Allow packager to provide a default configuration file for clang

[serge-sans-paille]

Summary

Such file can be set at configuration time through -DCLANG_DEFAULT_CONFIG_FILE
and is used if no other configuration is provided.

It allows package maintainer to provide per-distribution default flags like
default gcc toolchain, linker error script etc.

[sylvestre.ledru]

Great idea. Do you have an example of what the configuration could look like on a distro?
Merci!

[serge-sans-paille]

I was thinking of `-Wl,--error-handling-script=/etc/clang/linker-script.sh` or something along the line. The initial goal would be to suggest installing the asan runtime if some asan symbols are missing.

[tstellar]

@sylvestre.ledru Can you drop the picture?

[tstellar]

Can you give an example of how this would be used to select the gcc-toolchain?

[serge-sans-paille]

Can you give an example of how this would be used to select the gcc-toolchain?

That would mean shipping a configuration file with a

--gcc-toolchain=/path/to/the/toolchain

[sylvestre.ledru]

In D91598#2400042, @tstellar wrote:

@sylvestre.ledru Can you drop the picture?

Sure, this is one of the phab macro and I wasn't expecting it to be that big :)

[sylvestre.ledru]

It would be great to:

• provide examples
• mention it in the release notes
• have some tests

clang/docs/UsersManual.rst

treated as a file name and is searched for sequentially in the directories:
     - system directory,
     - the directory where Clang executable resides.
 
 Both user and system directories for configuration files are specified during
 clang build using CMake parameters, CLANG_CONFIG_FILE_USER_DIR and
 CLANG_CONFIG_FILE_SYSTEM_DIR respectively. The first file found is used. It is
 an error if the required file cannot be found.
 
+If clang was built with the CMake parameter CLANG_DEFAULT_CONFIG_FILE, and no
+other configuration file is specified, that file is always looked up for.
+
 Another way to specify a configuration file is to encode it in executable name.
 For example, if the Clang executable is named `armv7l-clang` (it may be a
 symbolic link to `clang`), then Clang will search for file `armv7l.cfg` in the
 directory where Clang resides.
 
 If a driver mode is specified in invocation, Clang tries to find a file specific
 for the specified mode. For example, if the executable file is named
 `x86_64-clang-cl`, Clang first looks for `x86_64-cl.cfg` and if it is not found,

clang/include/clang/Config/config.h.cmake

 
 /* Directories clang will search for headers */
 #define C_INCLUDE_DIRS "${C_INCLUDE_DIRS}"
 
 /* Directories clang will search for configuration files */
 #cmakedefine CLANG_CONFIG_FILE_SYSTEM_DIR "${CLANG_CONFIG_FILE_SYSTEM_DIR}"
 #cmakedefine CLANG_CONFIG_FILE_USER_DIR "${CLANG_CONFIG_FILE_USER_DIR}"
 
+/* Default config config for clang, prepended to every clang run */
+#cmakedefine CLANG_DEFAULT_CONFIG_FILE "${CLANG_DEFAULT_CONFIG_FILE}"
+
 /* Default <path> to all compiler invocations for --sysroot=<path>. */
 #define DEFAULT_SYSROOT "${DEFAULT_SYSROOT}"
 
 /* Directory where gcc is installed. */
 #define GCC_INSTALL_PREFIX "${GCC_INSTALL_PREFIX}"
 
 /* Define if we have libxml2 */
 #cmakedefine CLANG_HAVE_LIBXML ${CLANG_HAVE_LIBXML}

clang/lib/Driver/Driver.cpp

if (CLOptions) {
     if (ConfigFiles.size() > 1) {
       if (!std::all_of(
               ConfigFiles.begin(), ConfigFiles.end(),
               [ConfigFiles](std::string s) { return s == ConfigFiles[0]; })) {
         Diag(diag::err_drv_duplicate_config);
         return true;
       }
     }
+#ifdef CLANG_DEFAULT_CONFIG_FILE
+    if (ConfigFiles.empty())
+      ConfigFiles.push_back(CLANG_DEFAULT_CONFIG_FILE);
+#endif
 
     if (!ConfigFiles.empty()) {
       CfgFileName = ConfigFiles.front();
       assert(!CfgFileName.empty());
 
       // If argument contains directory separator, treat it as a path to
       // configuration file.
       if (llvm::sys::path::has_parent_path(CfgFileName)) {