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Subject: [Bug c++/99072] [modules] Compiling header unit with partial
 preprocessing (-E -fdirectives-only) twice causes CRC mismatch
Date: Wed, 24 Feb 2021 17:14:41 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D99072

--- Comment #6 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Nathan Sidwell <nathan@gcc.gnu.org>:

https://gcc.gnu.org/g:a13be187cb2987db851b3f096f5319d5fe3a7301

commit r11-7366-ga13be187cb2987db851b3f096f5319d5fe3a7301
Author: Nathan Sidwell <nathan@acm.org>
Date:   Wed Feb 24 05:50:12 2021 -0800

    c++: modules & -fpreprocessed [PR 99072]

    When we read preprocessed source, we deal with a couple of special
    location lines at the start of the file.  These provide information
    about the original filename of the source and the current directory,
    so we can process the source in the same manner.  When updating that
    code, I had a somewhat philosophical question: Should the line table
    contain evidence of the filename the user provided to the compiler?  I
    figured to leave it there, as it did no harm.  But this defect shows
    an issue.  It's in the line table and our (non optimizing) line table
    serializer emits that filename.  Which means if one re-preprocesses
    the original source to a differently-named intermediate file, the
    resultant CMI is different.  Boo.  That's a difference that doesn't
    matter, except the CRC matching then fails.  We should elide the
    filename, so that one can preprocess to mktemp intermediate filenames
    for whatever reason.

    This patch takes the approach of expunging it from the line table --
    so the line table will end up with exactly the same form.  That seems
    a better bet than trying to fix up mismatching line tables in CMI
    emission.

            PR c++/99072
            libcpp/
            * init.c (read_original_filename): Expunge all evidence of the
            original filename.
            gcc/testsuite/
            * g++.dg/modules/pr99072.H: New.=