170 /// shift is a manifest constant.
rotate(uint64_t val,size_t shift)171 inline uint64_t rotate(uint64_t val, size_t shift) {
172   // Avoid shifting by 64: doing so yields an undefined result.
173   return shift == 0 ? val : ((val >> shift) | (val << (64 - shift)));
174 }
175 
shift_mix(uint64_t val)176 inline uint64_t shift_mix(uint64_t val) {
177   return val ^ (val >> 47);
178 }
179 
hash_16_bytes(uint64_t low,uint64_t high)180 inline uint64_t hash_16_bytes(uint64_t low, uint64_t high) {
181   // Murmur-inspired hashing.
182   const uint64_t kMul = 0x9ddfea08eb382d69ULL;
183   uint64_t a = (low ^ high) * kMul;
184   a ^= (a >> 47);
185   uint64_t b = (high ^ a) * kMul;
186   b ^= (b >> 47);
187   b *= kMul;
188   return b;
189 }
190 
hash_1to3_bytes(const char * s,size_t len,uint64_t seed)191 inline uint64_t hash_1to3_bytes(const char *s, size_t len, uint64_t seed) {
192   uint8_t a = s[0];
193   uint8_t b = s[len >> 1];
194   uint8_t c = s[len - 1];
195   uint32_t y = static_cast<uint32_t>(a) + (static_cast<uint32_t>(b) << 8);
196   uint32_t z = len + (static_cast<uint32_t>(c) << 2);
197   return shift_mix(y * k2 ^ z * k3 ^ seed) * k2;
198 }
199 
hash_4to8_bytes(const char * s,size_t len,uint64_t seed)200 inline uint64_t hash_4to8_bytes(const char *s, size_t len, uint64_t seed) {
201   uint64_t a = fetch32(s);
202   return hash_16_bytes(len + (a << 3), seed ^ fetch32(s + len - 4));
203 }
204 
hash_9to16_bytes(const char * s,size_t len,uint64_t seed)205 inline uint64_t hash_9to16_bytes(const char *s, size_t len, uint64_t seed) {
206   uint64_t a = fetch64(s);
207   uint64_t b = fetch64(s + len - 8);
208   return hash_16_bytes(seed ^ a, rotate(b + len, len)) ^ b;
209 }
210 
hash_17to32_bytes(const char * s,size_t len,uint64_t seed)211 inline uint64_t hash_17to32_bytes(const char *s, size_t len, uint64_t seed) {
212   uint64_t a = fetch64(s) * k1;
213   uint64_t b = fetch64(s + 8);
214   uint64_t c = fetch64(s + len - 8) * k2;
215   uint64_t d = fetch64(s + len - 16) * k0;
216   return hash_16_bytes(rotate(a - b, 43) + rotate(c ^ seed, 30) + d,
217                        a + rotate(b ^ k3, 20) - c + len + seed);
218 }
219 
hash_33to64_bytes(const char * s,size_t len,uint64_t seed)220 inline uint64_t hash_33to64_bytes(const char *s, size_t len, uint64_t seed) {
221   uint64_t z = fetch64(s + 24);
222   uint64_t a = fetch64(s) + (len + fetch64(s + len - 16)) * k0;
223   uint64_t b = rotate(a + z, 52);
224   uint64_t c = rotate(a, 37);
225   a += fetch64(s + 8);
226   c += rotate(a, 7);
227   a += fetch64(s + 16);
228   uint64_t vf = a + z;
229   uint64_t vs = b + rotate(a, 31) + c;
230   a = fetch64(s + 16) + fetch64(s + len - 32);
231   z = fetch64(s + len - 8);
232   b = rotate(a + z, 52);
233   c = rotate(a, 37);
234   a += fetch64(s + len - 24);
235   c += rotate(a, 7);
236   a += fetch64(s + len - 16);
237   uint64_t wf = a + z;
238   uint64_t ws = b + rotate(a, 31) + c;
239   uint64_t r = shift_mix((vf + ws) * k2 + (wf + vs) * k0);
240   return shift_mix((seed ^ (r * k0)) + vs) * k2;
241 }
242 
hash_short(const char * s,size_t length,uint64_t seed)243 inline uint64_t hash_short(const char *s, size_t length, uint64_t seed) {
244   if (length >= 4 && length <= 8)
245     return hash_4to8_bytes(s, length, seed);
246   if (length > 8 && length <= 16)
247     return hash_9to16_bytes(s, length, seed);
248   if (length > 16 && length <= 32)
249     return hash_17to32_bytes(s, length, seed);
250   if (length > 32)
251     return hash_33to64_bytes(s, length, seed);
252   if (length != 0)
253     return hash_1to3_bytes(s, length, seed);
254 
255   return k2 ^ seed;
256 }
257 
258 /// \brief The intermediate state used during hashing.
259 /// Currently, the algorithm for computing hash codes is based on CityHash and
260 /// keeps 56 bytes of arbitrary state.
261 struct hash_state {
262   uint64_t h0, h1, h2, h3, h4, h5, h6;
263 
264   /// \brief Create a new hash_state structure and initialize it based on the
265   /// seed and the first 64-byte chunk.
266   /// This effectively performs the initial mix.
createhash_state267   static hash_state create(const char *s, uint64_t seed) {
268     hash_state state = {
269       0, seed, hash_16_bytes(seed, k1), rotate(seed ^ k1, 49),
270       seed * k1, shift_mix(seed), 0 };
271     state.h6 = hash_16_bytes(state.h4, state.h5);
272     state.mix(s);
273     return state;
274   }
275 
276   /// \brief Mix 32-bytes from the input sequence into the 16-bytes of 'a'
277   /// and 'b', including whatever is already in 'a' and 'b'.
mix_32_byteshash_state278   static void mix_32_bytes(const char *s, uint64_t &a, uint64_t &b) {
279     a += fetch64(s);
280     uint64_t c = fetch64(s + 24);
281     b = rotate(b + a + c, 21);
282     uint64_t d = a;
283     a += fetch64(s + 8) + fetch64(s + 16);
284     b += rotate(a, 44) + d;
285     a += c;
286   }
287 
288   /// \brief Mix in a 64-byte buffer of data.
289   /// We mix all 64 bytes even when the chunk length is smaller, but we
290   /// record the actual length.
mixhash_state291   void mix(const char *s) {
292     h0 = rotate(h0 + h1 + h3 + fetch64(s + 8), 37) * k1;
293     h1 = rotate(h1 + h4 + fetch64(s + 48), 42) * k1;
294     h0 ^= h6;
295     h1 += h3 + fetch64(s + 40);
296     h2 = rotate(h2 + h5, 33) * k1;
297     h3 = h4 * k1;
298     h4 = h0 + h5;
299     mix_32_bytes(s, h3, h4);
300     h5 = h2 + h6;
301     h6 = h1 + fetch64(s + 16);
302     mix_32_bytes(s + 32, h5, h6);
303     std::swap(h2, h0);
304   }
305 
306   /// \brief Compute the final 64-bit hash code value based on the current
307   /// state and the length of bytes hashed.
finalizehash_state308   uint64_t finalize(size_t length) {
309     return hash_16_bytes(hash_16_bytes(h3, h5) + shift_mix(h1) * k1 + h2,
310                          hash_16_bytes(h4, h6) + shift_mix(length) * k1 + h0);
311   }
312 };
313 
314 
315 /// \brief A global, fixed seed-override variable.
316 ///
317 /// This variable can be set using the \see llvm::set_fixed_execution_seed
318 /// function. See that function for details. Do not, under any circumstances,
319 /// set or read this variable.
320 extern size_t fixed_seed_override;
321 
get_execution_seed()322 inline size_t get_execution_seed() {
323   // FIXME: This needs to be a per-execution seed. This is just a placeholder
324   // implementation. Switching to a per-execution seed is likely to flush out
325   // instability bugs and so will happen as its own commit.
326   //
327   // However, if there is a fixed seed override set the first time this is
328   // called, return that instead of the per-execution seed.
329   const uint64_t seed_prime = 0xff51afd7ed558ccdULL;
330   static size_t seed = fixed_seed_override ? fixed_seed_override
331                                            : (size_t)seed_prime;
332   return seed;
333 }
334 
335 
336 /// \brief Trait to indicate whether a type's bits can be hashed directly.
337 ///
338 /// A type trait which is true if we want to combine values for hashing by
339 /// reading the underlying data. It is false if values of this type must
340 /// first be passed to hash_value, and the resulting hash_codes combined.
341 //
342 // FIXME: We want to replace is_integral_or_enum and is_pointer here with
343 // a predicate which asserts that comparing the underlying storage of two
344 // values of the type for equality is equivalent to comparing the two values
345 // for equality. For all the platforms we care about, this holds for integers
346 // and pointers, but there are platforms where it doesn't and we would like to
347 // support user-defined types which happen to satisfy this property.
348 template <typename T> struct is_hashable_data
349   : std::integral_constant<bool, ((is_integral_or_enum<T>::value ||
350                                    std::is_pointer<T>::value) &&
351                                   64 % sizeof(T) == 0)> {};
352 
353 // Special case std::pair to detect when both types are viable and when there
354 // is no alignment-derived padding in the pair. This is a bit of a lie because
355 // std::pair isn't truly POD, but it's close enough in all reasonable
356 // implementations for our use case of hashing the underlying data.
357 template <typename T, typename U> struct is_hashable_data<std::pair<T, U> >
358   : std::integral_constant<bool, (is_hashable_data<T>::value &&
359                                   is_hashable_data<U>::value &&
360                                   (sizeof(T) + sizeof(U)) ==
361                                    sizeof(std::pair<T, U>))> {};
362 
363 /// \brief Helper to get the hashable data representation for a type.
364 /// This variant is enabled when the type itself can be used.
365 template <typename T>
366 typename std::enable_if<is_hashable_data<T>::value, T>::type
367 get_hashable_data(const T &value) {
368   return value;
369 }
370 /// \brief Helper to get the hashable data representation for a type.
371 /// This variant is enabled when we must first call hash_value and use the
372 /// result as our data.
373 template <typename T>
374 typename std::enable_if<!is_hashable_data<T>::value, size_t>::type
375 get_hashable_data(const T &value) {
376   using ::llvm::hash_value;
377   return hash_value(value);
378 }
379 
380 /// \brief Helper to store data from a value into a buffer and advance the
381 /// pointer into that buffer.
382 ///
383 /// This routine first checks whether there is enough space in the provided
384 /// buffer, and if not immediately returns false. If there is space, it
385 /// copies the underlying bytes of value into the buffer, advances the
386 /// buffer_ptr past the copied bytes, and returns true.
387 template <typename T>
388 bool store_and_advance(char *&buffer_ptr, char *buffer_end, const T& value,
389                        size_t offset = 0) {
390   size_t store_size = sizeof(value) - offset;
391   if (buffer_ptr + store_size > buffer_end)
392     return false;
393   const char *value_data = reinterpret_cast<const char *>(&value);
394   memcpy(buffer_ptr, value_data + offset, store_size);
395   buffer_ptr += store_size;
396   return true;
397 }
398 
399 /// \brief Implement the combining of integral values into a hash_code.
400 ///
401 /// This overload is selected when the value type of the iterator is
402 /// integral. Rather than computing a hash_code for each object and then
403 /// combining them, this (as an optimization) directly combines the integers.
404 template <typename InputIteratorT>
405 hash_code hash_combine_range_impl(InputIteratorT first, InputIteratorT last) {
406   const size_t seed = get_execution_seed();
407   char buffer[64], *buffer_ptr = buffer;
408   char *const buffer_end = std::end(buffer);
409   while (first != last && store_and_advance(buffer_ptr, buffer_end,
410                                             get_hashable_data(*first)))
411     ++first;
412   if (first == last)
413     return hash_short(buffer, buffer_ptr - buffer, seed);
414   assert(buffer_ptr == buffer_end);
415 
416   hash_state state = state.create(buffer, seed);
417   size_t length = 64;
418   while (first != last) {
419     // Fill up the buffer. We don't clear it, which re-mixes the last round
420     // when only a partial 64-byte chunk is left.
421     buffer_ptr = buffer;
422     while (first != last && store_and_advance(buffer_ptr, buffer_end,
423                                               get_hashable_data(*first)))
424       ++first;
425 
426     // Rotate the buffer if we did a partial fill in order to simulate doing
427     // a mix of the last 64-bytes. That is how the algorithm works when we
428     // have a contiguous byte sequence, and we want to emulate that here.
429     std::rotate(buffer, buffer_ptr, buffer_end);
430 
431     // Mix this chunk into the current state.
432     state.mix(buffer);
433     length += buffer_ptr - buffer;
434   };
435 
436   return state.finalize(length);
437 }
438 
439 /// \brief Implement the combining of integral values into a hash_code.
440 ///
441 /// This overload is selected when the value type of the iterator is integral
442 /// and when the input iterator is actually a pointer. Rather than computing
443 /// a hash_code for each object and then combining them, this (as an
444 /// optimization) directly combines the integers. Also, because the integers
445 /// are stored in contiguous memory, this routine avoids copying each value
446 /// and directly reads from the underlying memory.
447 template <typename ValueT>
448 typename std::enable_if<is_hashable_data<ValueT>::value, hash_code>::type
449 hash_combine_range_impl(ValueT *first, ValueT *last) {
450   const size_t seed = get_execution_seed();
451   const char *s_begin = reinterpret_cast<const char *>(first);
452   const char *s_end = reinterpret_cast<const char *>(last);
453   const size_t length = std::distance(s_begin, s_end);
454   if (length <= 64)
455     return hash_short(s_begin, length, seed);
456 
457   const char *s_aligned_end = s_begin + (length & ~63);
458   hash_state state = state.create(s_begin, seed);
459   s_begin += 64;
460   while (s_begin != s_aligned_end) {
461     state.mix(s_begin);
462     s_begin += 64;
463   }
464   if (length & 63)
465     state.mix(s_end - 64);
466 
467   return state.finalize(length);
468 }
469 
470 } // namespace detail
471 } // namespace hashing
472 
473 
474 /// \brief Compute a hash_code for a sequence of values.
475 ///
476 /// This hashes a sequence of values. It produces the same hash_code as
477 /// 'hash_combine(a, b, c, ...)', but can run over arbitrary sized sequences
478 /// and is significantly faster given pointers and types which can be hashed as
479 /// a sequence of bytes.
480 template <typename InputIteratorT>
481 hash_code hash_combine_range(InputIteratorT first, InputIteratorT last) {
482   return ::llvm::hashing::detail::hash_combine_range_impl(first, last);
483 }
484 
485 
486 // Implementation details for hash_combine.
487 namespace hashing {
488 namespace detail {
489 
490 /// \brief Helper class to manage the recursive combining of hash_combine
491 /// arguments.
492 ///
493 /// This class exists to manage the state and various calls involved in the
494 /// recursive combining of arguments used in hash_combine. It is particularly
495 /// useful at minimizing the code in the recursive calls to ease the pain
496 /// caused by a lack of variadic functions.
497 struct hash_combine_recursive_helper {
498   char buffer[64];
499   hash_state state;
500   const size_t seed;
501 
502 public:
503   /// \brief Construct a recursive hash combining helper.
504   ///
505   /// This sets up the state for a recursive hash combine, including getting
506   /// the seed and buffer setup.
507   hash_combine_recursive_helper()
508     : seed(get_execution_seed()) {}
509 
510   /// \brief Combine one chunk of data into the current in-flight hash.
511   ///
512   /// This merges one chunk of data into the hash. First it tries to buffer
513   /// the data. If the buffer is full, it hashes the buffer into its
514   /// hash_state, empties it, and then merges the new chunk in. This also
515   /// handles cases where the data straddles the end of the buffer.
516   template <typename T>
517   char *combine_data(size_t &length, char *buffer_ptr, char *buffer_end, T data) {
518     if (!store_and_advance(buffer_ptr, buffer_end, data)) {
519       // Check for skew which prevents the buffer from being packed, and do
520       // a partial store into the buffer to fill it. This is only a concern
521       // with the variadic combine because that formation can have varying
522       // argument types.
523       size_t partial_store_size = buffer_end - buffer_ptr;
524       memcpy(buffer_ptr, &data, partial_store_size);
525 
526       // If the store fails, our buffer is full and ready to hash. We have to
527       // either initialize the hash state (on the first full buffer) or mix
528       // this buffer into the existing hash state. Length tracks the *hashed*
529       // length, not the buffered length.
530       if (length == 0) {
531         state = state.create(buffer, seed);
532         length = 64;
533       } else {
534         // Mix this chunk into the current state and bump length up by 64.
535         state.mix(buffer);
536         length += 64;
537       }
538       // Reset the buffer_ptr to the head of the buffer for the next chunk of
539       // data.
540       buffer_ptr = buffer;
541 
542       // Try again to store into the buffer -- this cannot fail as we only
543       // store types smaller than the buffer.
544       if (!store_and_advance(buffer_ptr, buffer_end, data,
545                              partial_store_size))
546         abort();
547     }
548     return buffer_ptr;
549   }
550 
551   /// \brief Recursive, variadic combining method.
552   ///
553   /// This function recurses through each argument, combining that argument
554   /// into a single hash.
555   template <typename T, typename ...Ts>
556   hash_code combine(size_t length, char *buffer_ptr, char *buffer_end,
557                     const T &arg, const Ts &...args) {
558     buffer_ptr = combine_data(length, buffer_ptr, buffer_end, get_hashable_data(arg));
559 
560     // Recurse to the next argument.
561     return combine(length, buffer_ptr, buffer_end, args...);
562   }
563 
564   /// \brief Base case for recursive, variadic combining.
565   ///
566   /// The base case when combining arguments recursively is reached when all
567   /// arguments have been handled. It flushes the remaining buffer and
568   /// constructs a hash_code.
569   hash_code combine(size_t length, char *buffer_ptr, char *buffer_end) {
570     // Check whether the entire set of values fit in the buffer. If so, we'll
571     // use the optimized short hashing routine and skip state entirely.
572     if (length == 0)
573       return hash_short(buffer, buffer_ptr - buffer, seed);
574 
575     // Mix the final buffer, rotating it if we did a partial fill in order to
576     // simulate doing a mix of the last 64-bytes. That is how the algorithm
577     // works when we have a contiguous byte sequence, and we want to emulate
578     // that here.
579     std::rotate(buffer, buffer_ptr, buffer_end);
580 
581     // Mix this chunk into the current state.
582     state.mix(buffer);
583     length += buffer_ptr - buffer;
584 
585     return state.finalize(length);
586   }
587 };
588 
589 } // namespace detail
590 } // namespace hashing
591 
592 /// \brief Combine values into a single hash_code.
593 ///
594 /// This routine accepts a varying number of arguments of any type. It will
595 /// attempt to combine them into a single hash_code. For user-defined types it
596 /// attempts to call a \see hash_value overload (via ADL) for the type. For
597 /// integer and pointer types it directly combines their data into the
598 /// resulting hash_code.
599 ///
600 /// The result is suitable for returning from a user's hash_value
601 /// *implementation* for their user-defined type. Consumers of a type should
602 /// *not* call this routine, they should instead call 'hash_value'.
603 template <typename ...Ts> hash_code hash_combine(const Ts &...args) {
604   // Recursively hash each argument using a helper class.
605   ::llvm::hashing::detail::hash_combine_recursive_helper helper;
606   return helper.combine(0, helper.buffer, helper.buffer + 64, args...);
607 }
608 
609 // Implementation details for implementations of hash_value overloads provided
610 // here.
611 namespace hashing {
612 namespace detail {
613 
614 /// \brief Helper to hash the value of a single integer.
615 ///
616 /// Overloads for smaller integer types are not provided to ensure consistent
617 /// behavior in the presence of integral promotions. Essentially,
618 /// "hash_value('4')" and "hash_value('0' + 4)" should be the same.
619 inline hash_code hash_integer_value(uint64_t value) {
620   // Similar to hash_4to8_bytes but using a seed instead of length.
621   const uint64_t seed = get_execution_seed();
622   const char *s = reinterpret_cast<const char *>(&value);
623   const uint64_t a = fetch32(s);
624   return hash_16_bytes(seed + (a << 3), fetch32(s + 4));
625 }
626 
627 } // namespace detail
628 } // namespace hashing
629 
630 // Declared and documented above, but defined here so that any of the hashing
631 // infrastructure is available.
632 template <typename T>
633 typename std::enable_if<is_integral_or_enum<T>::value, hash_code>::type
634 hash_value(T value) {
635   return ::llvm::hashing::detail::hash_integer_value(value);
636 }
637 
638 // Declared and documented above, but defined here so that any of the hashing
639 // infrastructure is available.
640 template <typename T> hash_code hash_value(const T *ptr) {
641   return ::llvm::hashing::detail::hash_integer_value(
642     reinterpret_cast<uintptr_t>(ptr));
643 }
644 
645 // Declared and documented above, but defined here so that any of the hashing
646 // infrastructure is available.
647 template <typename T, typename U>
648 hash_code hash_value(const std::pair<T, U> &arg) {
649   return hash_combine(arg.first, arg.second);
650 }
651 
652 // Declared and documented above, but defined here so that any of the hashing
653 // infrastructure is available.
654 template <typename T>
655 hash_code hash_value(const std::basic_string<T> &arg) {
656   return hash_combine_range(arg.begin(), arg.end());
657 }
658 
659 } // namespace llvm
660 
661 #endif
662