[Bug libfortran/77278] Use LTO for libgfortran

[rguenther at suse dot de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77278

--- Comment #15 from rguenther at suse dot de <rguenther at suse dot de> ---
On Tue, 4 Jun 2019, hubicka at ucw dot cz wrote:

> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77278
> 
> --- Comment #14 from Jan Hubicka <hubicka at ucw dot cz> ---
> > 
> > Yeah, I do remember this.  I think we settled on the above
> > (previously you had dim[7] in the library I think) to be
> > compatible.  Still a C simple testcase complains:
> > 
> > typedef struct { int ndim; int dim[]; } *descp;
> > void foo (descp d);
> > 
> > void bar()
> > {
> >   struct { int ndim; int dim[2]; } desc;
> >   desc.ndim = 2;
> >   foo (&desc);
> > }
> > 
> > t2.c: In function ‘bar’:
> > t2.c:8:8: warning: passing argument 1 of ‘foo’ from incompatible pointer 
> > type [-Wincompatible-pointer-types]
> >    foo (&desc);
> >         ^~~~~
> > t2.c:2:17: note: expected ‘descp’ {aka ‘struct <anonymous> *’} but 
> > argument is of type ‘struct <anonymous> *’
> >  void foo (descp d);
> >            ~~~~~~^
> > 
> > and we probably assign different alias sets to both.
> 
> Curiously enough in C version of the LTO testcase I get no warning now
> since we simplify function type and thus both pointers are turned into
> incomplete pointers and considered TBAA compatible.
> After lunch I will check why the warning triggers here.
> 
> Still the alias set is different, so I think we more or less get lucky
> by using base+offset tests first since array descriptors are accessed
> directly after constant propagation of poointers.
> > 
> > Now to make aliasing happy both the Frontend and LTO have to
> > compute the same TYPE_CANONICAL for _all_ of the array
> > descriptor types.  You can either go and allocate
> > dim always to the max size statically or in the Fortran
> > FE use self-referential types (not sure if you then can
> > statically instantiate an object of such type...) or
> > rewrite all accesses to the fixed-dimension statically
> > allocated array descriptors to go via the dim[] type
> > (I think I suggested the latter elsewhere).
> > 
> > So instantiate my_descriptor and then store for further
> > use VIEW_CONVERT_EXPR <generic-descriptor-type> (my_descriptor).
> > 
> > I hope that doesn't defeat [IPA] optimization ...
> I think Martin's code generally gives up on type mismaches so it
> is quite possible it gives up already.
> 
> An option would be also to teach LTO that array descriptors are
> special types aliasing with each other but not aliasing with
> anything else.

I'd rather not do that ;)  Btw, I wonder what happens at
the call boundary inside a single fortran module where
the caller passes a dim[2] array to a subroutine
handling arbitrary dimension arrays?  I suspect the
IL would have the very same TBAA issue.  Can you produce
a fortran testcase that exposes such a case so we can have a
look into the details?