[Bug fortran/56293] New: I/O: Segfault in write_float when trying to print a not-word-aligned REAL(16) / -fno-align-commons

[Bug fortran/56293] New: I/O: Segfault in write_float when trying to print a not-word-aligned REAL(16) / -fno-align-commons

[burnus at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56293

             Bug #: 56293
           Summary: I/O: Segfault in write_float when trying to print a
                    not-word-aligned REAL(16) /  -fno-align-commons
    Classification: Unclassified
           Product: gcc
           Version: 4.8.0
            Status: UNCONFIRMED
          Keywords: wrong-code
          Severity: normal
          Priority: P3
         Component: fortran
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: burnus@gcc.gnu.org
                CC: jb@gcc.gnu.org, jvdelisle@gcc.gnu.org


Reported by Maciej Skrzypek at
http://gcc.gnu.org/ml/fortran/2013-02/msg00058.html

In the following code, "p" is not aligned; using it fails in GCC 4.8 with a
segfault in

#0  0x00007ffff7ba5e16 in write_float (dtp=dtp@entry=0x7fffffffde40,
f=f@entry=0x7fffffffdd70, source=source@entry=0x601084 <sss_+4> "",
len=len@entry=16, 
    comp_d=comp_d@entry=1) at libgfortran/io/write_float.def:1259
#1  0x00007ffff7ba8365 in _gfortrani_write_real (dtp=dtp@entry=0x7fffffffde40,
source=source@entry=0x601084 <sss_+4> "", length=length@entry=16)
    at libgfortran/io/write.c:1470
#2  0x00007ffff7ba8dcf in list_formatted_write_scalar (size=16, kind=16,
p=0x601084 <sss_+4>, type=BT_REAL, dtp=0x7fffffffde40) at
libgfortran/io/write.c:1571
#3  _gfortrani_list_formatted_write (dtp=0x7fffffffde40, type=BT_REAL,
p=<optimized out>, kind=16, size=16, nelems=1) at libgfortran/io/write.c:1599



Compile with:  -fno-align-commons

      program main
      real(16) p
      integer i
      common/sss/ i, p
      print*, p
      end

[Bug fortran/56293] I/O: Segfault in write_float when trying to print a not-word-aligned REAL(16) / -fno-align-commons

[burnus at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56293

Tobias Burnus <burnus at gcc dot gnu.org> changed:

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

--- Comment #1 from Tobias Burnus <burnus at gcc dot gnu.org> 2013-02-12 09:52:09 UTC ---
Ditto for READ:

0x00007ffff7b9b55f in _gfortrani_convert_real (dtp=dtp@entry=0x7fffffffde40,
dest=dest@entry=0x601084 <sss_+4>, buffer=0x604fb0 "445.54",
length=length@entry=16)  at libgfortran/io/read.c:155

[Bug fortran/56293] I/O: Segfault in write_float when trying to print a not-word-aligned REAL(16) / -fno-align-commons

[tobi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56293

Tobias Schlüter <tobi at gcc dot gnu.org> changed:

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

--- Comment #2 from Tobias Schlüter <tobi at gcc dot gnu.org> 2013-02-12 10:23:32 UTC ---
This is valid behavior, -fno-align-common is documented to potentially break
stuff (though the documentation could be more explicit on that).

[Bug fortran/56293] Segfault when trying to access pass-by-reference value of a not-word-aligned REAL(16) / -fno-align-commons

[burnus at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56293

Tobias Burnus <burnus at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|I/O: Segfault in            |Segfault when trying to
                   |write_float when trying to  |access pass-by-reference
                   |print a not-word-aligned    |value of a not-word-aligned
                   |REAL(16) /                  |REAL(16) /
                   |-fno-align-commons          |-fno-align-commons

--- Comment #3 from Tobias Burnus <burnus at gcc dot gnu.org> 2013-02-12 12:35:19 UTC ---
Also occurs if one calls ("call foo(p)"):
      subroutine foo(x)
        real(16) :: x, y
        y = x  ! FAILS HERE
        ! print *, y
      end subroutine foo
instead of "print *, p".


The bug is probably somewhere in gcc/fortran/trans-common.c's. For instance at
build_common_decl, which uses:

      if (!com->is_bind_c)
        DECL_ALIGN (decl) = BIGGEST_ALIGNMENT;

or/and in "build_field", which has:

  known_align = (offset & -offset) * BITS_PER_UNIT;
  if (known_align == 0 || known_align > BIGGEST_ALIGNMENT)
    known_align = BIGGEST_ALIGNMENT;

  desired_align = update_alignment_for_field (rli, field, known_align);
  if (desired_align > known_align)
       (field) = 1;

or …

[Bug fortran/56293] Segfault when trying to access pass-by-reference value of a not-word-aligned REAL(16) / -fno-align-commons

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56293

--- Comment #4 from Richard Biener <rguenth at gcc dot gnu.org> 2013-02-12 12:39:55 UTC ---
(In reply to comment #3)
> Also occurs if one calls ("call foo(p)"):
>       subroutine foo(x)
>         real(16) :: x, y
>         y = x  ! FAILS HERE
>         ! print *, y
>       end subroutine foo
> instead of "print *, p".
> 
> 
> The bug is probably somewhere in gcc/fortran/trans-common.c's. For instance at
> build_common_decl, which uses:
> 
>       if (!com->is_bind_c)
>         DECL_ALIGN (decl) = BIGGEST_ALIGNMENT;
> 
> or/and in "build_field", which has:
> 
>   known_align = (offset & -offset) * BITS_PER_UNIT;
>   if (known_align == 0 || known_align > BIGGEST_ALIGNMENT)
>     known_align = BIGGEST_ALIGNMENT;
> 
>   desired_align = update_alignment_for_field (rli, field, known_align);
>   if (desired_align > known_align)
>        (field) = 1;
> 
> or …

The problem is that you are in no way dealing with the interface
mismatches in a correct way.  Not that I think you can.

I suggest to remove -fno-align-commons.

Or is there a fortran way to say that

       subroutine foo(x)
         real(16) :: x, y

may receive an unaligned x?

It seems to me that with an -fno-align-commons common you cannot do very
much.  You'd have to make sure to always access it though the common
and never pass pieces of it to another function (maybe the frontend could
warn about this).  Can you specify -fno-align-commons per common block?

[Bug fortran/56293] Segfault when trying to access pass-by-reference value of a not-word-aligned REAL(16) / -fno-align-commons

[burnus at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56293

--- Comment #5 from Tobias Burnus <burnus at gcc dot gnu.org> 2013-02-12 13:07:10 UTC ---
Some tests with ifort, which by default uses unaligned commons: The first test
case works, i.e. I/O with the unaligned "p" works. However, if one calls a user
procedure ("call foo(p)"), it segfaults.


Possible solutions:

a) Copy-in/copy-out around procedure calls - which is fun with large arrays.

b) Require the user to annotate the functions to possibly accept unaligned
input – or do so automatically. (With all the fun of third-party libraries like
LAPACK, BLAS, ...)

c) Simply deprecating the  -fno-align-commons flag, improving the wording of
the diagnostic and the description in the man page.

Somehow, I am in favour of (c).

The question is also whether one can construct a fully standard-conform example
which fails without -fno-align-commons – and whether some real-world code uses
COMMON in a way that would fails with alignments/padding.

[Bug fortran/56293] Segfault when trying to access pass-by-reference value of a not-word-aligned REAL(16) / -fno-align-commons

[tkoenig at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56293

Thomas Koenig <tkoenig at gcc dot gnu.org> changed:

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

--- Comment #6 from Thomas Koenig <tkoenig at gcc dot gnu.org> 2013-03-10 15:32:14 UTC ---

> The question is also whether one can construct a fully standard-conform example
> which fails without -fno-align-commons – and whether some real-world code uses
> COMMON in a way that would fails with alignments/padding.

      program main
      real a,f1,f2,d
      common /foo/ a,f1,f2,d
      a = 1.0
      d = 1.0
      call s1
      end
      subroutine s1
      real a,b
      double precision d
      common /foo/ a,d,b
      print *,a
      print *,b
      end

[Bug fortran/56293] Segfault when trying to access pass-by-reference value of a not-word-aligned REAL(16) / -fno-align-commons

[tobi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56293

--- Comment #7 from Tobias Schlüter <tobi at gcc dot gnu.org> 2013-03-11 00:15:43 UTC ---
(In reply to comment #6)
> > The question is also whether one can construct a fully standard-conform example
> > which fails without -fno-align-commons – and whether some real-world code uses
> > COMMON in a way that would fails with alignments/padding.
> 
>       program main
>       real a,f1,f2,d
>       common /foo/ a,f1,f2,d
>       a = 1.0
>       d = 1.0
>       call s1
>       end
>       subroutine s1
>       real a,b
>       double precision d
>       common /foo/ a,d,b
>       print *,a
>       print *,b
>       end

That's not valid.  I'm looking at the F95 working draft:
according to note 5.33
"""Association in different scoping units between objects of default type,
objects of double precision real type, and sequence structures is permitted
according to the rules for equivalence objects (5.5.1)."""

and in 5.5.1. we have
"""If an equivalence-object is of type default integer, default real, double
precision real, default complex, default logical, or numeric sequence type, all
of the objects in the equivalence set shall be of these types."""

[Bug fortran/56293] Segfault when trying to access pass-by-reference value of a not-word-aligned REAL(16) / -fno-align-commons

[dominiq at lps dot ens.fr]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56293

Dominique d'Humieres <dominiq at lps dot ens.fr> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |WAITING
   Last reconfirmed|                            |2014-01-07
     Ever confirmed|0                           |1

--- Comment #8 from Dominique d'Humieres <dominiq at lps dot ens.fr> ---
> c) Simply deprecating the  -fno-align-commons flag, improving the wording 
> of the diagnostic and the description in the man page.
>
> Somehow, I am in favour of (c).

Me too.