[Bug c/60819] dse1 removing not-dead insn (aliasing issue?)

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60819

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2014-04-11
          Component|rtl-optimization            |c
     Ever confirmed|0                           |1

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
I think you are violating aliasing in your testcase:

typedef int __m64 __attribute__ ((__vector_size__ (8), __may_alias__));
typedef short __v4hi __attribute__ ((__vector_size__ (8)));

you access __m64 with a __v4hi (__may_alias__ doesn't make the __m64 parameter
have alias-set zero!).

Now, we lower this to

  return (int) *((short int *) &VIEW_CONVERT_EXPR<vector(4) short int>(__A) +
(sizetype) ((long unsigned int) __N * 2));

because of the variable indexing which makes this a memory access.  So
eventually that lowering should make sure no TBAA issues appear or we
should reject variable indexes here.

That is, variable indexing should use a MEM_REF with alias-set zero
(if we are sure that non-variable indexing will never result in a memory
access,
it seems that the FE lowers constant indexes the same and only later during
gimplification we fold that to use a BIT_FIELD_REF).

Confirmed as a Frontend bug sofar, just in case the testcase is valid.