Optimize 0-sized array iterators

Author: ldionne

libcxx/include/__iterator/static_bounded_iter.h

@@ -31,6 +31,37 @@ _LIBCPP_PUSH_MACROS
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
+template <class _Iterator, size_t _Size>
+struct __static_bounded_iter_storage {
+  _LIBCPP_HIDE_FROM_ABI
+  _LIBCPP_CONSTEXPR_SINCE_CXX14 explicit __static_bounded_iter_storage(_Iterator __current, _Iterator __begin)
+      : __current_(__current), __begin_(__begin) {}
+
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR _Iterator& __current() _NOEXCEPT { return __current_; }
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR _Iterator __current() const _NOEXCEPT { return __current_; }
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR _Iterator __begin() const _NOEXCEPT { return __begin_; }
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR _Iterator __end() const _NOEXCEPT { return __begin_ + _Size; }
+
+private:
+  _Iterator __current_; // current iterator
+  _Iterator __begin_;   // start of the valid range, which is [__begin_, __begin_ + _Size)
+};
+
+template <class _Iterator, size_t _Size>
+struct __static_bounded_iter_storage<_Iterator, 0> {
+  _LIBCPP_HIDE_FROM_ABI
+  _LIBCPP_CONSTEXPR_SINCE_CXX14 explicit __static_bounded_iter_storage(_Iterator __current, _Iterator /* __begin */)
+      : __current_(__current) {}
+
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR _Iterator& __current() _NOEXCEPT { return __current_; }
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR _Iterator __current() const _NOEXCEPT { return __current_; }
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR _Iterator __begin() const _NOEXCEPT { return __current_; }
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR _Iterator __end() const _NOEXCEPT { return __current_ + _Size; }
+
+private:
+  _Iterator __current_; // current iterator
+};
+
 // This is an iterator wrapper for contiguous iterators that points within a range
 // whose size is known at compile-time. This is very similar to `__bounded_iter`,
 // except that we don't have to store the end of the range in physical memory since
@@ -60,8 +91,7 @@ struct __static_bounded_iter {
   template <class _OtherIterator, __enable_if_t<is_convertible<_OtherIterator, _Iterator>::value, int> = 0>
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR
   __static_bounded_iter(__static_bounded_iter<_OtherIterator, _Size> const& __other) _NOEXCEPT
-      : __current_(__other.__current_),
-        __begin_(__other.__begin_) {}
+      : __storage_(__other.__storage_.__current(), __other.__storage_.__begin()) {}
 
   // Assign a bounded iterator to another one, rebinding the bounds of the iterator as well.
   _LIBCPP_HIDE_FROM_ABI __static_bounded_iter& operator=(__static_bounded_iter const&) = default;
@@ -72,7 +102,7 @@ struct __static_bounded_iter {
   // by the provided [begin, begin + _Size] range.
   _LIBCPP_HIDE_FROM_ABI
   _LIBCPP_CONSTEXPR_SINCE_CXX14 explicit __static_bounded_iter(_Iterator __current, _Iterator __begin)
-      : __current_(__current), __begin_(__begin) {
+      : __storage_(__current, __begin) {
     _LIBCPP_ASSERT_INTERNAL(
         __begin <= __current, "__static_bounded_iter(current, begin): current and begin are inconsistent");
     _LIBCPP_ASSERT_INTERNAL(
@@ -86,32 +116,33 @@ struct __static_bounded_iter {
   // Dereference and indexing operations.
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 reference operator*() const _NOEXCEPT {
     _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(
-        __current_ != __end(), "__static_bounded_iter::operator*: Attempt to dereference an iterator at the end");
-    return *__current_;
+        __current() != __end(), "__static_bounded_iter::operator*: Attempt to dereference an iterator at the end");
+    return *__current();
   }
 
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 pointer operator->() const _NOEXCEPT {
     _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(
-        __current_ != __end(), "__static_bounded_iter::operator->: Attempt to dereference an iterator at the end");
-    return std::__to_address(__current_);
+        __current() != __end(), "__static_bounded_iter::operator->: Attempt to dereference an iterator at the end");
+    return std::__to_address(__current());
   }
 
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 reference operator[](difference_type __n) const _NOEXCEPT {
     _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(
-        __n >= __begin_ - __current_, "__static_bounded_iter::operator[]: Attempt to index an iterator past the start");
+        __n >= __begin() - __current(),
+        "__static_bounded_iter::operator[]: Attempt to index an iterator past the start");
     _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(
-        __n < __end() - __current_,
+        __n < __end() - __current(),
         "__static_bounded_iter::operator[]: Attempt to index an iterator at or past the end");
-    return __current_[__n];
+    return __current()[__n];
   }
 
   // Arithmetic operations.
   //
   // These operations check that the iterator remains within `[begin, end]`.
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 __static_bounded_iter& operator++() _NOEXCEPT {
     _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(
-        __current_ != __end(), "__static_bounded_iter::operator++: Attempt to advance an iterator past the end");
-    ++__current_;
+        __current() != __end(), "__static_bounded_iter::operator++: Attempt to advance an iterator past the end");
+    ++__current();
     return *this;
   }
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 __static_bounded_iter operator++(int) _NOEXCEPT {
@@ -122,8 +153,8 @@ struct __static_bounded_iter {
 
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 __static_bounded_iter& operator--() _NOEXCEPT {
     _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(
-        __current_ != __begin_, "__static_bounded_iter::operator--: Attempt to rewind an iterator past the start");
-    --__current_;
+        __current() != __begin(), "__static_bounded_iter::operator--: Attempt to rewind an iterator past the start");
+    --__current();
     return *this;
   }
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 __static_bounded_iter operator--(int) _NOEXCEPT {
@@ -134,11 +165,11 @@ struct __static_bounded_iter {
 
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 __static_bounded_iter& operator+=(difference_type __n) _NOEXCEPT {
     _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(
-        __n >= __begin_ - __current_,
+        __n >= __begin() - __current(),
         "__static_bounded_iter::operator+=: Attempt to rewind an iterator past the start");
     _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(
-        __n <= __end() - __current_, "__static_bounded_iter::operator+=: Attempt to advance an iterator past the end");
-    __current_ += __n;
+        __n <= __end() - __current(), "__static_bounded_iter::operator+=: Attempt to advance an iterator past the end");
+    __current() += __n;
     return *this;
   }
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 friend __static_bounded_iter
@@ -156,11 +187,11 @@ struct __static_bounded_iter {
 
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 __static_bounded_iter& operator-=(difference_type __n) _NOEXCEPT {
     _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(
-        __n <= __current_ - __begin_,
+        __n <= __current() - __begin(),
         "__static_bounded_iter::operator-=: Attempt to rewind an iterator past the start");
     _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(
-        __n >= __current_ - __end(), "__static_bounded_iter::operator-=: Attempt to advance an iterator past the end");
-    __current_ -= __n;
+        __n >= __current() - __end(), "__static_bounded_iter::operator-=: Attempt to advance an iterator past the end");
+    __current() -= __n;
     return *this;
   }
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 friend __static_bounded_iter
@@ -171,7 +202,7 @@ struct __static_bounded_iter {
   }
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 friend difference_type
   operator-(__static_bounded_iter const& __x, __static_bounded_iter const& __y) _NOEXCEPT {
-    return __x.__current_ - __y.__current_;
+    return __x.__current() - __y.__current();
   }
 
   // Comparison operations.
@@ -182,42 +213,42 @@ struct __static_bounded_iter {
   // if they have different validity ranges.
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR friend bool
   operator==(__static_bounded_iter const& __x, __static_bounded_iter const& __y) _NOEXCEPT {
-    return __x.__current_ == __y.__current_;
+    return __x.__current() == __y.__current();
   }
 
 #if _LIBCPP_STD_VER <= 17
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR friend bool
   operator!=(__static_bounded_iter const& __x, __static_bounded_iter const& __y) _NOEXCEPT {
-    return __x.__current_ != __y.__current_;
+    return __x.__current() != __y.__current();
   }
 
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR friend bool
   operator<(__static_bounded_iter const& __x, __static_bounded_iter const& __y) _NOEXCEPT {
-    return __x.__current_ < __y.__current_;
+    return __x.__current() < __y.__current();
   }
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR friend bool
   operator>(__static_bounded_iter const& __x, __static_bounded_iter const& __y) _NOEXCEPT {
-    return __x.__current_ > __y.__current_;
+    return __x.__current() > __y.__current();
   }
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR friend bool
   operator<=(__static_bounded_iter const& __x, __static_bounded_iter const& __y) _NOEXCEPT {
-    return __x.__current_ <= __y.__current_;
+    return __x.__current() <= __y.__current();
   }
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR friend bool
   operator>=(__static_bounded_iter const& __x, __static_bounded_iter const& __y) _NOEXCEPT {
-    return __x.__current_ >= __y.__current_;
+    return __x.__current() >= __y.__current();
   }
 
 #else
   _LIBCPP_HIDE_FROM_ABI constexpr friend strong_ordering
   operator<=>(__static_bounded_iter const& __x, __static_bounded_iter const& __y) noexcept {
     if constexpr (three_way_comparable<_Iterator, strong_ordering>) {
-      return __x.__current_ <=> __y.__current_;
+      return __x.__current() <=> __y.__current();
     } else {
-      if (__x.__current_ < __y.__current_)
+      if (__x.__current() < __y.__current())
         return strong_ordering::less;
 
-      if (__x.__current_ == __y.__current_)
+      if (__x.__current() == __y.__current())
         return strong_ordering::equal;
 
       return strong_ordering::greater;
@@ -230,10 +261,12 @@ struct __static_bounded_iter {
   friend struct pointer_traits;
   template <class, size_t, class>
   friend struct __static_bounded_iter;
-  _Iterator __current_; // current iterator
-  _Iterator __begin_;   // start of the valid range, which is [__begin_, __begin_ + _Size)
+  __static_bounded_iter_storage<_Iterator, _Size> __storage_;
 
-  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR _Iterator __end() const _NOEXCEPT { return __begin_ + _Size; }
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR _Iterator& __current() _NOEXCEPT { return __storage_.__current(); }
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR _Iterator __current() const _NOEXCEPT { return __storage_.__current(); }
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR _Iterator __begin() const _NOEXCEPT { return __storage_.__begin(); }
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR _Iterator __end() const _NOEXCEPT { return __storage_.__end(); }
 };
 
 template <size_t _Size, class _It>
@@ -254,7 +287,7 @@ struct pointer_traits<__static_bounded_iter<_Iterator, _Size> > {
   using difference_type = typename pointer_traits<_Iterator>::difference_type;
 
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR static element_type* to_address(pointer __it) _NOEXCEPT {
-    return std::__to_address(__it.__current_);
+    return std::__to_address(__it.__current());
   }
 };