[libc-commits] [libc] [libc] Updated header_generation.rst (PR #99712)

[Michael Jones via libc-commits]

================
@@ -1,120 +1,160 @@
+.. role:: raw-html(raw)
+    :format: html
+
 Generating Public and Internal headers
 ======================================
 
-.. warning::
-  This page is severely out of date. Much of the information it contains may be
-  incorrect. Please only remove this warning once the page has been updated.
-
-Other libc implementations make use of preprocessor macro tricks to make header
-files platform agnostic. When macros aren't suitable, they rely on build
-system tricks to pick the right set of files to compile and export. While these
-approaches have served them well, parts of their systems have become extremely
-complicated making it hard to modify, extend or maintain. To avoid these
-problems in llvm-libc, we use a header generation mechanism. The mechanism is
-driven by a *header configuration language*.
-
-Header Configuration Language
------------------------------
-
-Header configuration language consists of few special *commands*. The header
-generation mechanism takes an input file, which has an extension of
-``.h.def``, and produces a header file with ``.h`` extension. The header
-configuration language commands are listed in the input ``.h.def`` file. While
-reading a ``.h.def`` file, the header generation tool does two things:
-
-1. Copy the lines not containing commands as is into the output ``.h`` file.
-2. Replace the line on which a command occurs with some other text as directed
-   by the command. The replacement text can span multiple lines.
-
-Command syntax
-~~~~~~~~~~~~~~
-
-A command should be listed on a line by itself, and should not span more than
-one line. The first token to appear on the line is the command name prefixed
-with ``%%``. For example, a line with the ``include_file`` command should start
-with ``%%include_file``. There can be indentation spaces before the ``%%``
-prefix.
+This is a new implementation of the previous libc header generator. The old
+header generator (libc-hdrgen aka “headergen”) is based on tablegen, which
+creates an awkward dependency on the rest of LLVM for our build system. By
+creating a new standalone headergen we can eliminate these dependencies for
+easier cross compatibility.
 
-Most commands typically take arguments. They are listed as a comma separated
-list of named identifiers within parenthesis, similar to the C function call
-syntax. Before performing the action corresponding to the command, the header
-generator replaces the arguments with concrete values.
+There are 3 main components of the new Headergen. The first component are the
+yaml files that contain all the function header information and are separated by
+header specification and standard. The second component are the classes that are
+created for each component of the function header: macros, enumerations, types,
+function, arguments, and objects. The third component is the python script that
+uses the class representation to deserialize yaml files into its specific
+components and then reserializes the components into the function header. The
+python script also combines the generated header content with header definitions
+and extra macro and type inclusions from the .h.def file.
 
-Argument Syntax
-~~~~~~~~~~~~~~~
 
-Arguments are named indentifiers but prefixed with ``$`` and enclosed in ``{``
-and ``}``. For example, ``${path_to_constants}``.
-
-Comments
-~~~~~~~~
-
-There can be cases wherein one wants to add comments in the .h.def file but
-does not want them to be copied into the generated header file. Such comments
-can be added by beginning the comment lines with the ``<!>`` prefix. Currently,
-comments have to be on lines of their own. That is, they cannot be suffixes like
-this:
-
-```
-%%include_file(a/b/c) <!> Path to c in b of a.  !!! WRONG SYNTAX
-```
-
-Available Commands
-------------------
-
-Sub-sections below describe the commands currently available. Under each command
-is the description of the arguments to the command, and the action taken by the
-header generation tool when processing a command.
+Instructions
+-----------------------------
----------------
michaelrj-google wrote:

nit: this line should be the same length as the one above it.

https://github.com/llvm/llvm-project/pull/99712