[Bug tree-optimization/55334] [4.8 Regression] mgrid regression (ipa-cp disables vectorization)

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55334

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

           What    |Removed                     |Added
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                 CC|                            |rguenth at gcc dot gnu.org

--- Comment #23 from Richard Biener <rguenth at gcc dot gnu.org> 2012-12-21 14:27:18 UTC ---
In fact there are a few dependences we can solve:

191: versioning for alias required: can't determine dependence between
MEM[(real(kind=8)[0:D.1986] *)&x + 20247552B][_20] and
MEM[(real(kind=8)[0:D.1989] *)&x + 37823552B][_20]

from

Creating dr for MEM[(real(kind=8)[0:D.1986] *)&x + 20247552B][_20]
analyze_innermost: success.
        base_address: &x
        offset from base address: (ssizetype) ((sizetype) (((_7 - stride.11_6)
+ pretmp_791) + pretmp_813) * 8)
        constant offset from base address: 20247568
        step: 8
        aligned to: 8 
        base_object: MEM[(real(kind=8)[0:D.1986] *)&x + 20247552B]
        Access function 0: {{(stride.9_4 * 2 + pretmp_792) + 2, +,
stride.9_4}_2, +, 1}_3

and

Creating dr for MEM[(real(kind=8)[0:D.1983] *)&x][_20]
analyze_innermost: success.
        base_address: &x
        offset from base address: (ssizetype) ((sizetype) (((_7 - stride.11_6)
+ pretmp_791) + pretmp_813) * 8)
        constant offset from base address: 16
        step: 8
        aligned to: 8
        base_object: MEM[(real(kind=8)[0:D.1983] *)&x]
        Access function 0: {{(stride.9_4 * 2 + pretmp_792) + 2, +,
stride.9_4}_2, +, 1}_3

we hit:

  /* If the references do not access the same object, we do not know
     whether they alias or not.  */
  if (!operand_equal_p (DR_BASE_OBJECT (a), DR_BASE_OBJECT (b), 0))
    {
      DDR_ARE_DEPENDENT (res) = chrec_dont_know;
      return res;
    }

but for the case the access functions are the same we can conclude the
accesses cannot alias if the base_objects cannot alias (danger!  data-ref
does not record an "access size", so with the same access function
but only 1-byte offset in base_object this can lead to false disambiguations,
so it might in the end not be that easy ...)  This size issue is also
why we cannot translate the base_object internal offset into an access
function (see testcases I added for bugs we had there)