Add hashing support for std::tuple

[MForster]

Summary

All tuple values are passed directly to hash_combine. This is inspired by the implementation used for Swift:

https://github.com/llvm/llvm-project-staging/commit/4a1b4edbe1d1969284c1528e2950ac81b25edc8f
https://github.com/llvm/llvm-project-staging/commit/845f3829b91522920a59c351b9011af01c5c7f87

[MForster]

This is a prerequisite for future changes that I am making. The implementation is supposed to be generic and reusable.

[gribozavr2]

llvm/include/llvm/ADT/Hashing.h

125

Could you move it right next to the implementation of hash_value for std::pair above?

685

Ditto, would be nice to maintain the same order as in the header file (right next to the std::pair overload).

688

Could you add a TODO to replace this custom machinery with a call to std::apply when LLVM starts using C++17?

Right now in C++14 I think we can use std::index_sequence_for instead of MakeTupleIndexSet. The helper can also be moved from a side namespace into a local lambda.

llvm/unittests/ADT/HashingTest.cpp

84

Could you imitate these tests for std::tuple as well?

[MForster]

Address review comments.

[MForster]

llvm/include/llvm/ADT/Hashing.h

688

Added a TODO and used std::index_sequence_for. That's neat.

The helper can also be moved from a side namespace into a local lambda.

How would you do that? I know that you can use auto to create a generic lambda, but how do you destructure the type to extract the Indices type parameter?

[MForster]

This comment was removed by MForster.

[MForster]

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[MForster]

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[gribozavr2]

llvm/include/llvm/ADT/Hashing.h

688

Ah right, we need a named template parameter type pack.

llvm/include/llvm/ADT/Hashing.h

 //===-- 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

 ///
 /// 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 tuple.
+template <typename... Ts>
+hash_code hash_value(const std::tuple<Ts...> &arg);
+
 /// Compute a hash_code for a standard string.
 template <typename T>
 hash_code hash_value(const std::basic_string<T> &arg);
 
-/// Compute a hash_code for a tuple.
-template <typename... Ts> hash_code hash_value(const std::tuple<Ts...> &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

 
 // 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());
-}
-
 // Implementation details for the hash_value overload for std::tuple<...>(...).
 namespace hashing {
 namespace detail {
 
-// The indices of a tuple (0..n-1) are collected as the Indices template
-// parameter of the TupleIndexSet template.
-template <unsigned... Indices> struct TupleIndexSet {};
-
-// Collecting the indices uses recursive template metaprogramming via the
-// MakeTupleIndexSet struct. The I template parameter recursively
-// counts up to N-1. The values are collected in the Indices template parameter
-// and then returned in the second template specialization when I reaches N.
-template <unsigned I, unsigned N, unsigned... Indices>
-struct MakeTupleIndexSet {
-  typedef typename MakeTupleIndexSet<I + 1, N, Indices..., I>::Type Type;
-};
-
-template <unsigned N, unsigned... Indices>
-struct MakeTupleIndexSet<N, N, Indices...> {
-  typedef TupleIndexSet<Indices...> Type;
-};
-
-// The indices are then expanded in the hash_value_tuple_helper function into
-// std::get<...>(...) calls.
-template <typename... Ts, unsigned... Indices>
+template <typename... Ts, std::size_t... Indices>
 hash_code hash_value_tuple_helper(const std::tuple<Ts...> &arg,
-                                  TupleIndexSet<Indices...> indices) {
+                                  std::index_sequence<Indices...> indices) {
   return hash_combine(std::get<Indices>(arg)...);
 }
 
 } // namespace detail
 } // namespace hashing
 
 template <typename... Ts> hash_code hash_value(const std::tuple<Ts...> &arg) {
-  typename ::llvm::hashing::detail::MakeTupleIndexSet<0, sizeof...(Ts)>::Type
-      indices;
-  return hash_value_tuple_helper(arg, indices);
+  // TODO: Use std::apply when LLVM starts using C++17.
+  return ::llvm::hashing::detail::hash_value_tuple_helper(
+      arg, typename std::index_sequence_for<Ts...>());
+}
+
+// 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

llvm/unittests/ADT/HashingTest.cpp

TEST(HashingTest, HashValueBasicTest) {
   EXPECT_EQ(hash_value(addr), hash_value(&y));
 }
 
 TEST(HashingTest, HashValueStdPair) {
   EXPECT_EQ(hash_combine(42, 43), hash_value(std::make_pair(42, 43)));
   EXPECT_NE(hash_combine(43, 42), hash_value(std::make_pair(42, 43)));
   EXPECT_NE(hash_combine(42, 43), hash_value(std::make_pair(42ull, 43ull)));
   EXPECT_NE(hash_combine(42, 43), hash_value(std::make_pair(42, 43ull)));
   EXPECT_NE(hash_combine(42, 43), hash_value(std::make_pair(42ull, 43)));
 
   // Note that pairs are implicitly flattened to a direct sequence of data and
   // hashed efficiently as a consequence.
   EXPECT_EQ(hash_combine(42, 43, 44),
             hash_value(std::make_pair(42, std::make_pair(43, 44))));
   EXPECT_EQ(hash_value(std::make_pair(42, std::make_pair(43, 44))),
             hash_value(std::make_pair(std::make_pair(42, 43), 44)));
 
   // Ensure that pairs which have padding bytes *inside* them don't get treated
   // this way.
   EXPECT_EQ(hash_combine('0', hash_combine(1ull, '2')),
             hash_value(std::make_pair('0', std::make_pair(1ull, '2'))));
 
   // Ensure that non-POD pairs don't explode the traits used.
   NonPOD obj1(1, 2), obj2(3, 4), obj3(5, 6);
   EXPECT_EQ(hash_combine(obj1, hash_combine(obj2, obj3)),
             hash_value(std::make_pair(obj1, std::make_pair(obj2, obj3))));
 }
 
 TEST(HashingTest, HashValueStdTuple) {
   EXPECT_EQ(hash_combine(), hash_value(std::make_tuple()));
   EXPECT_EQ(hash_combine(42), hash_value(std::make_tuple(42)));
   EXPECT_EQ(hash_combine(42, 'c'), hash_value(std::make_tuple(42, 'c')));
+
+  EXPECT_NE(hash_combine(43, 42), hash_value(std::make_tuple(42, 43)));
+  EXPECT_NE(hash_combine(42, 43), hash_value(std::make_tuple(42ull, 43ull)));
+  EXPECT_NE(hash_combine(42, 43), hash_value(std::make_tuple(42, 43ull)));
+  EXPECT_NE(hash_combine(42, 43), hash_value(std::make_tuple(42ull, 43)));
 }
 
 TEST(HashingTest, HashValueStdString) {
   std::string s = "Hello World!";
   EXPECT_EQ(hash_combine_range(s.c_str(), s.c_str() + s.size()), hash_value(s));
   EXPECT_EQ(hash_combine_range(s.c_str(), s.c_str() + s.size() - 1),
             hash_value(s.substr(0, s.size() - 1)));
   EXPECT_EQ(hash_combine_range(s.c_str() + 1, s.c_str() + s.size() - 1),