[Bug tree-optimization/48052] loop not vectorized if index is "unsigned int"

["rguenth at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48052

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2011.03.10 09:46:16
                 CC|                            |spop at gcc dot gnu.org
     Ever Confirmed|0                           |1

--- Comment #2 from Richard Guenther <rguenth at gcc dot gnu.org> 2011-03-10 09:46:17 UTC ---
This is a known issue with POINTER_PLUS_EXPR semantics, how the C frontend
handles pointer-based array accesses and fold.  And in the end SCEV analysis.
The issue is we end up with

  *(c + (((long unsigned int)i) * 8))

with that 'long unsigned int' being sizetype.  At the point of SCEV
analysis we do not factor in the fact that i does not wrap around and
that because of this the evolution is
{ c, +, 8 }

With signed integers we simply exploit undefined behavior.

So yes, it's a known problem (but I always fail to remember a testcase
where it matters ;)).

In the very end my plan was to fix this all with no-undefined-overflow
branch, but maybe Sebastian can think of a way to use number-of-iteration
analysis in SCEV?  (Ugh, that's a chicken-and-egg problem, no?)