[Bug preprocessor/108101] New: "#pragma once" causes other files not be included

[Bug preprocessor/108101] New: "#pragma once" causes other files not be included

[terra at gnome dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108101

            Bug ID: 108101
           Summary: "#pragma once" causes other files not be included
           Product: gcc
           Version: 12.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: preprocessor
          Assignee: unassigned at gcc dot gnu.org
          Reporter: terra at gnome dot org
  Target Milestone: ---

# (echo '#pragma once'; echo 'Killroy was here') > a.h
# (echo '#pragma once'; echo 'Killroy was here') > b.h
# (echo '#include "a.h"'; echo '#include "b.h"') > c.c
# gcc -E c.c | grep Killroy
1
# clang -E c.c | grep Killroy
2

According to gcc, Killroy was only there once.  Clang is keeping better track
and spots him there twice.

This seems to be version independent: tried 7.5.0 and 12.1.0

[Bug preprocessor/108101] "#pragma once" causes other files not be included

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108101

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Iirc pragma once uses a hash of the content and that is what is going on here.
That or the underlying file system is using some inode being the same and copy
on write.

[Bug preprocessor/108101] "#pragma once" causes other files not be included

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108101

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |DUPLICATE
             Status|UNCONFIRMED                 |RESOLVED

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Dup of bug 52566

*** This bug has been marked as a duplicate of bug 52566 ***

[Bug preprocessor/108101] "#pragma once" causes other files not be included

[terra at gnome dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108101

--- Comment #3 from M Welinder <terra at gnome dot org> ---
Note also:

# gcc -MM c.c
c.o: c.c a.h

No b.h present!