[Bug tree-optimization/33136] [4.1/4.2/4.3 Regression] wrong code due to alias with allocation in loop

["jakub at redhat dot com" <gcc-bugzilla at gcc dot gnu dot org>]

------- Comment #18 from jakub at redhat dot com  2007-08-23 14:49 -------
Subject: Re:  [4.1/4.2/4.3 Regression] wrong code due to alias with allocation
in loop

On Thu, Aug 23, 2007 at 01:45:11PM -0000, dberlin at dberlin dot org wrote:
> > If you take address of the whole struct rather than some specific field and
> > that address doesn't escape the CU, then that still doesn't explain how
> > could a pointer var with first field's type point to the struct.
> 
> > Say for
> > struct A { int i; float f; } you still need (int *) ptrA
> 
> Uh, no.
> 
> &ptrA will do just fine. You can clobber all fields through it if it escapes.

See "that address doesn't escape the CU" above I wrote.  But if so,
it will be in global_types_full_escape and so 
ipa_type_escape_field_does_not_clobber_p will return false.

> > If ipa_type_escape_star_count_of_interesting_type (var_type) > 0 (i.e. var is
> > a pointer to struct/union rather than struct/union itself, how is the fact that
> > something took address of fields within the struct itself relevant to whether
> > some pointer may point to the pointer var or not?
> 
> If the address was never taken anywhere, it can't be pointed to.
> Type-escape tries to go a little further, and see if, when the address
> of a field is taken, if that address is ever cast'd,
> incremented/decremented, or escapes.  If not, then only that field is
> clobbbered, not the entire structure.  Otherwise, it is equivalent to
> calculating TREE_ADDRESSABLE.

I was talking about say
struct S { int i; float f; struct S **s1; struct S ***s2; struct S ****s3; };
struct S ***var;
struct S ****ptr;

If we are asking whether
may_alias (ptr, get_alias_set (TREE_TYPE (TREE_TYPE (ptr))), var, get_alias_set
(var), false)
then we are only interested in whether var is TREE_ADDRESSABLE,
and this is IMHO exactly the same as asking in case of:
void ***var;
void ****ptr;

But in the former case ipa_type_escape_star_count_of_interesting_type
(var_type) == 3 and so we will call
ipa_type_escape_field_does_not_clobber_p, in the latter case
not.  How are these two different?

In the c#8 testcase this is similar,
ipa_type_escape_star_count_of_interesting_type (var_type) == 1,
we have
struct S *var;
struct S **ptr;
again, why does it matter if *var is a struct or not?  What matters
is if var is TREE_ADDRESSABLE, if there is no &var, then it can't
point to it, otherwise it can.  The same as for
void *var;
void **ptr;

But if var isn't TREE_ADDRESSABLE, I'd bet nothing ever calls
may_alias_p with it as third argument.

So, do you agree the only case may_alias_p should handle with
ipa_type_escape_field_does_not_clobber_p is
ipa_type_escape_star_count_of_interesting_type (var_type) == 0 ?

        Jakub


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=33136