[Bug tree-optimization/49413] over-optimization that causes valid code to segfault

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49413

Richard Guenther <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|                            |INVALID

--- Comment #10 from Richard Guenther <rguenth at gcc dot gnu.org> 2011-08-04 09:35:13 UTC ---
There are two camps of users, some want the compiler to treat *(double *)
as aligned because, damn, they said so (especially on targets where
accessing unaligned data is costly)!  Others want the compiler to figure
out misalignment properly, thus, treat *(double *) as aligned only if
it knows (for sure?).

We can't serve both camps.

Consider

void foo (double *p, double *q, int n)
{
  int i;
  for (i = 0; i < n; ++i)
    p[i] += q[i];
}

do you want the compiler to vectorize the above or not?  The C standard
says we can assume *p and *q are properly aligned for what the ABI
says about doubles.  But you say, well, I may als well call this function
with some misaligned data where I think the compiler should figure this
out for me.

Now, I say not so.