[Bug fortran/114922] New: fsyntax-only need the modules

[Bug fortran/114922] New: fsyntax-only need the modules

[axel.ehrich@tu-clausthal.de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114922

            Bug ID: 114922
           Summary: fsyntax-only need the modules
           Product: gcc
           Version: 13.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: fortran
          Assignee: unassigned at gcc dot gnu.org
          Reporter: axel.ehrich@tu-clausthal.de
  Target Milestone: ---

Created attachment 58087
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=58087&action=edit
Short Example for this problem

I encounter the following problem with gfortran: When I try to compile a file
(such as the one attached) with the options -c -fsyntax-only,
that is

"gfortran -c -fsyntax-only syntax-only_test.f90"

I receive the following error message:
------------------------------------------------------
syntax-only_test.f90:2:5:

    2 | use module
      |     1
Fatal Error: Cannot open module file 'module.mod' for reading at (1): No such
file or directory
compilation terminated.
-----------------------------------------------

One purpose of the option "-fsyntax-only" is to create the module files (.mod),
however, without doing the compilation that would require module files. The
observed behaviour appears like a bug in gfortran.

I used 
https://aoterodelaroza.github.io/devnotes/modern-fortran-makefiles/
to look how i can solve the problem with the depends of mod files.

Best regards,
Axel

[Bug fortran/114922] fsyntax-only need the modules

[anlauf at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114922

anlauf at gcc dot gnu.org changed:

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

--- Comment #1 from anlauf at gcc dot gnu.org ---
The documentation says:

-fsyntax-only

    Check the code for syntax errors, but do not actually compile it. This will
generate module files for each module present in the code, but no other output
file.


So if you do not provide the needed modules, you should get exactly the same
errors as when compiling without -fsyntax-only.

[Bug fortran/114922] fsyntax-only need the modules

[kargls at comcast dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114922

kargls at comcast dot net changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |kargls at comcast dot net

--- Comment #2 from kargls at comcast dot net ---
While Harald gave one reason, consider the simply program

   use foo, only bar
   x = bar(1.)
   end

gfortran needs information about 'bar'.  Is it an array,
function, or subroutine?

[Bug fortran/114922] fsyntax-only need the modules

[axel.ehrich@tu-clausthal.de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114922

--- Comment #3 from Axel Ehrich <axel.ehrich@tu-clausthal.de> ---
The question is related to the actual purpose of the option -fsyntax only:

In my understanding, the intention of the option -fsyntax-only is to construct
the module files needed to compile the code. A  build process would involve two
steps: Step 1: construct all module files with gfortran -fsyntax only. Step 2:
construct the object files without this option, while relying on the presence
of all module files. (I have found this recipe on the internet and consider it
valuable.)

Alternatively, the purpose of -fsyntax-only could be to save compile time by a
doing quick first check before entering the time consuming parts of the
compilation.  If this syntax check goes so far that it requires the module
files already, the goal mentioned above cannot be reached.

[Bug fortran/114922] fsyntax-only need the modules

[mikael at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114922

Mikael Morin <mikael at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |mikael at gcc dot gnu.org

--- Comment #4 from Mikael Morin <mikael at gcc dot gnu.org> ---
(In reply to Axel Ehrich from comment #3)

> In my understanding, the intention of the option -fsyntax-only is to
> construct the module files needed to compile the code. A  build process
> would involve two steps: Step 1: construct all module files with gfortran
> -fsyntax only. Step 2: construct the object files without this option, while
> relying on the presence of all module files. (I have found this recipe on
> the internet and consider it valuable.)
> 
The normal process is to compile directly the files (without -fsyntax-only) in
the right order, in a single step.  Some people prefer to do it in two steps,
depending on their needs.

> Alternatively, the purpose of -fsyntax-only could be to save compile time by
> a doing quick first check before entering the time consuming parts of the
> compilation. 
Yes, that's the main purpose.

> If this syntax check goes so far that it requires the module
> files already, the goal mentioned above cannot be reached.

Yes, it can.  The files have to be compiled in the right order.

Keep in mind that the compiler processes one file at a time and doesn't have
a global knowledge of all the files together.

Consider this example:
  module m2
    use m1
  end module m2

What symbols should be made available in the module file for m2?
Don't you think that the knowledge of the content of m1 is needed?

[Bug fortran/114922] fsyntax-only need the modules

[axel.ehrich@tu-clausthal.de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114922

--- Comment #5 from Axel Ehrich <axel.ehrich@tu-clausthal.de> ---
Thank you for the explanation. This resolves the issue.