[libc] Generate configure.rst from the JSON config information. (#65791)

libc/CMakeLists.txt

@@ -143,6 +143,7 @@ foreach(opt IN LISTS global_config)
   message(STATUS "${opt_name}: ${opt_value}")
   set(${opt_name} ${opt_value})
 endforeach()
+generate_config_doc(${main_config_file} ${LIBC_SOURCE_DIR}/docs/configure.rst)
 load_libc_config(${LIBC_SOURCE_DIR}/config/${LIBC_TARGET_OS}/config.json ${cmd_line_conf})
 load_libc_config(${LIBC_SOURCE_DIR}/config/${LIBC_TARGET_OS}/${LIBC_TARGET_ARCHITECTURE}/config.json ${cmd_line_conf})
 

libc/cmake/modules/LibcConfig.cmake

@@ -47,7 +47,6 @@ function(read_libc_config config_file opt_list)
     # to load. If there are no config options, it is better to remove that
     # config.json file instead of including an empty file.
     message(FATAL_ERROR "${config_file}: Does not contain any config option groups")
-    return()
   endif()
   math(EXPR group_count_1 "${group_count} - 1")
 
@@ -135,3 +134,84 @@ function(load_libc_config config_file)
     set(${opt_name} ${opt_value} PARENT_SCOPE)
   endforeach()
 endfunction()
+
+function(generate_config_doc config_file doc_file)
+  if(NOT EXISTS ${config_file})
+    message(FATAL_ERROR "${config_file} does not exist")
+  endif()
+  file(READ ${config_file} json_config)
+  string(JSON group_count ERROR_VARIABLE json_error LENGTH ${json_config})
+  if(json_error)
+    message(FATAL_ERROR "${config_file}: ${json_error}")
+  endif()
+  if(${group_count} EQUAL 0)
+    message(FATAL_ERROR "${config_file}: Does not contain any config option groups")
+  endif()
+  math(EXPR group_count_1 "${group_count} - 1")
+
+  set(doc_string ".. _configure:\n"
+                 "..\n"
+                 "   Do not edit this file directly. CMake will auto generate it.\n"
+                 "   If the changes are intended, add this file to your commit.\n"
+                 "\n"
+                 "==========================\n"
+                 "Configure Options\n"
+                 "==========================\n"
+                 "\n"
+                 "Below is the full set of options one can use to configure the libc build.\n"
+                 "An option can be given an explicit value on the CMake command line using\n"
+                 "the following syntax:\n"
+                 "\n"
+                 ".. code-block:: sh\n"
+                 "\n"
+                 "  $> cmake <other build options> -D<libc config option name>=<option value> <more options>\n"
+                 "\n"
+                 "For example:\n"
+                 "\n"
+                 ".. code-block:: sh\n"
+                 "\n"
+                 "  $> cmake <other build options> -DLIBC_CONF_PRINTF_DISABLE_FLOAT=ON <more options>\n"
+                 "\n"
+                 "See the main ``config/config.json``, and the platform and architecture specific\n"
+                 "overrides in ``config/<platform>/config.json`` and ``config/<platform>/<arch>/config.json,``\n"
+                 "to learn about the defaults for your platform and target.\n"
+                 "\n")
+
+  foreach(group_num RANGE ${group_count_1})
+    string(JSON group_name ERROR_VARIABLE json_error MEMBER ${json_config} ${group_num})
+    if(json_error)
+      message(FATAL_ERROR "${config_file}: ${json_error}")
+    endif()
+    string(APPEND doc_string "* **\"${group_name}\" options**\n")
+    string(JSON option_map ERROR_VARIABLE json_error GET ${json_config} ${group_name})
+    if(json_error)
+      message(FATAL_ERROR ${json_error})
+    endif()
+    string(JSON option_count ERROR_VARIABLE jsor_error LENGTH ${option_map})
+    if(json_error)
+      message(FATAL_ERROR ${json_error})
+    endif()
+    if(${option_count} EQUAL 0)
+      message(FATAL_ERROR "${config_file}: No options listed against the config option group '${group_name}'")
+    endif()
+
+    math(EXPR option_count_1 "${option_count} - 1")
+    foreach(opt_num RANGE ${option_count_1})
+      string(JSON option_name ERROR_VARIABLE json_error MEMBER ${option_map} ${opt_num})
+      if(json_error)
+        message(FATAL_ERROR ${json_error})
+      endif()
+      string(JSON opt_object ERROR_VARIABLE json_error GET ${option_map} ${option_name})
+      if(json_error)
+        message(FATAL_ERROR "Error generating ${doc_file}: ${json_error}\n${opt_object}")
+      endif()
+      string(JSON opt_doc ERROR_VARIABLE json_error GET ${opt_object} "doc")
+      if(json_error)
+        message(FATAL_ERROR "Error generating ${doc_file}: ${json_error}")
+      endif()
+      string(APPEND doc_string "    - ``${option_name}``: ${opt_doc}\n")
+    endforeach()
+  endforeach()
+  message(STATUS "Writing config doc to ${doc_file}")
+  file(WRITE ${doc_file} ${doc_string})
+endfunction()

libc/docs/configure.rst

@@ -0,0 +1,31 @@
+.. _configure:
+..
+   Do not edit this file directly. CMake will auto generate it.
+   If the changes are intended, add this file to your commit.
+
+==========================
+Configure Options
+==========================
+
+Below is the full set of options one can use to configure the libc build.
+An option can be given an explicit value on the CMake command line using
+the following syntax:
+
+.. code-block:: sh
+
+  $> cmake <other build options> -D<libc config option name>=<option value> <more options>
+
+For example:
+
+.. code-block:: sh
+
+  $> cmake <other build options> -DLIBC_CONF_PRINTF_DISABLE_FLOAT=ON <more options>
+
+See the main ``config/config.json``, and the platform and architecture specific
+overrides in ``config/<platform>/config.json`` and ``config/<platform>/<arch>/config.json,``
+to learn about the defaults for your platform and target.
+
+* **"printf" options**
+    - ``LIBC_CONF_PRINTF_DISABLE_FLOAT``: Disable printing floating point values in printf and friends.
+    - ``LIBC_CONF_PRINTF_DISABLE_INDEX_MODE``: Disable index mode in the printf format string.
+    - ``LIBC_CONF_PRINTF_DISABLE_WRITE_INT``: Disable handling of %n in printf format string.

libc/docs/index.rst

@@ -52,6 +52,7 @@ stages there is no ABI stability in any form.
    usage_modes
    overlay_mode
    fullbuild_mode
+   configure
    gpu/index.rst
 
 .. toctree::