[Bug middle-end/40194] fortran rules for optimizing

[matz at gcc dot gnu dot org]

------- Comment #9 from matz at gcc dot gnu dot org  2009-08-13 12:16 -------
It is indeed somewhat related, but currently the patchset doesn't help here.
The first issue is, that I create new types with unconflicting alias sets
only for global variables, not local ones (as I1 is in this case).  I believe
I can change this meanwhile (the restriction came from earlier versions of
the patch).  But even if this is fixed we can't figure out the difference
between INTENT(IN) and INTENT(OUT), i.e. even then we still believe that 
I1 can possibly be changed by the call.

We have three ways out of this:
1) make the frontend not pass INTENT(IN) parameters by reference, maybe
   at least for scalars.  That would be the preferred way, but unfortunately
   this probably would break a lot of code assuming such calling convention
   especially when interfacing with C code :-/
2) invent a new attribute that we can attach to the individual arguments
   that would indicate non-clobbering via this reference
3) find a combination of existing things (DECL_BY_REFERENCE/TREE_READONLY/etc)
   that could be used by the middle end to conclude non-clobbering.
   Something like a const reference comes to mind, but unfortunately (the
   middle-end coming mostly from the C/C++ side) we don't conclude many
   interesting things about const refs/pointers.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40194