Re: Enable pointer TBAA for LTO

[Jan Hubicka <hubicka at ucw dot cz>]

> 
> Please in future leave patches for review again if you do such
> big changes before committing...

Uhm, sorry, next time I will be more cureful.  It seemed rather easy after
debugging it but indeed it is somewhat surprising issue.
> 
> I don't understand why we need this (testcase?) because get_alias_set ()
> is supposed to do the alias-set of pointer globbing for us.

The situation is as follows.  You can have

struct a {
  int *ptr;
}

struct b {
  float *ptr;
}

Now, if becase type is ignored by TYPE_CANONICAL, we have

   TYPE_CANONICAL (sturct b) = struct a.

and while building alias set of "struct a" we record compoents as:

   struct a
      ^
      |
    int *

Now do

struct b b = {NULL};
float *p=&b->ptr;
*p=nonnull;
return b.ptr;

Now alias set of the initialization is alias set of "struct a". The alias set
of of the pointer store is "float *".  We ask alias oracle if "struct a" can
conflict with "float *" and answer is no, because "int *" (component of struct
b) and "float *" are not conflicting.  With the change we record component
alias as follows:

   struct a
      ^
      |
   void *

which makes the answer to be correct, because "int *" conflicts with all pointers,
so all such queries about a structure gimple_canonical_types_compatible with "struct a"
will be handled correctly.

I will add aritifical testcase for this after double checking that the code above
miscompiles without that conditional.

I found that having warning about TBAA incompatibility when doing merigng in
lto-symtab is great to catch issues like this.

I had this patch for quite some time, but it was producing obviously wrong
positives (in Fortran, Ada and also sometimes in Firefox on array initializes
of style int a[]={1,2,3}).  I wrongly assumed tha the bug is because we compute
TYPE_CANONICAL sensitively to array size and there are permited transitions
of array sizes between units.  THis is not the case.

Yesterday I found that the problem is with interaction of get_alias_set
globbing and gimple_canonical_types_compatible globbing.  We can't have
get_alias_set globbing more or less coarse than
gimple_canonical_types_compatible does.

Here the situation is reverse to the case above : because array type is
inherited from element type, we can't have TYPE_CANONICAL more globbed for
array than we have get_alias_set.  In this case the problem appeared when
TYPE_NONALIASED_COMPONENT array previaled in type merging other arrays.  The
situation got worse with pointer, becuase array of pointers of one type could
prevail array of pointers of other type and then the array type gets different
alias sets in different units.  This seems to work for C programs, but is
wrong.  I will send patch and separate explanation after re-testing final
version shortly.

Honza