[ADT] Make ArrayRef constexpr friendly

[njames93]

Summary

This class is similar to std::span which uses constexpr for most member functions, theres no reason ArrayRef shouldn't be the same.

[dblaikie]

Test coverage? (possibly just modifying much of the existing testing to occur in a constexpr context, if possible)

[njames93]

In D125837#3529746, @dblaikie wrote:

Test coverage? (possibly just modifying much of the existing testing to occur in a constexpr context, if possible)

I was wondering what the best way for tests would be. Guessing static asserts is the correct way

[njames93]

Gonna put this on postpone until we start using c++17 as many things which should be constexpr just aren't as we are using c++14

llvm/include/llvm/ADT/ArrayRef.h

private:
     /// The number of elements.
     size_type Length = 0;
 
   public:
     /// @name Constructors
     /// @{
 
     /// Construct an empty ArrayRef.
-    /*implicit*/ ArrayRef() = default;
+    /*implicit*/ constexpr ArrayRef() = default;
 
     /// Construct an empty ArrayRef from None.
-    /*implicit*/ ArrayRef(NoneType) {}
+    /*implicit*/ constexpr ArrayRef(NoneType) {}
 
     /// Construct an ArrayRef from a single element.
-    /*implicit*/ ArrayRef(const T &OneElt)
+    /*implicit*/ constexpr ArrayRef(const T &OneElt)
         : Data(&OneElt), Length(1) {}
 
     /// Construct an ArrayRef from a pointer and length.
-    /*implicit*/ ArrayRef(const T *data, size_t length)
+    /*implicit*/ constexpr ArrayRef(const T *data, size_t length)
         : Data(data), Length(length) {}
 
     /// Construct an ArrayRef from a range.
-    ArrayRef(const T *begin, const T *end)
+    constexpr ArrayRef(const T *begin, const T *end)
         : Data(begin), Length(end - begin) {}
 
     /// Construct an ArrayRef from a SmallVector. This is templated in order to
     /// avoid instantiating SmallVectorTemplateCommon<T> whenever we
     /// copy-construct an ArrayRef.
-    template<typename U>
-    /*implicit*/ ArrayRef(const SmallVectorTemplateCommon<T, U> &Vec)
-      : Data(Vec.data()), Length(Vec.size()) {
-    }
+    template <typename U>
+    /*implicit*/ constexpr ArrayRef(const SmallVectorTemplateCommon<T, U> &Vec)
+        : Data(Vec.data()), Length(Vec.size()) {}
 
     /// Construct an ArrayRef from a std::vector.
-    template<typename A>
-    /*implicit*/ ArrayRef(const std::vector<T, A> &Vec)
+    template <typename A>
+    /*implicit*/ constexpr ArrayRef(const std::vector<T, A> &Vec)
         : Data(Vec.data()), Length(Vec.size()) {}
 
     /// Construct an ArrayRef from a std::array
     template <size_t N>
     /*implicit*/ constexpr ArrayRef(const std::array<T, N> &Arr)
         : Data(Arr.data()), Length(N) {}
 
     /// Construct an ArrayRef from a C array.
     template <size_t N>
: Data(Vec.begin() == Vec.end() ? (T*)nullptr : Vec.begin()),
       Length(Vec.size()) {}
 #if LLVM_GNUC_PREREQ(9, 0, 0)
 #pragma GCC diagnostic pop
 #endif
 
     /// Construct an ArrayRef<const T*> from ArrayRef<T*>. This uses SFINAE to
     /// ensure that only ArrayRefs of pointers can be converted.
     template <typename U>
-    ArrayRef(const ArrayRef<U *> &A,
-             std::enable_if_t<std::is_convertible<U *const *, T const *>::value>
-                 * = nullptr)
+    constexpr ArrayRef(
+        const ArrayRef<U *> &A,
+        std::enable_if_t<std::is_convertible<U *const *, T const *>::value> * =
+            nullptr)
         : Data(A.data()), Length(A.size()) {}
 
     /// Construct an ArrayRef<const T*> from a SmallVector<T*>. This is
     /// templated in order to avoid instantiating SmallVectorTemplateCommon<T>
     /// whenever we copy-construct an ArrayRef.
     template <typename U, typename DummyT>
-    /*implicit*/ ArrayRef(
+    /*implicit*/ constexpr ArrayRef(
         const SmallVectorTemplateCommon<U *, DummyT> &Vec,
         std::enable_if_t<std::is_convertible<U *const *, T const *>::value> * =
             nullptr)
         : Data(Vec.data()), Length(Vec.size()) {}
 
     /// Construct an ArrayRef<const T*> from std::vector<T*>. This uses SFINAE
     /// to ensure that only vectors of pointers can be converted.
     template <typename U, typename A>
-    ArrayRef(const std::vector<U *, A> &Vec,
-             std::enable_if_t<std::is_convertible<U *const *, T const *>::value>
-                 * = nullptr)
+    constexpr ArrayRef(
+        const std::vector<U *, A> &Vec,
+        std::enable_if_t<std::is_convertible<U *const *, T const *>::value> * =
+            nullptr)
         : Data(Vec.data()), Length(Vec.size()) {}
 
     /// @}
     /// @name Simple Operations
     /// @{
 
-    iterator begin() const { return Data; }
-    iterator end() const { return Data + Length; }
+    constexpr iterator begin() const { return Data; }
+    constexpr iterator end() const { return Data + Length; }
 
-    reverse_iterator rbegin() const { return reverse_iterator(end()); }
-    reverse_iterator rend() const { return reverse_iterator(begin()); }
+    constexpr reverse_iterator rbegin() const {
+      return reverse_iterator(end());
+    }
+    constexpr reverse_iterator rend() const {
+      return reverse_iterator(begin());
+    }
 
     /// empty - Check if the array is empty.
-    bool empty() const { return Length == 0; }
+    constexpr bool empty() const { return Length == 0; }
 
-    const T *data() const { return Data; }
+    constexpr const T *data() const { return Data; }
 
     /// size - Get the array size.
-    size_t size() const { return Length; }
+    constexpr size_t size() const { return Length; }
 
     /// front - Get the first element.
-    const T &front() const {
+    constexpr const T &front() const {
       assert(!empty());
       return Data[0];
     }
 
     /// back - Get the last element.
-    const T &back() const {
+    constexpr const T &back() const {
       assert(!empty());
       return Data[Length-1];
     }
 
     // copy - Allocate copy in Allocator and return ArrayRef<T> to it.
     template <typename Allocator> MutableArrayRef<T> copy(Allocator &A) {
       T *Buff = A.template Allocate<T>(Length);
       std::uninitialized_copy(begin(), end(), Buff);
       return MutableArrayRef<T>(Buff, Length);
     }
 
     /// equals - Check for element-wise equality.
-    bool equals(ArrayRef RHS) const {
+    constexpr bool equals(ArrayRef RHS) const {
       if (Length != RHS.Length)
         return false;
       return std::equal(begin(), end(), RHS.begin());
     }
 
     /// slice(n, m) - Chop off the first N elements of the array, and keep M
     /// elements in the array.
-    ArrayRef<T> slice(size_t N, size_t M) const {
+    constexpr ArrayRef<T> slice(size_t N, size_t M) const {
       assert(N+M <= size() && "Invalid specifier");
       return ArrayRef<T>(data()+N, M);
     }
 
     /// slice(n) - Chop off the first N elements of the array.
-    ArrayRef<T> slice(size_t N) const { return slice(N, size() - N); }
+    constexpr ArrayRef<T> slice(size_t N) const { return slice(N, size() - N); }
 
     /// Drop the first \p N elements of the array.
-    ArrayRef<T> drop_front(size_t N = 1) const {
+    constexpr ArrayRef<T> drop_front(size_t N = 1) const {
       assert(size() >= N && "Dropping more elements than exist");
       return slice(N, size() - N);
     }
 
     /// Drop the last \p N elements of the array.
-    ArrayRef<T> drop_back(size_t N = 1) const {
+    constexpr ArrayRef<T> drop_back(size_t N = 1) const {
       assert(size() >= N && "Dropping more elements than exist");
       return slice(0, size() - N);
     }
 
     /// Return a copy of *this with the first N elements satisfying the
     /// given predicate removed.
-    template <class PredicateT> ArrayRef<T> drop_while(PredicateT Pred) const {
+    template <class PredicateT>
+    constexpr ArrayRef<T> drop_while(PredicateT Pred) const {
       return ArrayRef<T>(find_if_not(*this, Pred), end());
     }
 
     /// Return a copy of *this with the first N elements not satisfying
     /// the given predicate removed.
-    template <class PredicateT> ArrayRef<T> drop_until(PredicateT Pred) const {
+    template <class PredicateT>
+    constexpr ArrayRef<T> drop_until(PredicateT Pred) const {
       return ArrayRef<T>(find_if(*this, Pred), end());
     }
 
     /// Return a copy of *this with only the first \p N elements.
-    ArrayRef<T> take_front(size_t N = 1) const {
+    constexpr ArrayRef<T> take_front(size_t N = 1) const {
       if (N >= size())
         return *this;
       return drop_back(size() - N);
     }
 
     /// Return a copy of *this with only the last \p N elements.
-    ArrayRef<T> take_back(size_t N = 1) const {
+    constexpr ArrayRef<T> take_back(size_t N = 1) const {
       if (N >= size())
         return *this;
       return drop_front(size() - N);
     }
 
     /// Return the first N elements of this Array that satisfy the given
     /// predicate.
-    template <class PredicateT> ArrayRef<T> take_while(PredicateT Pred) const {
+    template <class PredicateT>
+    constexpr ArrayRef<T> take_while(PredicateT Pred) const {
       return ArrayRef<T>(begin(), find_if_not(*this, Pred));
     }
 
     /// Return the first N elements of this Array that don't satisfy the
     /// given predicate.
-    template <class PredicateT> ArrayRef<T> take_until(PredicateT Pred) const {
+    template <class PredicateT>
+    constexpr ArrayRef<T> take_until(PredicateT Pred) const {
       return ArrayRef<T>(begin(), find_if(*this, Pred));
     }
 
     /// @}
     /// @name Operator Overloads
     /// @{
-    const T &operator[](size_t Index) const {
+    constexpr const T &operator[](size_t Index) const {
       assert(Index < Length && "Invalid index!");
       return Data[Index];
     }
 
     /// Disallow accidental assignment from a temporary.
     ///
     /// The declaration here is extra complicated so that "arrayRef = {}"
     /// continues to select the move assignment operator.
public:
     using iterator = pointer;
     using const_iterator = const_pointer;
     using reverse_iterator = std::reverse_iterator<iterator>;
     using const_reverse_iterator = std::reverse_iterator<const_iterator>;
     using size_type = size_t;
     using difference_type = ptrdiff_t;
 
     /// Construct an empty MutableArrayRef.
-    /*implicit*/ MutableArrayRef() = default;
+    /*implicit*/ constexpr MutableArrayRef() = default;
 
     /// Construct an empty MutableArrayRef from None.
-    /*implicit*/ MutableArrayRef(NoneType) : ArrayRef<T>() {}
+    /*implicit*/ constexpr MutableArrayRef(NoneType) : ArrayRef<T>() {}
 
     /// Construct a MutableArrayRef from a single element.
-    /*implicit*/ MutableArrayRef(T &OneElt) : ArrayRef<T>(OneElt) {}
+    /*implicit*/ constexpr MutableArrayRef(T &OneElt) : ArrayRef<T>(OneElt) {}
 
     /// Construct a MutableArrayRef from a pointer and length.
-    /*implicit*/ MutableArrayRef(T *data, size_t length)
+    /*implicit*/ constexpr MutableArrayRef(T *data, size_t length)
         : ArrayRef<T>(data, length) {}
 
     /// Construct a MutableArrayRef from a range.
-    MutableArrayRef(T *begin, T *end) : ArrayRef<T>(begin, end) {}
+    constexpr MutableArrayRef(T *begin, T *end) : ArrayRef<T>(begin, end) {}
 
     /// Construct a MutableArrayRef from a SmallVector.
-    /*implicit*/ MutableArrayRef(SmallVectorImpl<T> &Vec)
+    /*implicit*/ constexpr MutableArrayRef(SmallVectorImpl<T> &Vec)
         : ArrayRef<T>(Vec) {}
 
     /// Construct a MutableArrayRef from a std::vector.
-    /*implicit*/ MutableArrayRef(std::vector<T> &Vec)
+    /*implicit*/ constexpr MutableArrayRef(std::vector<T> &Vec)
         : ArrayRef<T>(Vec) {}
 
     /// Construct a MutableArrayRef from a std::array
     template <size_t N>
     /*implicit*/ constexpr MutableArrayRef(std::array<T, N> &Arr)
         : ArrayRef<T>(Arr) {}
 
     /// Construct a MutableArrayRef from a C array.
     template <size_t N>
     /*implicit*/ constexpr MutableArrayRef(T (&Arr)[N]) : ArrayRef<T>(Arr) {}
 
-    T *data() const { return const_cast<T*>(ArrayRef<T>::data()); }
+    constexpr T *data() const { return const_cast<T *>(ArrayRef<T>::data()); }
 
-    iterator begin() const { return data(); }
-    iterator end() const { return data() + this->size(); }
+    constexpr iterator begin() const { return data(); }
+    constexpr iterator end() const { return data() + this->size(); }
 
-    reverse_iterator rbegin() const { return reverse_iterator(end()); }
-    reverse_iterator rend() const { return reverse_iterator(begin()); }
+    constexpr reverse_iterator rbegin() const {
+      return reverse_iterator(end());
+    }
+    constexpr reverse_iterator rend() const {
+      return reverse_iterator(begin());
+    }
 
     /// front - Get the first element.
-    T &front() const {
+    constexpr T &front() const {
       assert(!this->empty());
       return data()[0];
     }
 
     /// back - Get the last element.
-    T &back() const {
+    constexpr T &back() const {
       assert(!this->empty());
       return data()[this->size()-1];
     }
 
     /// slice(n, m) - Chop off the first N elements of the array, and keep M
     /// elements in the array.
-    MutableArrayRef<T> slice(size_t N, size_t M) const {
+    constexpr MutableArrayRef<T> slice(size_t N, size_t M) const {
       assert(N + M <= this->size() && "Invalid specifier");
       return MutableArrayRef<T>(this->data() + N, M);
     }
 
     /// slice(n) - Chop off the first N elements of the array.
-    MutableArrayRef<T> slice(size_t N) const {
+    constexpr MutableArrayRef<T> slice(size_t N) const {
       return slice(N, this->size() - N);
     }
 
     /// Drop the first \p N elements of the array.
-    MutableArrayRef<T> drop_front(size_t N = 1) const {
+    constexpr MutableArrayRef<T> drop_front(size_t N = 1) const {
       assert(this->size() >= N && "Dropping more elements than exist");
       return slice(N, this->size() - N);
     }
 
-    MutableArrayRef<T> drop_back(size_t N = 1) const {
+    constexpr MutableArrayRef<T> drop_back(size_t N = 1) const {
       assert(this->size() >= N && "Dropping more elements than exist");
       return slice(0, this->size() - N);
     }
 
     /// Return a copy of *this with the first N elements satisfying the
     /// given predicate removed.
     template <class PredicateT>
-    MutableArrayRef<T> drop_while(PredicateT Pred) const {
+    constexpr MutableArrayRef<T> drop_while(PredicateT Pred) const {
       return MutableArrayRef<T>(find_if_not(*this, Pred), end());
     }
 
     /// Return a copy of *this with the first N elements not satisfying
     /// the given predicate removed.
     template <class PredicateT>
-    MutableArrayRef<T> drop_until(PredicateT Pred) const {
+    constexpr MutableArrayRef<T> drop_until(PredicateT Pred) const {
       return MutableArrayRef<T>(find_if(*this, Pred), end());
     }
 
     /// Return a copy of *this with only the first \p N elements.
-    MutableArrayRef<T> take_front(size_t N = 1) const {
+    constexpr MutableArrayRef<T> take_front(size_t N = 1) const {
       if (N >= this->size())
         return *this;
       return drop_back(this->size() - N);
     }
 
     /// Return a copy of *this with only the last \p N elements.
-    MutableArrayRef<T> take_back(size_t N = 1) const {
+    constexpr MutableArrayRef<T> take_back(size_t N = 1) const {
       if (N >= this->size())
         return *this;
       return drop_front(this->size() - N);
     }
 
     /// Return the first N elements of this Array that satisfy the given
     /// predicate.
     template <class PredicateT>
-    MutableArrayRef<T> take_while(PredicateT Pred) const {
+    constexpr MutableArrayRef<T> take_while(PredicateT Pred) const {
       return MutableArrayRef<T>(begin(), find_if_not(*this, Pred));
     }
 
     /// Return the first N elements of this Array that don't satisfy the
     /// given predicate.
     template <class PredicateT>
-    MutableArrayRef<T> take_until(PredicateT Pred) const {
+    constexpr MutableArrayRef<T> take_until(PredicateT Pred) const {
       return MutableArrayRef<T>(begin(), find_if(*this, Pred));
     }
 
     /// @}
     /// @name Operator Overloads
     /// @{
-    T &operator[](size_t Index) const {
+    constexpr T &operator[](size_t Index) const {
       assert(Index < this->size() && "Invalid index!");
       return data()[Index];
     }
   };
 
   /// This is a MutableArrayRef that owns its array.
   template <typename T> class OwningArrayRef : public MutableArrayRef<T> {
   public:
public:
 
     ~OwningArrayRef() { delete[] this->data(); }
   };
 
   /// @name ArrayRef Convenience constructors
   /// @{
 
   /// Construct an ArrayRef from a single element.
-  template<typename T>
-  ArrayRef<T> makeArrayRef(const T &OneElt) {
+  template <typename T> constexpr ArrayRef<T> makeArrayRef(const T &OneElt) {
     return OneElt;
   }
 
   /// Construct an ArrayRef from a pointer and length.
-  template<typename T>
-  ArrayRef<T> makeArrayRef(const T *data, size_t length) {
+  template <typename T>
+  constexpr ArrayRef<T> makeArrayRef(const T *data, size_t length) {
     return ArrayRef<T>(data, length);
   }
 
   /// Construct an ArrayRef from a range.
-  template<typename T>
-  ArrayRef<T> makeArrayRef(const T *begin, const T *end) {
+  template <typename T>
+  constexpr ArrayRef<T> makeArrayRef(const T *begin, const T *end) {
     return ArrayRef<T>(begin, end);
   }
 
   /// Construct an ArrayRef from a SmallVector.
   template <typename T>
-  ArrayRef<T> makeArrayRef(const SmallVectorImpl<T> &Vec) {
+  constexpr ArrayRef<T> makeArrayRef(const SmallVectorImpl<T> &Vec) {
     return Vec;
   }
 
   /// Construct an ArrayRef from a SmallVector.
   template <typename T, unsigned N>
-  ArrayRef<T> makeArrayRef(const SmallVector<T, N> &Vec) {
+  constexpr ArrayRef<T> makeArrayRef(const SmallVector<T, N> &Vec) {
     return Vec;
   }
 
   /// Construct an ArrayRef from a std::vector.
-  template<typename T>
-  ArrayRef<T> makeArrayRef(const std::vector<T> &Vec) {
+  template <typename T>
+  constexpr ArrayRef<T> makeArrayRef(const std::vector<T> &Vec) {
     return Vec;
   }
 
   /// Construct an ArrayRef from a std::array.
   template <typename T, std::size_t N>
-  ArrayRef<T> makeArrayRef(const std::array<T, N> &Arr) {
+  constexpr ArrayRef<T> makeArrayRef(const std::array<T, N> &Arr) {
     return Arr;
   }
 
   /// Construct an ArrayRef from an ArrayRef (no-op) (const)
-  template <typename T> ArrayRef<T> makeArrayRef(const ArrayRef<T> &Vec) {
+  template <typename T>
+  constexpr ArrayRef<T> makeArrayRef(const ArrayRef<T> &Vec) {
     return Vec;
   }
 
   /// Construct an ArrayRef from an ArrayRef (no-op)
-  template <typename T> ArrayRef<T> &makeArrayRef(ArrayRef<T> &Vec) {
+  template <typename T> constexpr ArrayRef<T> &makeArrayRef(ArrayRef<T> &Vec) {
     return Vec;
   }
 
   /// Construct an ArrayRef from a C array.
-  template<typename T, size_t N>
-  ArrayRef<T> makeArrayRef(const T (&Arr)[N]) {
+  template <typename T, size_t N>
+  ArrayRef<T> constexpr makeArrayRef(const T (&Arr)[N]) {
     return ArrayRef<T>(Arr);
   }
 
   /// Construct a MutableArrayRef from a single element.
-  template<typename T>
-  MutableArrayRef<T> makeMutableArrayRef(T &OneElt) {
+  template <typename T>
+  MutableArrayRef<T> constexpr makeMutableArrayRef(T &OneElt) {
     return OneElt;
   }
 
   /// Construct a MutableArrayRef from a pointer and length.
-  template<typename T>
-  MutableArrayRef<T> makeMutableArrayRef(T *data, size_t length) {
+  template <typename T>
+  MutableArrayRef<T> constexpr makeMutableArrayRef(T *data, size_t length) {
     return MutableArrayRef<T>(data, length);
   }
 
   /// @}
   /// @name ArrayRef Comparison Operators
   /// @{
 
-  template<typename T>
-  inline bool operator==(ArrayRef<T> LHS, ArrayRef<T> RHS) {
+  template <typename T>
+  inline constexpr bool operator==(ArrayRef<T> LHS, ArrayRef<T> RHS) {
     return LHS.equals(RHS);
   }
 
   template <typename T>
-  inline bool operator==(SmallVectorImpl<T> &LHS, ArrayRef<T> RHS) {
+  inline constexpr bool operator==(SmallVectorImpl<T> &LHS, ArrayRef<T> RHS) {
     return ArrayRef<T>(LHS).equals(RHS);
   }
 
   template <typename T>
-  inline bool operator!=(ArrayRef<T> LHS, ArrayRef<T> RHS) {
+  inline constexpr bool operator!=(ArrayRef<T> LHS, ArrayRef<T> RHS) {
     return !(LHS == RHS);
   }
 
   template <typename T>
-  inline bool operator!=(SmallVectorImpl<T> &LHS, ArrayRef<T> RHS) {
+  inline constexpr bool operator!=(SmallVectorImpl<T> &LHS, ArrayRef<T> RHS) {
     return !(LHS == RHS);
   }
 
   /// @}
 
   template <typename T> hash_code hash_value(ArrayRef<T> S) {
     return hash_combine_range(S.begin(), S.end());
   }