[libc] Add a JSON based config option system.

[sivachandra]

Summary

Few printf config options have been setup using this new config system
along with their baremetal overrides. A follow up patch will add generation
of doc/config.rst, which will contain the full list of libc config options
and short description explaining how they affect the libc.

[sivachandra]

This patch implements the core of the scheme proposed in https://discourse.llvm.org/t/rfc-systematic-way-to-introduce-and-use-libc-config-options/72943.

[gchatelet]

libc/CMakeLists.txt

99–100

Maybe it's clearer with a direct form "CMake command line option takes precedence over 'config.json' options".

109

typo

libc/cmake/modules/LibcConfig.cmake

26–28

Maybe state in the documentation that the function does nothing if the file is missing.

98–102

This is not super clear, can you rephrase without double negation? Also maybe an exemple would help.

[sivachandra]

Address comments.

[sivachandra]

Remove few debug commands.

[phosek]

libc/CMakeLists.txt

116

What's the reason for removing the variable from cache?

[gchatelet]

libc/cmake/modules/LibcConfig.cmake

27

typo

102

typo existence

104

its

123

?

[sivachandra]

Fix typos identified by the review.

[sivachandra]

libc/CMakeLists.txt

116

I think you saw the previous version which had those commands. I put them in to debug but are removed in the latest version.

libc/cmake/modules/LibcConfig.cmake

123

Didn't understand :)

[gchatelet]

libc/cmake/modules/LibcConfig.cmake

123

Sorry I should have been more explicit.

I can't parse This option is not be overriden, is this This option is not *to* be *overridden*?

[sivachandra]

Fix another typo.

libc/cmake/modules/LibcConfig.cmake

123

Oh! Fixed now.

[gchatelet]

libc/cmake/modules/LibcConfig.cmake

123

I think there's still one d missing in overriden, should be overridden :)

[sivachandra]

Fix yet another typo.

[sivachandra]

Rebase

libc/CMakeLists.txt

 set(LIBC_TARGET_TRIPLE "" CACHE STRING "The target triple for the libc build.")
 
 set(LIBC_ENABLE_UNITTESTS ON)
 set(LIBC_ENABLE_HERMETIC_TESTS ${LLVM_LIBC_FULL_BUILD})
 
 # Defines LIBC_TARGET_ARCHITECTURE and associated macros.
 include(LLVMLibCArchitectures)
 
+include(LibcConfig)
+# Config loading happens in three steps:
+# 1. Load the config file config/config.json and set up config vars.
+# 2. Load config/${LIBC_TARGET_OS}/config.json if available and override
+#    vars as suitable.
+# 3. Load config/${LIBC_TARGET_OS}/${LIBC_TARGET_ARCH}/config.json is
+#    available and override vars as suitable.
+# All the three steps will not override options already set from the
+# CMake command line.
+set(main_config_file ${LIBC_SOURCE_DIR}/config/config.json)
+read_libc_config(${main_config_file} global_config)
+foreach(opt IN LISTS global_config)
+  string(JSON opt_name ERROR_VARIABLE json_error MEMBER ${opt} 0)
+  if(json_error)
+    message(FATAL_ERROR ${json_error})
+  endif()
+  if(DEFINED ${opt_name})
+    # The option is alredy defined from the command line so we ignore it here.
+    # We still make note of it so that further config load will also ignore
+    # this option.
+    message(STATUS "${opt_name}: ${${opt_name}} (from command line)")
+    list(APPEND cmd_line_conf ${opt_name})
+    # Remove the variable from the cache.
+    set(temp ${${opt_name}})
+    unset(${opt_name} CACHE)
+    set(${opt_name} ${temp})
+    continue()
+  endif()
+
+  string(JSON opt_object ERROR_VARIABLE json_error GET ${opt} ${opt_name})
+  if(json_error)
+    message(FATAL_ERROR "Error reading info of option '${opt_name}': ${json_error}")
+  endif()
+  string(JSON opt_value ERROR_VARIABLE json_error GET ${opt_object} "value")
+  if(json_error)
+    message(FATAL_ERROR ${json_error})
+  endif()
+  message(STATUS "${opt_name}: ${opt_value}")
+  set(${opt_name} ${opt_value})
+endforeach()
+load_libc_config(${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})
+
 if(LIBC_TARGET_ARCHITECTURE_IS_GPU)
   set(LIBC_INCLUDE_DIR ${CMAKE_CURRENT_BINARY_DIR}/include)
   set(LIBC_INSTALL_INCLUDE_DIR ${CMAKE_INSTALL_INCLUDEDIR}/gpu-none-llvm)
   set(LIBC_LIBRARY_DIR ${LLVM_LIBRARY_OUTPUT_INTDIR})
 elseif(LLVM_ENABLE_PER_TARGET_RUNTIME_DIR AND LIBC_ENABLE_USE_BY_CLANG)
   set(LIBC_INCLUDE_DIR ${LLVM_BINARY_DIR}/include/${LLVM_DEFAULT_TARGET_TRIPLE})
   set(LIBC_INSTALL_INCLUDE_DIR ${CMAKE_INSTALL_INCLUDEDIR}/${LLVM_DEFAULT_TARGET_TRIPLE})
   set(LIBC_LIBRARY_DIR ${LLVM_LIBRARY_OUTPUT_INTDIR}/${LLVM_DEFAULT_TARGET_TRIPLE})

libc/cmake/modules/LibcConfig.cmake

+# This cmake module contains utilities to handle libc config options.
+# The information about the config options is stored in the JSON format.
+# The JSON parsing commands that CMake provides are rather tedious to use.
+# Below is a quick reference which tries to map the CMake JSON parsing
+# commands to the Python dictionary API.
+#
+# * There is no way to iterate over the JSON items. One will first
+#   have to find the number of items using string(JSON ... LENGTH ...)
+#   command, and then iterate over the items using foreach(... RANGE ...).
+# * The way to get the key from the JSON dictionary is to use the index
+#   of the item and the string(JSON ... MEMBER ... $<index>) function.
+# * Once you have the key, you can use the string(JSON ... GET ... $<key>)
+#   function to get the value corresponding to the key.
+
+# Fill |opt_list| with all options listed in |config_file|. For each option,
+# the item added to |opt_list| is the dictionary of the form:
+#   {
+#     "<option name>": {
+#       "value: <option value>,
+#       "doc": "<option doc string>",
+#     }
+#   }
+# Each of the above items can be parsed again with the string(JSON ...)
+# command.
+function(read_libc_config config_file opt_list)
+  if(NOT EXISTS ${config_file})
+    return()
+  endif()
+  # We will assume that a config file is loaded only once and that
+  # each config file loaded will affect config information. Since
+  # we want a change to config information to trigger reconfiguration,
+  # we added the |config_file| to the list of file the configure itself
+  # should depend on.
+  set_property(
+    DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}
+    PROPERTY CMAKE_CONFIGURE_DEPENDS ${config_file})
+
+  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)
+    # This "if" conditions becomes active if there are no config options
+    # 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")
+
+  set(optname_list)
+  foreach(group_num RANGE ${group_count_1})
+    # The group names are the keys of the global dictionary. So, we first
+    # lookup the group name or the key for each item in the dictionary.
+    string(JSON group_name ERROR_VARIABLE json_error MEMBER ${json_config} ${group_num})
+    if(json_error)
+      message(FATAL_ERROR "${config_file}: ${json_error}")
+    endif()
+
+    # Once we have the group name, we GET the option map for that group, which
+    # is the value corresponding to the group name key.
+    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()
+      list(FIND optname_list ${option_name} optname_exists)
+      if(${optname_exists} GREATER -1)
+        message(FATAL_ERROR "${config_file}: Found duplicate option name: ${option_name}")
+      endif()
+      list(APPEND optname_list ${option_name})
+
+      string(JSON optdata ERROR_VARIABLE json_error GET ${option_map} ${option_name})
+      if(json_error)
+        message(FATAL_ERROR ${json_error})
+      endif()
+      set(opt "{\"${option_name}\": ${optdata}}")
+      list(APPEND all_opts ${opt})
+    endforeach()
+  endforeach()
+  set(${opt_list} ${all_opts} PARENT_SCOPE)
+endfunction()
+
+# Loads the config options listed in |config_file|. If any options are not to
+# be overridden, then their names should be passed after |config_file|. For
+# names not present in the not-to-be-overriden list, a cmake var with that
+# name is created if not already present and set to the value listed in the
+# config file.
+function(load_libc_config config_file)
+  read_libc_config(${config_file} file_opts)
+  foreach(opt IN LISTS file_opts)
+    string(JSON opt_name ERROR_VARIABLE json_error MEMBER ${opt} 0)
+    if(json_error)
+      message(FATAL_ERROR ${json_error})
+    endif()
+    if(NOT DEFINED ${opt_name})
+      message(FATAL_ERROR: " Option ${opt_name} defined in ${config_file} is invalid.")
+    endif()
+    if(ARGN)
+      list(FIND ARGN ${opt_name} optname_exists)
+      if(${optname_exists} GREATER -1)
+        continue()
+      endif()
+    endif()
+    string(JSON opt_object ERROR_VARIABLE json_error GET ${opt} ${opt_name})
+    if(json_error)
+      message(FATAL_ERROR ${json_error})
+    endif()
+    string(JSON opt_value ERROR_VARIABLE jsor_error GET ${opt_object} "value")
+    if(json_error)
+      message(FATAL_ERROR ${json_error})
+    endif()
+    message(STATUS "Overriding - ${opt_name}: ${opt_value} (Previous value: ${${opt_name}})")
+    set(${opt_name} ${opt_value} PARENT_SCOPE)
+  endforeach()
+endfunction()

libc/config/baremetal/config.json

+{
+  "printf": {
+    "LIBC_CONF_PRINTF_DISABLE_FLOAT": {
+      "value": true
+    },
+    "LIBC_CONF_PRINTF_DISABLE_INDEX_MODE": {
+      "value": true
+    },
+    "LIBC_CONF_PRINTF_DISABLE_WRITE_INT": {
+      "value": true
+    }
+  }
+}

libc/config/config.json

+{
+  "printf": {
+    "LIBC_CONF_PRINTF_DISABLE_FLOAT": {
+      "value": false,
+      "doc": "Disable printing floating point values in printf and friends."
+    },
+    "LIBC_CONF_PRINTF_DISABLE_INDEX_MODE": {
+      "value": false,
+      "doc": "Disable index mode in the printf format string."
+    },
+    "LIBC_CONF_PRINTF_DISABLE_WRITE_INT": {
+      "value": false,
+      "doc": "Disable handling of %n in printf format string."
+    }
+  }
+}

libc/src/stdio/CMakeLists.txt

DEPENDS
     libc.src.stdio.printf_core.printf_main
     libc.src.stdio.printf_core.writer
 )
 
 list(APPEND printf_deps
       libc.src.__support.arg_list
       libc.src.stdio.printf_core.vfprintf_internal
 )
+
+if(LIBC_CONF_PRINTF_DISABLE_FLOAT)
+  list(APPEND printf_copts "-DLIBC_COPT_PRINTF_DISABLE_FLOAT")
+endif()
+if(LIBC_CONF_PRINTF_DISABLE_INDEX_MODE)
+  list(APPEND printf_copts "-DLIBC_COPT_PRINTF_DISABLE_INDEX_MODE")
+endif()
+if(LIBC_CONF_PRINTF_DISABLE_WRITE_INT)
+  list(APPEND printf_copts "-DLIBC_COPT_PRINTF_DISABLE_WRITE_INT")
+endif()
+
 if(LLVM_LIBC_FULL_BUILD)
  list(APPEND printf_deps
       libc.src.__support.File.file
       libc.src.__support.File.platform_file
       libc.src.__support.File.platform_stdout
   )
 else()
- set(printf_copts "-DLIBC_COPT_PRINTF_USE_SYSTEM_FILE")
+ list(APPEND printf_copts "-DLIBC_COPT_PRINTF_USE_SYSTEM_FILE")
 endif()
 
 add_entrypoint_object(
   printf
   SRCS
     printf.cpp
   HDRS
     printf.h