[Bug tree-optimization/38011] vectorizer ignores alignment, useless versioning

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38011

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

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

--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> 2013-03-27 12:35:44 UTC ---
Not true - you aligned the pointer, not the data it points to.  There isn't
a good way to do that with an aligned attribute, the closest you can get at
is with

typedef double aligned_double __attribute__((aligned (16)));
void assignMultiplyVec(aligned_double* restrict a, aligned_double* restrict b,
                       double coef, unsigned count)
{
  for(unsigned i=0; i<count; i++) {
      a[i] = b[i]*coef;
  }
}

but that has the issue that a[1] is not aligned but technically you
still say so (the issue is that the array has no gaps according to
the C standard but the alignment of the element type is bigger than
its size ...).

So instead we now have an assume_aligned builtin which you can use like

void assignMultiplyVec(double* restrict a_, const double * restrict b_,
                       double coef, unsigned count)
{
  double* restrict a = __builtin_assume_aligned (a_, 16);
  double* restrict b = __builtin_assume_aligned (b_, 16);
  for(unsigned i=0; i<count; i++) {
    a[i] = b[i]*coef;
  }
}

which does not have this issue.