DWARF parsing and writing support using LLVM (#2520)

This imports LLVM code for DWARF handling. That code has the
Apache 2 license like us. It's also the same code used to
emit DWARF in the common toolchain, so it seems like a safe choice.

This adds two passes: --dwarfdump which runs the same code LLVM
runs for llvm-dwarfdump. This shows we can parse it ok, and will
be useful for debugging. And --dwarfupdate writes out the DWARF
sections (unchanged from what we read, so it just roundtrips - for
updating we need #2515).

This puts LLVM in thirdparty which is added here.

All the LLVM code is behind USE_LLVM_DWARF, which is on
by default, but off in JS for now, as it increases code size by 20%.

This current approach imports the LLVM files directly. This is not
how they are intended to be used, so it required a bunch of
local changes - more than I expected actually, for the platform-specific
stuff. For now this seems to work, so it may be good enough, but
in the long term we may want to switch to linking against libllvm.
A downside to doing that is that binaryen users would need to
have an LLVM build, and even in the waterfall builds we'd have a
problem - while we ship LLVM there anyhow, we constantly update
it, which means that binaryen would need to be on latest llvm all
the time too (which otherwise, given DWARF is quite stable, we
might not need to constantly update).

An even larger issue is that as I did this work I learned about how
DWARF works in LLVM, and while the reading code is easy to
reuse, the writing code is trickier. The main code path is heavily
integrated with the MC layer, which we don't have - we might want
to create a "fake MC layer" for that, but it sounds hard. Instead,
there is the YAML path which is used mostly for testing, and which
can convert DWARF to and from YAML and from binary. Using
the non-YAML parts there, we can convert binary DWARF to
the YAML layer's nice Info data, then convert that to binary. This
works, however, this is not the path LLVM uses normally, and it
supports only some basic DWARF sections - I had to add ranges
support, in fact. So if we need more complex things, we may end
up needing to use the MC layer approach, or consider some other
DWARF library. However, hopefully that should not affect the core
binaryen code which just calls a library for DWARF stuff.

Helps #2400

1 files changed, 659 insertions, 0 deletions

diff --git a/third_party/llvm-project/include/llvm/ADT/Hashing.h b/third_party/llvm-project/include/llvm/ADT/Hashing.h
new file mode 100644
index 000000000..adcc5cf54
--- /dev/null
+++ b/third_party/llvm-project/include/llvm/ADT/Hashing.h
@@ -0,0 +1,659 @@
+//===-- llvm/ADT/Hashing.h - Utilities for hashing --------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// This file implements the newly proposed standard C++ interfaces for hashing
+// arbitrary data and building hash functions for user-defined types. This
+// interface was originally proposed in N3333[1] and is currently under review
+// for inclusion in a future TR and/or standard.
+//
+// The primary interfaces provide are comprised of one type and three functions:
+//
+//  -- 'hash_code' class is an opaque type representing the hash code for some
+//     data. It is the intended product of hashing, and can be used to implement
+//     hash tables, checksumming, and other common uses of hashes. It is not an
+//     integer type (although it can be converted to one) because it is risky
+//     to assume much about the internals of a hash_code. In particular, each
+//     execution of the program has a high probability of producing a different
+//     hash_code for a given input. Thus their values are not stable to save or
+//     persist, and should only be used during the execution for the
+//     construction of hashing datastructures.
+//
+//  -- 'hash_value' is a function designed to be overloaded for each
+//     user-defined type which wishes to be used within a hashing context. It
+//     should be overloaded within the user-defined type's namespace and found
+//     via ADL. Overloads for primitive types are provided by this library.
+//
+//  -- 'hash_combine' and 'hash_combine_range' are functions designed to aid
+//      programmers in easily and intuitively combining a set of data into
+//      a single hash_code for their object. They should only logically be used
+//      within the implementation of a 'hash_value' routine or similar context.
+//
+// Note that 'hash_combine_range' contains very special logic for hashing
+// a contiguous array of integers or pointers. This logic is *extremely* fast,
+// on a modern Intel "Gainestown" Xeon (Nehalem uarch) @2.2 GHz, these were
+// benchmarked at over 6.5 GiB/s for large keys, and <20 cycles/hash for keys
+// under 32-bytes.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_ADT_HASHING_H
+#define LLVM_ADT_HASHING_H
+
+#include "llvm/Support/DataTypes.h"
+#include "llvm/Support/ErrorHandling.h"
+#include "llvm/Support/SwapByteOrder.h"
+#include "llvm/Support/type_traits.h"
+#include <algorithm>
+#include <cassert>
+#include <cstring>
+#include <string>
+#include <utility>
+
+namespace llvm {
+
+/// An opaque object representing a hash code.
+///
+/// This object represents the result of hashing some entity. It is intended to
+/// be used to implement hashtables or other hashing-based data structures.
+/// While it wraps and exposes a numeric value, this value should not be
+/// trusted to be stable or predictable across processes or executions.
+///
+/// In order to obtain the hash_code for an object 'x':
+/// \code
+///   using llvm::hash_value;
+///   llvm::hash_code code = hash_value(x);
+/// \endcode
+class hash_code {
+  size_t value;
+
+public:
+  /// Default construct a hash_code.
+  /// Note that this leaves the value uninitialized.
+  hash_code() = default;
+
+  /// Form a hash code directly from a numerical value.
+  hash_code(size_t value) : value(value) {}
+
+  /// Convert the hash code to its numerical value for use.
+  /*explicit*/ operator size_t() const { return value; }
+
+  friend bool operator==(const hash_code &lhs, const hash_code &rhs) {
+    return lhs.value == rhs.value;
+  }
+  friend bool operator!=(const hash_code &lhs, const hash_code &rhs) {
+    return lhs.value != rhs.value;
+  }
+
+  /// Allow a hash_code to be directly run through hash_value.
+  friend size_t hash_value(const hash_code &code) { return code.value; }
+};
+
+/// Compute a hash_code for any integer value.
+///
+/// Note that this function is intended to compute the same hash_code for
+/// a particular value without regard to the pre-promotion type. This is in
+/// contrast to hash_combine which may produce different hash_codes for
+/// differing argument types even if they would implicit promote to a common
+/// type without changing the value.
+template <typename T>
+typename std::enable_if<is_integral_or_enum<T>::value, hash_code>::type
+hash_value(T value);
+
+/// Compute a hash_code for a pointer's address.
+///
+/// N.B.: This hashes the *address*. Not the value and not the type.
+template <typename T> hash_code hash_value(const T *ptr);
+
+/// Compute a hash_code for a pair of objects.
+template <typename T, typename U>
+hash_code hash_value(const std::pair<T, U> &arg);
+
+/// Compute a hash_code for a standard string.
+template <typename T>
+hash_code hash_value(const std::basic_string<T> &arg);
+
+
+/// Override the execution seed with a fixed value.
+///
+/// This hashing library uses a per-execution seed designed to change on each
+/// run with high probability in order to ensure that the hash codes are not
+/// attackable and to ensure that output which is intended to be stable does
+/// not rely on the particulars of the hash codes produced.
+///
+/// That said, there are use cases where it is important to be able to
+/// reproduce *exactly* a specific behavior. To that end, we provide a function
+/// which will forcibly set the seed to a fixed value. This must be done at the
+/// start of the program, before any hashes are computed. Also, it cannot be
+/// undone. This makes it thread-hostile and very hard to use outside of
+/// immediately on start of a simple program designed for reproducible
+/// behavior.
+void set_fixed_execution_hash_seed(uint64_t fixed_value);
+
+
+// All of the implementation details of actually computing the various hash
+// code values are held within this namespace. These routines are included in
+// the header file mainly to allow inlining and constant propagation.
+namespace hashing {
+namespace detail {
+
+inline uint64_t fetch64(const char *p) {
+  uint64_t result;
+  memcpy(&result, p, sizeof(result));
+  if (sys::IsBigEndianHost)
+    sys::swapByteOrder(result);
+  return result;
+}
+
+inline uint32_t fetch32(const char *p) {
+  uint32_t result;
+  memcpy(&result, p, sizeof(result));
+  if (sys::IsBigEndianHost)
+    sys::swapByteOrder(result);
+  return result;
+}
+
+/// Some primes between 2^63 and 2^64 for various uses.
+static const uint64_t k0 = 0xc3a5c85c97cb3127ULL;
+static const uint64_t k1 = 0xb492b66fbe98f273ULL;
+static const uint64_t k2 = 0x9ae16a3b2f90404fULL;
+static const uint64_t k3 = 0xc949d7c7509e6557ULL;
+
+/// Bitwise right rotate.
+/// Normally this will compile to a single instruction, especially if the
+/// shift is a manifest constant.
+inline uint64_t rotate(uint64_t val, size_t shift) {
+  // Avoid shifting by 64: doing so yields an undefined result.
+  return shift == 0 ? val : ((val >> shift) | (val << (64 - shift)));
+}
+
+inline uint64_t shift_mix(uint64_t val) {
+  return val ^ (val >> 47);
+}
+
+inline uint64_t hash_16_bytes(uint64_t low, uint64_t high) {
+  // Murmur-inspired hashing.
+  const uint64_t kMul = 0x9ddfea08eb382d69ULL;
+  uint64_t a = (low ^ high) * kMul;
+  a ^= (a >> 47);
+  uint64_t b = (high ^ a) * kMul;
+  b ^= (b >> 47);
+  b *= kMul;
+  return b;
+}
+
+inline uint64_t hash_1to3_bytes(const char *s, size_t len, uint64_t seed) {
+  uint8_t a = s[0];
+  uint8_t b = s[len >> 1];
+  uint8_t c = s[len - 1];
+  uint32_t y = static_cast<uint32_t>(a) + (static_cast<uint32_t>(b) << 8);
+  uint32_t z = static_cast<uint32_t>(len) + (static_cast<uint32_t>(c) << 2);
+  return shift_mix(y * k2 ^ z * k3 ^ seed) * k2;
+}
+
+inline uint64_t hash_4to8_bytes(const char *s, size_t len, uint64_t seed) {
+  uint64_t a = fetch32(s);
+  return hash_16_bytes(len + (a << 3), seed ^ fetch32(s + len - 4));
+}
+
+inline uint64_t hash_9to16_bytes(const char *s, size_t len, uint64_t seed) {
+  uint64_t a = fetch64(s);
+  uint64_t b = fetch64(s + len - 8);
+  return hash_16_bytes(seed ^ a, rotate(b + len, len)) ^ b;
+}
+
+inline uint64_t hash_17to32_bytes(const char *s, size_t len, uint64_t seed) {
+  uint64_t a = fetch64(s) * k1;
+  uint64_t b = fetch64(s + 8);
+  uint64_t c = fetch64(s + len - 8) * k2;
+  uint64_t d = fetch64(s + len - 16) * k0;
+  return hash_16_bytes(rotate(a - b, 43) + rotate(c ^ seed, 30) + d,
+                       a + rotate(b ^ k3, 20) - c + len + seed);
+}
+
+inline uint64_t hash_33to64_bytes(const char *s, size_t len, uint64_t seed) {
+  uint64_t z = fetch64(s + 24);
+  uint64_t a = fetch64(s) + (len + fetch64(s + len - 16)) * k0;
+  uint64_t b = rotate(a + z, 52);
+  uint64_t c = rotate(a, 37);
+  a += fetch64(s + 8);
+  c += rotate(a, 7);
+  a += fetch64(s + 16);
+  uint64_t vf = a + z;
+  uint64_t vs = b + rotate(a, 31) + c;
+  a = fetch64(s + 16) + fetch64(s + len - 32);
+  z = fetch64(s + len - 8);
+  b = rotate(a + z, 52);
+  c = rotate(a, 37);
+  a += fetch64(s + len - 24);
+  c += rotate(a, 7);
+  a += fetch64(s + len - 16);
+  uint64_t wf = a + z;
+  uint64_t ws = b + rotate(a, 31) + c;
+  uint64_t r = shift_mix((vf + ws) * k2 + (wf + vs) * k0);
+  return shift_mix((seed ^ (r * k0)) + vs) * k2;
+}
+
+inline uint64_t hash_short(const char *s, size_t length, uint64_t seed) {
+  if (length >= 4 && length <= 8)
+    return hash_4to8_bytes(s, length, seed);
+  if (length > 8 && length <= 16)
+    return hash_9to16_bytes(s, length, seed);
+  if (length > 16 && length <= 32)
+    return hash_17to32_bytes(s, length, seed);
+  if (length > 32)
+    return hash_33to64_bytes(s, length, seed);
+  if (length != 0)
+    return hash_1to3_bytes(s, length, seed);
+
+  return k2 ^ seed;
+}
+
+/// The intermediate state used during hashing.
+/// Currently, the algorithm for computing hash codes is based on CityHash and
+/// keeps 56 bytes of arbitrary state.
+struct hash_state {
+  uint64_t h0 = 0, h1 = 0, h2 = 0, h3 = 0, h4 = 0, h5 = 0, h6 = 0;
+
+  /// Create a new hash_state structure and initialize it based on the
+  /// seed and the first 64-byte chunk.
+  /// This effectively performs the initial mix.
+  static hash_state create(const char *s, uint64_t seed) {
+    hash_state state = {
+      0, seed, hash_16_bytes(seed, k1), rotate(seed ^ k1, 49),
+      seed * k1, shift_mix(seed), 0 };
+    state.h6 = hash_16_bytes(state.h4, state.h5);
+    state.mix(s);
+    return state;
+  }
+
+  /// Mix 32-bytes from the input sequence into the 16-bytes of 'a'
+  /// and 'b', including whatever is already in 'a' and 'b'.
+  static void mix_32_bytes(const char *s, uint64_t &a, uint64_t &b) {
+    a += fetch64(s);
+    uint64_t c = fetch64(s + 24);
+    b = rotate(b + a + c, 21);
+    uint64_t d = a;
+    a += fetch64(s + 8) + fetch64(s + 16);
+    b += rotate(a, 44) + d;
+    a += c;
+  }
+
+  /// Mix in a 64-byte buffer of data.
+  /// We mix all 64 bytes even when the chunk length is smaller, but we
+  /// record the actual length.
+  void mix(const char *s) {
+    h0 = rotate(h0 + h1 + h3 + fetch64(s + 8), 37) * k1;
+    h1 = rotate(h1 + h4 + fetch64(s + 48), 42) * k1;
+    h0 ^= h6;
+    h1 += h3 + fetch64(s + 40);
+    h2 = rotate(h2 + h5, 33) * k1;
+    h3 = h4 * k1;
+    h4 = h0 + h5;
+    mix_32_bytes(s, h3, h4);
+    h5 = h2 + h6;
+    h6 = h1 + fetch64(s + 16);
+    mix_32_bytes(s + 32, h5, h6);
+    std::swap(h2, h0);
+  }
+
+  /// Compute the final 64-bit hash code value based on the current
+  /// state and the length of bytes hashed.
+  uint64_t finalize(size_t length) {
+    return hash_16_bytes(hash_16_bytes(h3, h5) + shift_mix(h1) * k1 + h2,
+                         hash_16_bytes(h4, h6) + shift_mix(length) * k1 + h0);
+  }
+};
+
+
+/// A global, fixed seed-override variable.
+///
+/// This variable can be set using the \see llvm::set_fixed_execution_seed
+/// function. See that function for details. Do not, under any circumstances,
+/// set or read this variable.
+extern uint64_t fixed_seed_override;
+
+inline uint64_t get_execution_seed() {
+  // FIXME: This needs to be a per-execution seed. This is just a placeholder
+  // implementation. Switching to a per-execution seed is likely to flush out
+  // instability bugs and so will happen as its own commit.
+  //
+  // However, if there is a fixed seed override set the first time this is
+  // called, return that instead of the per-execution seed.
+  const uint64_t seed_prime = 0xff51afd7ed558ccdULL;
+  static uint64_t seed = fixed_seed_override ? fixed_seed_override : seed_prime;
+  return seed;
+}
+
+
+/// Trait to indicate whether a type's bits can be hashed directly.
+///
+/// A type trait which is true if we want to combine values for hashing by
+/// reading the underlying data. It is false if values of this type must
+/// first be passed to hash_value, and the resulting hash_codes combined.
+//
+// FIXME: We want to replace is_integral_or_enum and is_pointer here with
+// a predicate which asserts that comparing the underlying storage of two
+// values of the type for equality is equivalent to comparing the two values
+// for equality. For all the platforms we care about, this holds for integers
+// and pointers, but there are platforms where it doesn't and we would like to
+// support user-defined types which happen to satisfy this property.
+template <typename T> struct is_hashable_data
+  : std::integral_constant<bool, ((is_integral_or_enum<T>::value ||
+                                   std::is_pointer<T>::value) &&
+                                  64 % sizeof(T) == 0)> {};
+
+// Special case std::pair to detect when both types are viable and when there
+// is no alignment-derived padding in the pair. This is a bit of a lie because
+// std::pair isn't truly POD, but it's close enough in all reasonable
+// implementations for our use case of hashing the underlying data.
+template <typename T, typename U> struct is_hashable_data<std::pair<T, U> >
+  : std::integral_constant<bool, (is_hashable_data<T>::value &&
+                                  is_hashable_data<U>::value &&
+                                  (sizeof(T) + sizeof(U)) ==
+                                   sizeof(std::pair<T, U>))> {};
+
+/// Helper to get the hashable data representation for a type.
+/// This variant is enabled when the type itself can be used.
+template <typename T>
+typename std::enable_if<is_hashable_data<T>::value, T>::type
+get_hashable_data(const T &value) {
+  return value;
+}
+/// Helper to get the hashable data representation for a type.
+/// This variant is enabled when we must first call hash_value and use the
+/// result as our data.
+template <typename T>
+typename std::enable_if<!is_hashable_data<T>::value, size_t>::type
+get_hashable_data(const T &value) {
+  using ::llvm::hash_value;
+  return hash_value(value);
+}
+
+/// Helper to store data from a value into a buffer and advance the
+/// pointer into that buffer.
+///
+/// This routine first checks whether there is enough space in the provided
+/// buffer, and if not immediately returns false. If there is space, it
+/// copies the underlying bytes of value into the buffer, advances the
+/// buffer_ptr past the copied bytes, and returns true.
+template <typename T>
+bool store_and_advance(char *&buffer_ptr, char *buffer_end, const T& value,
+                       size_t offset = 0) {
+  size_t store_size = sizeof(value) - offset;
+  if (buffer_ptr + store_size > buffer_end)
+    return false;
+  const char *value_data = reinterpret_cast<const char *>(&value);
+  memcpy(buffer_ptr, value_data + offset, store_size);
+  buffer_ptr += store_size;
+  return true;
+}
+
+/// Implement the combining of integral values into a hash_code.
+///
+/// This overload is selected when the value type of the iterator is
+/// integral. Rather than computing a hash_code for each object and then
+/// combining them, this (as an optimization) directly combines the integers.
+template <typename InputIteratorT>
+hash_code hash_combine_range_impl(InputIteratorT first, InputIteratorT last) {
+  const uint64_t seed = get_execution_seed();
+  char buffer[64], *buffer_ptr = buffer;
+  char *const buffer_end = std::end(buffer);
+  while (first != last && store_and_advance(buffer_ptr, buffer_end,
+                                            get_hashable_data(*first)))
+    ++first;
+  if (first == last)
+    return hash_short(buffer, buffer_ptr - buffer, seed);
+  assert(buffer_ptr == buffer_end);
+
+  hash_state state = state.create(buffer, seed);
+  size_t length = 64;
+  while (first != last) {
+    // Fill up the buffer. We don't clear it, which re-mixes the last round
+    // when only a partial 64-byte chunk is left.
+    buffer_ptr = buffer;
+    while (first != last && store_and_advance(buffer_ptr, buffer_end,
+                                              get_hashable_data(*first)))
+      ++first;
+
+    // Rotate the buffer if we did a partial fill in order to simulate doing
+    // a mix of the last 64-bytes. That is how the algorithm works when we
+    // have a contiguous byte sequence, and we want to emulate that here.
+    std::rotate(buffer, buffer_ptr, buffer_end);
+
+    // Mix this chunk into the current state.
+    state.mix(buffer);
+    length += buffer_ptr - buffer;
+  };
+
+  return state.finalize(length);
+}
+
+/// Implement the combining of integral values into a hash_code.
+///
+/// This overload is selected when the value type of the iterator is integral
+/// and when the input iterator is actually a pointer. Rather than computing
+/// a hash_code for each object and then combining them, this (as an
+/// optimization) directly combines the integers. Also, because the integers
+/// are stored in contiguous memory, this routine avoids copying each value
+/// and directly reads from the underlying memory.
+template <typename ValueT>
+typename std::enable_if<is_hashable_data<ValueT>::value, hash_code>::type
+hash_combine_range_impl(ValueT *first, ValueT *last) {
+  const uint64_t seed = get_execution_seed();
+  const char *s_begin = reinterpret_cast<const char *>(first);
+  const char *s_end = reinterpret_cast<const char *>(last);
+  const size_t length = std::distance(s_begin, s_end);
+  if (length <= 64)
+    return hash_short(s_begin, length, seed);
+
+  const char *s_aligned_end = s_begin + (length & ~63);
+  hash_state state = state.create(s_begin, seed);
+  s_begin += 64;
+  while (s_begin != s_aligned_end) {
+    state.mix(s_begin);
+    s_begin += 64;
+  }
+  if (length & 63)
+    state.mix(s_end - 64);
+
+  return state.finalize(length);
+}
+
+} // namespace detail
+} // namespace hashing
+
+
+/// Compute a hash_code for a sequence of values.
+///
+/// This hashes a sequence of values. It produces the same hash_code as
+/// 'hash_combine(a, b, c, ...)', but can run over arbitrary sized sequences
+/// and is significantly faster given pointers and types which can be hashed as
+/// a sequence of bytes.
+template <typename InputIteratorT>
+hash_code hash_combine_range(InputIteratorT first, InputIteratorT last) {
+  return ::llvm::hashing::detail::hash_combine_range_impl(first, last);
+}
+
+
+// Implementation details for hash_combine.
+namespace hashing {
+namespace detail {
+
+/// Helper class to manage the recursive combining of hash_combine
+/// arguments.
+///
+/// This class exists to manage the state and various calls involved in the
+/// recursive combining of arguments used in hash_combine. It is particularly
+/// useful at minimizing the code in the recursive calls to ease the pain
+/// caused by a lack of variadic functions.
+struct hash_combine_recursive_helper {
+  char buffer[64] = {};
+  hash_state state;
+  const uint64_t seed;
+
+public:
+  /// Construct a recursive hash combining helper.
+  ///
+  /// This sets up the state for a recursive hash combine, including getting
+  /// the seed and buffer setup.
+  hash_combine_recursive_helper()
+    : seed(get_execution_seed()) {}
+
+  /// Combine one chunk of data into the current in-flight hash.
+  ///
+  /// This merges one chunk of data into the hash. First it tries to buffer
+  /// the data. If the buffer is full, it hashes the buffer into its
+  /// hash_state, empties it, and then merges the new chunk in. This also
+  /// handles cases where the data straddles the end of the buffer.
+  template <typename T>
+  char *combine_data(size_t &length, char *buffer_ptr, char *buffer_end, T data) {
+    if (!store_and_advance(buffer_ptr, buffer_end, data)) {
+      // Check for skew which prevents the buffer from being packed, and do
+      // a partial store into the buffer to fill it. This is only a concern
+      // with the variadic combine because that formation can have varying
+      // argument types.
+      size_t partial_store_size = buffer_end - buffer_ptr;
+      memcpy(buffer_ptr, &data, partial_store_size);
+
+      // If the store fails, our buffer is full and ready to hash. We have to
+      // either initialize the hash state (on the first full buffer) or mix
+      // this buffer into the existing hash state. Length tracks the *hashed*
+      // length, not the buffered length.
+      if (length == 0) {
+        state = state.create(buffer, seed);
+        length = 64;
+      } else {
+        // Mix this chunk into the current state and bump length up by 64.
+        state.mix(buffer);
+        length += 64;
+      }
+      // Reset the buffer_ptr to the head of the buffer for the next chunk of
+      // data.
+      buffer_ptr = buffer;
+
+      // Try again to store into the buffer -- this cannot fail as we only
+      // store types smaller than the buffer.
+      if (!store_and_advance(buffer_ptr, buffer_end, data,
+                             partial_store_size))
+        llvm_unreachable("buffer smaller than stored type");
+    }
+    return buffer_ptr;
+  }
+
+  /// Recursive, variadic combining method.
+  ///
+  /// This function recurses through each argument, combining that argument
+  /// into a single hash.
+  template <typename T, typename ...Ts>
+  hash_code combine(size_t length, char *buffer_ptr, char *buffer_end,
+                    const T &arg, const Ts &...args) {
+    buffer_ptr = combine_data(length, buffer_ptr, buffer_end, get_hashable_data(arg));
+
+    // Recurse to the next argument.
+    return combine(length, buffer_ptr, buffer_end, args...);
+  }
+
+  /// Base case for recursive, variadic combining.
+  ///
+  /// The base case when combining arguments recursively is reached when all
+  /// arguments have been handled. It flushes the remaining buffer and
+  /// constructs a hash_code.
+  hash_code combine(size_t length, char *buffer_ptr, char *buffer_end) {
+    // Check whether the entire set of values fit in the buffer. If so, we'll
+    // use the optimized short hashing routine and skip state entirely.
+    if (length == 0)
+      return hash_short(buffer, buffer_ptr - buffer, seed);
+
+    // Mix the final buffer, rotating it if we did a partial fill in order to
+    // simulate doing a mix of the last 64-bytes. That is how the algorithm
+    // works when we have a contiguous byte sequence, and we want to emulate
+    // that here.
+    std::rotate(buffer, buffer_ptr, buffer_end);
+
+    // Mix this chunk into the current state.
+    state.mix(buffer);
+    length += buffer_ptr - buffer;
+
+    return state.finalize(length);
+  }
+};
+
+} // namespace detail
+} // namespace hashing
+
+/// Combine values into a single hash_code.
+///
+/// This routine accepts a varying number of arguments of any type. It will
+/// attempt to combine them into a single hash_code. For user-defined types it
+/// attempts to call a \see hash_value overload (via ADL) for the type. For
+/// integer and pointer types it directly combines their data into the
+/// resulting hash_code.
+///
+/// The result is suitable for returning from a user's hash_value
+/// *implementation* for their user-defined type. Consumers of a type should
+/// *not* call this routine, they should instead call 'hash_value'.
+template <typename ...Ts> hash_code hash_combine(const Ts &...args) {
+  // Recursively hash each argument using a helper class.
+  ::llvm::hashing::detail::hash_combine_recursive_helper helper;
+  return helper.combine(0, helper.buffer, helper.buffer + 64, args...);
+}
+
+// Implementation details for implementations of hash_value overloads provided
+// here.
+namespace hashing {
+namespace detail {
+
+/// Helper to hash the value of a single integer.
+///
+/// Overloads for smaller integer types are not provided to ensure consistent
+/// behavior in the presence of integral promotions. Essentially,
+/// "hash_value('4')" and "hash_value('0' + 4)" should be the same.
+inline hash_code hash_integer_value(uint64_t value) {
+  // Similar to hash_4to8_bytes but using a seed instead of length.
+  const uint64_t seed = get_execution_seed();
+  const char *s = reinterpret_cast<const char *>(&value);
+  const uint64_t a = fetch32(s);
+  return hash_16_bytes(seed + (a << 3), fetch32(s + 4));
+}
+
+} // namespace detail
+} // namespace hashing
+
+// Declared and documented above, but defined here so that any of the hashing
+// infrastructure is available.
+template <typename T>
+typename std::enable_if<is_integral_or_enum<T>::value, hash_code>::type
+hash_value(T value) {
+  return ::llvm::hashing::detail::hash_integer_value(
+      static_cast<uint64_t>(value));
+}
+
+// Declared and documented above, but defined here so that any of the hashing
+// infrastructure is available.
+template <typename T> hash_code hash_value(const T *ptr) {
+  return ::llvm::hashing::detail::hash_integer_value(
+    reinterpret_cast<uintptr_t>(ptr));
+}
+
+// Declared and documented above, but defined here so that any of the hashing
+// infrastructure is available.
+template <typename T, typename U>
+hash_code hash_value(const std::pair<T, U> &arg) {
+  return hash_combine(arg.first, arg.second);
+}
+
+// Declared and documented above, but defined here so that any of the hashing
+// infrastructure is available.
+template <typename T>
+hash_code hash_value(const std::basic_string<T> &arg) {
+  return hash_combine_range(arg.begin(), arg.end());
+}
+
+} // namespace llvm
+
+#endif