[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
----------------
ldionne wrote:

```suggestion
///   inherits from __output_buffer it's safe to cast it to the subclass being
```

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