[llvm-branch-commits] [libcxx] [libc++][format][3/7] Improves std::format performance. (PR #101817)

[Louis Dionne via llvm-branch-commits]

================
@@ -58,23 +59,156 @@ namespace __format {
 /// This helper is used together with the @ref back_insert_iterator to offer
 /// type-erasure for the formatting functions. This reduces the number to
 /// template instantiations.
+///
+/// The design of the class is being changed to improve performance and do some
+/// code cleanups.
+/// The original design (as shipped up to LLVM-19) uses the following design:
+/// - There is an external object that connects the buffer to the output.
+/// - The class constructor stores a function pointer to a grow function and a
+///   type-erased pointer to the object that does the grow.
+/// - When writing data to the buffer would exceed the external buffer's
+///   capacity it requests the external buffer to flush its contents.
+///
+/// The new design tries to solve some issues with the current design:
+/// - The buffer used is a fixed-size buffer, benchmarking shows that using a
+///   dynamic allocated buffer has performance benefits.
+/// - Implementing P3107R5 "Permit an efficient implementation of std::print"
+///   is not trivial with the current buffers. Using the code from this series
+///   makes it trivial.
+///
+/// This class is ABI-tagged, still the new design does not change the size of
+/// objects of this class.
+///
+/// The new design contains information regarding format_to_n changes, these
+/// will be implemented in follow-up patch.
+///
+/// The new design is the following.
+/// - There is an external object that connects the buffer to the output.
+/// - This buffer object:
+///   - inherits publicly from this class.
+///   - has a static or dynamic buffer.
+///   - has a static member function to make space in its buffer write
+///     operations. This can be done by increasing the size of the internal
+///     buffer or by writing the contents of the buffer to the output iterator.
+///
+///     This member function is a constructor argument, so its name is not
+///     fixed. The code uses the name __prepare_write.
+/// - The number of output code units can be limited by a __max_output_size
+///   object. This is used in format_to_n This object:
+///   - Contains the maximum number of code units to be written.
+///   - Contains the number of code units that are requested to be written.
+///     This number is returned to the user of format_to_n.
+///   - The write functions call objects __request_write member function.
+///     This function:
+///     - Updates the number of code units that are requested to be written.
+///     - Returns the number of code units that can be written without
+///       exceeding the maximum number of code units to be written.
+///
+/// Documentation for the buffer usage members:
+/// - __ptr_ the start of the buffer.
+/// - __capacity_ the number of code units that can be written.
+///   This means [__ptr_, __ptr_ + __capacity_) is a valid range to write to.
+/// - __size_ the number of code units written in the buffer. The next code
+///   unit will be written at __ptr_ + __size_. This __size_ may NOT contain
+///   the total number of code units written by the __output_buffer. Whether or
+///   not it does depends on the sub-class used. Typically the total number of
+///   code units written is not interesting. It is interesting for format_to_n
+///   which has its own way to track this number.
+///
+/// Documentation for the buffer changes function:
+/// The subclasses have a function with the following signature:
+///
+///   static void __prepare_write(
+///     __output_buffer<_CharT>& __buffer, size_t __code_units);
+///
+/// This function is called when a write function writes more code units than
+/// the buffer' available space. When an __max_output_size object is provided
+/// the number of code units is the number of code units returned from
+/// __max_output_size::__request_write function.
+///
+/// - The __buffer contains *this. Since the class containing this function
+///   inherits from __output_buffer it's save to cast it to the subclass being
+///   used.
+/// - The __code_units is the number of code units the caller will write + 1.
+///   - This value does not take the avaiable space of the buffer into account.
+///   - The push_back function is more efficient when writing before resizing,
+///     this means the buffer should always have room for one code unit. Hence
+///     the + 1 is the size.
+/// - When the function returns there is room for at least one code unit. There
+///   is no requirement there is room for __code_units code units:
+///   - The class has some "bulk" operations. For example, __copy which copies
+///     the contents of a basic_string_view to the output. If the sub-class has
+///     a fixed size buffer the size of the basic_string_view may be larger
+///     than the buffer. In that case it's impossible to honor the requested
+///     size.
+///   - The at least one code unit makes sure the entire output can be written.
+///     (Obviously making room one code unit at a time is slow and
+///     it's recommended to return a larger available space.)
+///   - When the buffer has room for at least one code unit the function may be
+///     a no-op.
+/// - When the function makes space for more code units it uses one for these
+///   functions to signal the change:
+///   - __buffer_flushed()
+///     - This function is typically used for a fixed sized buffer.
+///     - The current contents of [__ptr_, __ptr_ + __size_) have been
+///       processed.
+///     - __ptr_ remains unchanged.
+///     - __capacity_ remains unchanged.
+///     - __size_ will be set to 0.
+///   - __buffer_moved(_CharT* __ptr, size_t __capacity)
+///     - This function is typically used for a dynamic sized buffer. There the
+///       location of the buffer changes due to reallocations.
+///     - __ptr_ will be set to __ptr. (This value may be the old value of
+///       __ptr_).
+///     - __capacity_ will be set to __capacity. (This value may be the old
+///       value of       __capacity_).
+///     - __size_ remains unchanged,
+///     - The range [__ptr, __ptr + __size_) contains the original data of the
+///       range  [__ptr_, __ptr_ + __size_).
+///
+/// The push_back function expects a valid buffer and a capacity of at least 1.
+/// This means:
+/// - The class is constructed with a valid buffer,
+/// - __buffer_moved is called with a valid buffer is used before the first
+///   write operation,
+/// - no write function is ever called, or
+/// - the class is constructed with a __max_output_size object with __max_size 0.
+///
+/// The latter option allows formatted_size to use the output buffer without
+/// ever writing anything to the buffer.
 template <__fmt_char_type _CharT>
 class _LIBCPP_TEMPLATE_VIS __output_buffer {
 public:
-  using value_type = _CharT;
+  using value_type           = _CharT;
+  using __prepare_write_type = void (*)(__output_buffer<_CharT>&, size_t);
 
-  template <class _Tp>
+  template <class _Tp> // Deprecated LLVM-19 function.
   _LIBCPP_HIDE_FROM_ABI explicit __output_buffer(_CharT* __ptr, size_t __capacity, _Tp* __obj)
       : __ptr_(__ptr),
         __capacity_(__capacity),
         __flush_([](_CharT* __p, size_t __n, void* __o) { static_cast<_Tp*>(__o)->__flush(__p, __n); }),
         __obj_(__obj) {}
 
+  // New LLVM-20 function.
+  [[nodiscard]]
+  _LIBCPP_HIDE_FROM_ABI explicit __output_buffer(_CharT* __ptr, size_t __capacity, __prepare_write_type __prepare_write)
+      : __ptr_(__ptr), __capacity_(__capacity), __prepare_write_(__prepare_write), __obj_(nullptr) {}
+
+  // Deprecated LLVM-19 function.
   _LIBCPP_HIDE_FROM_ABI void __reset(_CharT* __ptr, size_t __capacity) {
     __ptr_      = __ptr;
     __capacity_ = __capacity;
   }
 
+  // New LLVM-20 function.
+  _LIBCPP_HIDE_FROM_ABI void __buffer_flused() { __size_ = 0; }
+
+  // New LLVM-20 function.
+  _LIBCPP_HIDE_FROM_ABI void __buffer_moved(_CharT* __ptr, size_t __capacity) {
+    __ptr_      = __ptr;
+    __capacity_ = __capacity;
+  }
+
   _LIBCPP_HIDE_FROM_ABI auto __make_output_iterator() { return std::back_insert_iterator{*this}; }
 
   // Used in std::back_insert_iterator.
----------------
ldionne wrote:

Here I think it should be

```c++
  _LIBCPP_HIDE_FROM_ABI void push_back(_CharT __c) {
    std::__construct_at(__ptr_ + __size_, __c);
    ++__size_;
     // Profiling showed flushing after adding is more efficient than flushing
     // when entering the function.
     if (__size_ == __capacity_)
       __flush(0);
   }
```


https://github.com/llvm/llvm-project/pull/101817