1 //===- DenseMapInfoVariant.h - Type traits for DenseMap<variant> *- C++ -*-===//
2 //
3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4 // See https://llvm.org/LICENSE.txt for license information.
5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6 //
7 //===----------------------------------------------------------------------===//
8 ///
9 /// \file
10 /// This file defines DenseMapInfo traits for DenseMap<std::variant<Ts...>>.
11 ///
12 //===----------------------------------------------------------------------===//
13 
14 #ifndef LLVM_ADT_DENSEMAPINFOVARIANT_H
15 #define LLVM_ADT_DENSEMAPINFOVARIANT_H
16 
17 #include "llvm/ADT/DenseMapInfo.h"
18 #include <utility>
19 #include <variant>
20 
21 namespace llvm {
22 
23 // Provide DenseMapInfo for variants whose all alternatives have DenseMapInfo.
24 template <typename... Ts> struct DenseMapInfo<std::variant<Ts...>> {
25   using Variant = std::variant<Ts...>;
26   using FirstT = std::variant_alternative_t<0, Variant>;
27 
28   static inline Variant getEmptyKey() {
29     return Variant(std::in_place_index<0>, DenseMapInfo<FirstT>::getEmptyKey());
30   }
31 
32   static inline Variant getTombstoneKey() {
33     return Variant(std::in_place_index<0>,
34                    DenseMapInfo<FirstT>::getTombstoneKey());
35   }
36 
37   static unsigned getHashValue(const Variant &Val) {
38     return std::visit(
39         [&Val](auto &&Alternative) {
40           using T = std::decay_t<decltype(Alternative)>;
41           // Include index in hash to make sure same value as different
42           // alternatives don't collide.
43           return DenseMapInfo<std::pair<size_t, T>>::getHashValuePiecewise(
44               Val.index(), Alternative);
45         },
46         Val);
47   }
48 
49   static bool isEqual(const Variant &LHS, const Variant &RHS) {
50     if (LHS.index() != RHS.index())
51       return false;
52     if (LHS.valueless_by_exception())
53       return true;
54     // We want to dispatch to DenseMapInfo<T>::isEqual(LHS.get(I), RHS.get(I))
55     // We know the types are the same, but std::visit(V, LHS, RHS) doesn't.
56     // We erase the type held in LHS to void*, and dispatch over RHS.
57     const void *ErasedLHS =
58         std::visit([](const auto &LHS) -> const void * { return &LHS; }, LHS);
59     return std::visit(
60         [&](const auto &RHS) -> bool {
61           using T = std::remove_cv_t<std::remove_reference_t<decltype(RHS)>>;
62           return DenseMapInfo<T>::isEqual(*static_cast<const T *>(ErasedLHS),
63                                           RHS);
64         },
65         RHS);
66   }
67 };
68 
69 } // end namespace llvm
70 
71 #endif // LLVM_ADT_DENSEMAPINFOVARIANT_H
72