[cfe-dev] Configuration files

James Y Knight via cfe-dev cfe-dev at lists.llvm.org
Tue Sep 27 12:18:24 PDT 2016

I really don't see what the purpose of this feature is, if it's only going
to work effectively identically to an "@args-file" option.

If that's all you want, it doesn't even need to be in the clang driver, a 5
line shell script that looks at $0 to choose a config file to read can work
just as well.

But I don't think that's really what people want -- I believe that any
workable config-file scheme *must* interact with the various target
specification command-line arguments, and allow for setting options
dependent upon the actually-selected target.

On Tue, Sep 20, 2016 at 3:13 AM, Serge Pavlov via cfe-dev <
cfe-dev at lists.llvm.org> wrote:

> Thanks to all for feedback!
> I tried to summarize the feedback in the form of user-visible description
> (possibly a part of user documentation) and random set of development
> specific notes.
> ------ User documentation ------
> Using clang as a part of toolchain, especially in the case of
> cross-compilation, require setting up large amount of parameters - location
> of headers and libraries, set of enabled warnings, triplet names etc.
> Changing from debug to release build, or from host processor to accelerator
> or any other change of build configuration requires substantial changes of
> these parameters. To help maintaining option sets for various build
> variants, sets of options may be combined into configurations and stored in
> *configuration files*. Selection of particular configuration file activates
> all options it represents.
> Configuration file may be selected in several ways:
> - Using command line option --config,
> - By setting up environmental variable CLANGCFG,
> - As default configuration.
> Only one way may be used. If option ''--config" is specified, CLANGCFG is
> not checked and the default configuration is not be applied even if the
> requested configuration is not found. Similarly, if variable CLANGCFG
> exists, default configuration is never applied.
> Command line option --config expects argument either full path to
> configuration file, or a name of configuration, for instance:
> --config /home/user/cfgs/testing.cfg
> --config debug
> --config debug.cfg
> If full path is specified, options are read from that file. If
> configuration is specified by name with optional suffix ".cfg",
> corresponding configuration file is searched in the directories in the
> following order:
> - ~/.llvm
> - /etc/llvm
> If the option --config is absent, and environment variable CLANGCFG is
> set, content of CLANGCFG is used as full path to configuration file. If
> CLANGCFG is empty, config file is not used, no diagnostic produced.
> If neither --config nor CLANGCFG are specified, default config file is
> searched in the following order:
> - ~/.llvm/clang.cfg
> - /etc/llvm/clang.cfg
> - clang.cfg in the directory where clang executable resides.
> Configuration file is the sequence of compiler options specified in one or
> several lines. Lines started with '#' possibly prepended with space
> characters are comments, they are ignored. Lines started with '#" in the
> first column immediately followed by ':' are reserved for directives. The
> file may reference other files using usual syntax for response files:
> @included_file. Example of configuration file:
> # Frontend options
> -std=c++14 -fcxx-exceptions
> # Directories
> @include-dirs.cfg
> @library-dirs
> Name of the active configuration file and options it provided can be
> obtained by call of clang with options '-v' or '-###'.
> ------ End of user documentation ------
> Notes:
> 1. What should be the name of default config file, 'default.cfg' or
> 'clang.cfg'? If some tool other than clang will use the same mechanism,
> name 'clang.cfg' looks more appropriate.
> 2. Should compiler emit a warning (or even an error) if the specified
> configuration file was not found? Obviously absence of default config file
> should be silently ignored, as well as empty CLANGCFG variable. But what if
> configuration specified by --config is not found? This looks like a severe
> error, as user expectation are broken.
> 3. Default config file may be searched for in  the directory where clang
> executable resides. Should configuration specified by --config be searched
> in this directory as well? I would say no, because a user who need tuning
> configurations may prepare them in home directory or use those provided by
> installation in system directories. Ability to place default config into
> binary directory is convenient for compiler developer or CI tools, they may
> uses several variants of compiler simultaneously.
> 4. Format of proposed config file is in fact gnu response file. YAML
> format mentioned by Richard looks nice but it requires development of
> proper parser. I would propose at first implement the simple format. In
> future other format can be supported, we can distinguish formats by putting
> a directive like '#:format=yaml' or even automatically.
> 5. Some comments may be reserved for future use. Format directive
> mentioned above is an example. The sequence '#:' proposed as marker is
> absolutely arbitrary, any other prefix may be used. Now compiler may warn
> if it sees such comment. Probably we do not need to bother about the future
> extensions now.
> 6. Using response files in the form @file can imitate include directive.
> If a user do not want to copy-and-past pieces of config files, he may
> structure them using this feature.
> Thanks,
> --Serge
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