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.
Honza
