Merge pull request #2323 from div72/span

util: port upstream span changes

Author: jamescowens

src/span.h

@@ -1,60 +1,251 @@
-// Copyright (c) 2018 The Bitcoin Core developers
+// Copyright (c) 2018-2020 The Bitcoin Core developers
 // Distributed under the MIT software license, see the accompanying
-// file COPYING or https://opensource.org/licenses/mit-license.php.
+// file COPYING or https://www.opensource.org/licenses/mit-license.php.
 
 #ifndef BITCOIN_SPAN_H
 #define BITCOIN_SPAN_H
 
 #include <type_traits>
 #include <cstddef>
 #include <algorithm>
+#include <assert.h>
+
+#ifdef DEBUG
+#define CONSTEXPR_IF_NOT_DEBUG
+#define ASSERT_IF_DEBUG(x) assert((x))
+#else
+#define CONSTEXPR_IF_NOT_DEBUG constexpr
+#define ASSERT_IF_DEBUG(x)
+#endif
+
+#if defined(__clang__)
+#if __has_attribute(lifetimebound)
+#define SPAN_ATTR_LIFETIMEBOUND [[clang::lifetimebound]]
+#else
+#define SPAN_ATTR_LIFETIMEBOUND
+#endif
+#else
+#define SPAN_ATTR_LIFETIMEBOUND
+#endif
 
 /** A Span is an object that can refer to a contiguous sequence of objects.
  *
  * It implements a subset of C++20's std::span.
+ *
+ * Things to be aware of when writing code that deals with Spans:
+ *
+ * - Similar to references themselves, Spans are subject to reference lifetime
+ *   issues. The user is responsible for making sure the objects pointed to by
+ *   a Span live as long as the Span is used. For example:
+ *
+ *       std::vector<int> vec{1,2,3,4};
+ *       Span<int> sp(vec);
+ *       vec.push_back(5);
+ *       printf("%i\n", sp.front()); // UB!
+ *
+ *   may exhibit undefined behavior, as increasing the size of a vector may
+ *   invalidate references.
+ *
+ * - One particular pitfall is that Spans can be constructed from temporaries,
+ *   but this is unsafe when the Span is stored in a variable, outliving the
+ *   temporary. For example, this will compile, but exhibits undefined behavior:
+ *
+ *       Span<const int> sp(std::vector<int>{1, 2, 3});
+ *       printf("%i\n", sp.front()); // UB!
+ *
+ *   The lifetime of the vector ends when the statement it is created in ends.
+ *   Thus the Span is left with a dangling reference, and using it is undefined.
+ *
+ * - Due to Span's automatic creation from range-like objects (arrays, and data
+ *   types that expose a data() and size() member function), functions that
+ *   accept a Span as input parameter can be called with any compatible
+ *   range-like object. For example, this works:
+*
+ *       void Foo(Span<const int> arg);
+ *
+ *       Foo(std::vector<int>{1, 2, 3}); // Works
+ *
+ *   This is very useful in cases where a function truly does not care about the
+ *   container, and only about having exactly a range of elements. However it
+ *   may also be surprising to see automatic conversions in this case.
+ *
+ *   When a function accepts a Span with a mutable element type, it will not
+ *   accept temporaries; only variables or other references. For example:
+ *
+ *       void FooMut(Span<int> arg);
+ *
+ *       FooMut(std::vector<int>{1, 2, 3}); // Does not compile
+ *       std::vector<int> baz{1, 2, 3};
+ *       FooMut(baz); // Works
+ *
+ *   This is similar to how functions that take (non-const) lvalue references
+ *   as input cannot accept temporaries. This does not work either:
+ *
+ *       void FooVec(std::vector<int>& arg);
+ *       FooVec(std::vector<int>{1, 2, 3}); // Does not compile
+ *
+ *   The idea is that if a function accepts a mutable reference, a meaningful
+ *   result will be present in that variable after the call. Passing a temporary
+ *   is useless in that context.
  */
 template<typename C>
 class Span
 {
     C* m_data;
-    std::ptrdiff_t m_size;
+    std::size_t m_size;
+
+    template <class T>
+    struct is_Span_int : public std::false_type {};
+    template <class T>
+    struct is_Span_int<Span<T>> : public std::true_type {};
+    template <class T>
+    struct is_Span : public is_Span_int<typename std::remove_cv<T>::type>{};
+
 
 public:
     constexpr Span() noexcept : m_data(nullptr), m_size(0) {}
-    constexpr Span(C* data, std::ptrdiff_t size) noexcept : m_data(data), m_size(size) {}
-    constexpr Span(C* data, C* end) noexcept : m_data(data), m_size(end - data) {}
+
+    /** Construct a span from a begin pointer and a size.
+     *
+     * This implements a subset of the iterator-based std::span constructor in C++20,
+     * which is hard to implement without std::address_of.
+     */
+    template <typename T, typename std::enable_if<std::is_convertible<T (*)[], C (*)[]>::value, int>::type = 0>
+    constexpr Span(T* begin, std::size_t size) noexcept : m_data(begin), m_size(size) {}
+
+    /** Construct a span from a begin and end pointer.
+     *
+     * This implements a subset of the iterator-based std::span constructor in C++20,
+     * which is hard to implement without std::address_of.
+     */
+    template <typename T, typename std::enable_if<std::is_convertible<T (*)[], C (*)[]>::value, int>::type = 0>
+    CONSTEXPR_IF_NOT_DEBUG Span(T* begin, T* end) noexcept : m_data(begin), m_size(end - begin)
+    {
+        ASSERT_IF_DEBUG(end >= begin);
+    }
+
+    /** Implicit conversion of spans between compatible types.
+     *
+     *  Specifically, if a pointer to an array of type O can be implicitly converted to a pointer to an array of type
+     *  C, then permit implicit conversion of Span<O> to Span<C>. This matches the behavior of the corresponding
+     *  C++20 std::span constructor.
+     *
+     *  For example this means that a Span<T> can be converted into a Span<const T>.
+     */
+    template <typename O, typename std::enable_if<std::is_convertible<O (*)[], C (*)[]>::value, int>::type = 0>
+    constexpr Span(const Span<O>& other) noexcept : m_data(other.m_data), m_size(other.m_size) {}
+
+    /** Default copy constructor. */
+    constexpr Span(const Span&) noexcept = default;
+
+    /** Default assignment operator. */
+    Span& operator=(const Span& other) noexcept = default;
+
+    /** Construct a Span from an array. This matches the corresponding C++20 std::span constructor. */
+    template <int N>
+    constexpr Span(C (&a)[N]) noexcept : m_data(a), m_size(N) {}
+
+    /** Construct a Span for objects with .data() and .size() (std::string, std::array, std::vector, ...).
+     *
+     * This implements a subset of the functionality provided by the C++20 std::span range-based constructor.
+     *
+     * To prevent surprises, only Spans for constant value types are supported when passing in temporaries.
+     * Note that this restriction does not exist when converting arrays or other Spans (see above).
+     */
+    template <typename V>
+    constexpr Span(V& other SPAN_ATTR_LIFETIMEBOUND,
+        typename std::enable_if<!is_Span<V>::value &&
+                                std::is_convertible<typename std::remove_pointer<decltype(std::declval<V&>().data())>::type (*)[], C (*)[]>::value &&
+                                std::is_convertible<decltype(std::declval<V&>().size()), std::size_t>::value, std::nullptr_t>::type = nullptr)
+        : m_data(other.data()), m_size(other.size()){}
+
+    template <typename V>
+    constexpr Span(const V& other SPAN_ATTR_LIFETIMEBOUND,
+        typename std::enable_if<!is_Span<V>::value &&
+                                std::is_convertible<typename std::remove_pointer<decltype(std::declval<const V&>().data())>::type (*)[], C (*)[]>::value &&
+                                std::is_convertible<decltype(std::declval<const V&>().size()), std::size_t>::value, std::nullptr_t>::type = nullptr)
+        : m_data(other.data()), m_size(other.size()){}
 
     constexpr C* data() const noexcept { return m_data; }
     constexpr C* begin() const noexcept { return m_data; }
     constexpr C* end() const noexcept { return m_data + m_size; }
-    constexpr std::ptrdiff_t size() const noexcept { return m_size; }
-    constexpr C& operator[](std::ptrdiff_t pos) const noexcept { return m_data[pos]; }
-
-    constexpr Span<C> subspan(std::ptrdiff_t offset) const noexcept { return Span<C>(m_data + offset, m_size - offset); }
-    constexpr Span<C> subspan(std::ptrdiff_t offset, std::ptrdiff_t count) const noexcept { return Span<C>(m_data + offset, count); }
-    constexpr Span<C> first(std::ptrdiff_t count) const noexcept { return Span<C>(m_data, count); }
-    constexpr Span<C> last(std::ptrdiff_t count) const noexcept { return Span<C>(m_data + m_size - count, count); }
+    CONSTEXPR_IF_NOT_DEBUG C& front() const noexcept
+    {
+        ASSERT_IF_DEBUG(size() > 0);
+        return m_data[0];
+    }
+    CONSTEXPR_IF_NOT_DEBUG C& back() const noexcept
+    {
+        ASSERT_IF_DEBUG(size() > 0);
+        return m_data[m_size - 1];
+    }
+    constexpr std::size_t size() const noexcept { return m_size; }
+    constexpr bool empty() const noexcept { return size() == 0; }
+    CONSTEXPR_IF_NOT_DEBUG C& operator[](std::size_t pos) const noexcept
+    {
+        ASSERT_IF_DEBUG(size() > pos);
+        return m_data[pos];
+    }
+    CONSTEXPR_IF_NOT_DEBUG Span<C> subspan(std::size_t offset) const noexcept
+    {
+        ASSERT_IF_DEBUG(size() >= offset);
+        return Span<C>(m_data + offset, m_size - offset);
+    }
+    CONSTEXPR_IF_NOT_DEBUG Span<C> subspan(std::size_t offset, std::size_t count) const noexcept
+    {
+        ASSERT_IF_DEBUG(size() >= offset + count);
+        return Span<C>(m_data + offset, count);
+    }
+    CONSTEXPR_IF_NOT_DEBUG Span<C> first(std::size_t count) const noexcept
+    {
+        ASSERT_IF_DEBUG(size() >= count);
+        return Span<C>(m_data, count);
+    }
+    CONSTEXPR_IF_NOT_DEBUG Span<C> last(std::size_t count) const noexcept
+    {
+         ASSERT_IF_DEBUG(size() >= count);
+         return Span<C>(m_data + m_size - count, count);
+    }
 
     friend constexpr bool operator==(const Span& a, const Span& b) noexcept { return a.size() == b.size() && std::equal(a.begin(), a.end(), b.begin()); }
     friend constexpr bool operator!=(const Span& a, const Span& b) noexcept { return !(a == b); }
     friend constexpr bool operator<(const Span& a, const Span& b) noexcept { return std::lexicographical_compare(a.begin(), a.end(), b.begin(), b.end()); }
     friend constexpr bool operator<=(const Span& a, const Span& b) noexcept { return !(b < a); }
     friend constexpr bool operator>(const Span& a, const Span& b) noexcept { return (b < a); }
     friend constexpr bool operator>=(const Span& a, const Span& b) noexcept { return !(a < b); }
+
+    template <typename O> friend class Span;
 };
 
-/** Create a span to a container exposing data() and size().
- *
- * This correctly deals with constness: the returned Span's element type will be
- * whatever data() returns a pointer to. If either the passed container is const,
- * or its element type is const, the resulting span will have a const element type.
- *
- * std::span will have a constructor that implements this functionality directly.
- */
-template<typename A, int N>
-constexpr Span<A> MakeSpan(A (&a)[N]) { return Span<A>(a, N); }
+// MakeSpan helps constructing a Span of the right type automatically.
+/** MakeSpan for arrays: */
+template <typename A, int N> Span<A> constexpr MakeSpan(A (&a)[N]) { return Span<A>(a, N); }
+/** MakeSpan for temporaries / rvalue references, only supporting const output. */
+template <typename V> constexpr auto MakeSpan(V&& v SPAN_ATTR_LIFETIMEBOUND) -> typename std::enable_if<!std::is_lvalue_reference<V>::value, Span<const typename std::remove_pointer<decltype(v.data())>::type>>::type { return std::forward<V>(v); }
+/** MakeSpan for (lvalue) references, supporting mutable output. */
+template <typename V> constexpr auto MakeSpan(V& v SPAN_ATTR_LIFETIMEBOUND) -> Span<typename std::remove_pointer<decltype(v.data())>::type> { return v; }
+
+/** Pop the last element off a span, and return a reference to that element. */
+template <typename T>
+T& SpanPopBack(Span<T>& span)
+{
+    size_t size = span.size();
+    ASSERT_IF_DEBUG(size > 0);
+    T& back = span[size - 1];
+    span = Span<T>(span.data(), size - 1);
+    return back;
+}
+
+// Helper functions to safely cast to unsigned char pointers.
+inline unsigned char* UCharCast(char* c) { return (unsigned char*)c; }
+inline unsigned char* UCharCast(unsigned char* c) { return c; }
+inline const unsigned char* UCharCast(const char* c) { return (unsigned char*)c; }
+inline const unsigned char* UCharCast(const unsigned char* c) { return c; }
+
+// Helper function to safely convert a Span to a Span<[const] unsigned char>.
+template <typename T> constexpr auto UCharSpanCast(Span<T> s) -> Span<typename std::remove_pointer<decltype(UCharCast(s.data()))>::type> { return {UCharCast(s.data()), s.size()}; }
 
-template<typename V>
-constexpr Span<typename std::remove_pointer<decltype(std::declval<V>().data())>::type> MakeSpan(V& v) { return Span<typename std::remove_pointer<decltype(std::declval<V>().data())>::type>(v.data(), v.size()); }
+/** Like MakeSpan, but for (const) unsigned char member types only. Only works for (un)signed char containers. */
+template <typename V> constexpr auto MakeUCharSpan(V&& v) -> decltype(UCharSpanCast(MakeSpan(std::forward<V>(v)))) { return UCharSpanCast(MakeSpan(std::forward<V>(v))); }
 
 #endif