On 07/24/2017 10:18 AM, Serge Pavlov wrote:
I am thinking about reducing the patch further to leave only the ability to include config file when clang is called as `target-clang-drivermode`. It is still useful for cross compilation tasks because:
- It is a convenient way to switch between supported targets,
- SDK producer can ship compiler with a set of appropriate options or prepare them during installation. In this case if clang is called as `target-clang-drivermode`, it first tries to find file `target-drivermode.cfg` or `target.cfg` in a set of well-known directories, which in minimal case includes the directory where clang executable resides. If such file is found, options are read from it, otherwise only option --target is added as clang does it now.


This solution has obvious drawbacks:
- User cannot specify config file in command line in the same way as he can choose a target: `clang --target <target>`, - On Windows symlinks are implemented as file copy, the solution looks awkward. So more or less complete solution needs to allow specifying config file in command line.

I'd rather not reduce the patch in this way, and you didn't describe why you're considering reducing the patch. Can you please elaborate?


Using `@file` has some problems. Config file is merely a set of options, just as file included by `@file`. Different include file search is only a convenience and could be sacrificed. Comments and unused option warning suppression could be extended for all files included with `@file`. The real problem is the search path. To be useful, config files must be searched for in well-known directories, so that meaning of `clang @config_fille` does not depend on the current directory. So clang must have some rule to distinguish between config file and traditional use of `@file`. For instance, if file name ends with `.cfg` and there is a file with this name in config search directories, this is a config file and it is interpreted a bit differently. Of course, the file may be specified with full path, but this way is inconvenient.

I see no reason why we can't unify the processing but have different search-path rules for @file vs. --config file.


Another possible solution is to extend meaning of `--target` so that it fully matches with the use of `target-clang-drivermode`, that is the option `--target=hexagon` causes clang first to look for the file `hexagon.cfg` in well-known directories and use it if found. In this case treatment of `--target` is different if the option is specified in command line or in the content of config file (in the latter case it is processed as target name only), it may be confusing. Besides, use of config files is not restricted to the choice of target.

I think we should do this, so long as the implementation is reasonable, and the special case doesn't bother me in this regard. I don't view this as a replacement for '--config file', however, because, as you mention, the config files need not be restricted to target triples.

Thanks again,
Hal


Using special option for config files does not bring risk of compatibility breakage and does not change meaning of existing options.


Thanks,
--Serge

2017-05-10 11:25 GMT+07:00 Serge Pavlov <sepavl...@gmail.com <mailto:sepavl...@gmail.com>>:

    2017-05-10 3:46 GMT+07:00 Richard Smith <rich...@metafoo.co.uk
    <mailto:rich...@metafoo.co.uk>>:

        On 1 March 2017 at 02:50, Serge Pavlov via Phabricator
        <revi...@reviews.llvm.org <mailto:revi...@reviews.llvm.org>>
        wrote:


            Format of configuration file is similar to file used in
            the construct `@file`, it is a set of options.
            Configuration file have advantage over this construct:

            - it is searched for in well-known places rather than in
            current directory,


        This (and suppressing unused-argument warnings) might well be
        sufficient to justify a different command-line syntax rather
        than @file...


    Construct `@file` in this implementation is used only to read
    parts of config file inside containing file. Driver knows that it
    processes config file and can adjust treatment of `@file`. On the
    other hand, driver might parse config files in a more complicated
    way, for instance, it could treat line `# include(file_name)` as a
    command to include another file.

            - it may contain comments, long options may be split
            between lines using trailing backslashes,
            - other files may be included by `@file` and they will be
            resolved relative to the including file,


        ... but I think we should just add these extensions to our
        @file handling, and then use the exact same syntax and code to
        handle config files and @file files. That is, the difference
        between @ and --config would be that the latter looks in a
        different directory and suppresses "unused argument" warnings,
        but they would otherwise be identical.


    Changing treatment of `@file` can cause compatibility issues, in
    particular, both libiberty and cl resolves file name relative to
    current directory. So driver must deduce that `@file` is used to
    load config file rather than merely to organize arguments. Another
    difference is that `@file` inserts its content in the place where
    it occurs, while `--config` always puts arguments before user
    specified options. The following invocations:

        clang --config a.cfg -opt1 -opt2 file1.cpp
        clang -opt1 -opt2 file1.cpp --config a.cfg

    are equivalent, but variants with `@file` can have different effect.


            - the file may be encoded in executable name,
            - unused options from configuration file do not produce
            warnings.


            https://reviews.llvm.org/D24933
            <https://reviews.llvm.org/D24933>







--
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory

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