After my prior inquiry into the use of python as a build tool for
maintainers didn't produce any negative comments and several active and
even enthusiastic support message I'm going forward with submitting the
patch.

To repeat the detail, for the generation of the upcoming C++ standard
library module and the hints for missing definitions/declarations in the
std:: namespace we need a list of standard C++ APIs.  The information
needed for the two use cases is different but the actual APIs overlap
almost completely and therefore it would be a bad idea to have the data
separated.

We could opt for a file format that is easy to read in awk and writing the
appropriate scripts to transform the data into the appropriate output
format but this looks ugly, is hard to understand, and a nightmare to
maintain.  On the other hand, writing the code in Python is simple and
clean.


Therefore, Jonathan and I worked on a CSV file which contains the necessary
information and a Python to create the gperf input file to generate
std-name-hint.h and also, in future, the complete source of the export
interface description for the standard library module.  This mode is not
yet used because the module support isn't ready yet.  The output file
corresponds to the hand-coded version of the export code Jonathan uses
right now.

Note that in both of these cases the generated files are static, they don't
depend on the local configuration and therefore are checked into the source
code repository.  The script only has to run if the generated files are
explicitly removed or, in maintainer mode, if the CSV file has changed.
For normal compilation from a healthy source code tree the tool is not
needed.


One remaining issue is the responsibility for the CSV file.  The file needs
to live in the directory of the frontend and therefore nominally changes
need to be approved by the frontend maintainers.  The content entirely
consists of information from the standard library, though.  Any change that
doesn't break the build on one machine (i.e., the Python script doesn't
fail) will not course any problem because the output format of the script
is correct.  Therefore we have been wondering whether the CSV file should
at least have shared ownership between the frontend maintainers and the
libstdc++ maintainers.

The CSV file contain more hint information than the old hand-coded .gperf
file.  So, an additional effect of this patch is the extension of the hints
that are provided but given that the lookup is now fast this shouldn't have
any negative impact.  The file is not complete, though, this will come over
time and definitely before the module support is done.

I build my complete set of compilers with this patch without problems.

Any comments?

2022-09-12  Jonathan Wakely  <jwakely@redhat.com>
            Ulrich Drepper  <drepper@redhat.com>

contrib/ChangeLog
        * gcc_update: Add rule to generate gcc/cp/std-name-hint.gperf.

gcc/cp/ChangeLog
        * Make-lang.in: Add rule to generate gcc/cp/std-name-hint.gperf.
        Adjust rule to generate $(srcdir)/cp/std-name-hint.h.
        Add explicit rule to depend cp/name-lookup.o on
        $(srcdir)/cp/std-name-hint.h.
        * cxxapi-data.csv: New file.  Database of C++ APIs.
        * gen-cxxapi-file.py: New file.  Script to generate source code for
        C++ standard library exports and to generate C++ std:: namespace
        fix hints.
        * std-name-hint.gperf: Regenerated.
        * std-name-hint.h: Regenerated.