https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70138
--- Comment #8 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Further improved testcase (just decrease number of iterations somewhat, and
make sure the u elements that are summed are different in each outer loop
iteration, to verify the vectorizer doesn't just multiply value from some
iteration by the number of iterations):
double u[333 * 333];
__attribute__((noinline, noclone)) static void
foo (int *x)
{
double c = 0.0;
int a, b;
for (a = 0; a < 333; a++)
{
for (b = 0; b < 333; b++)
c = c + u[334 * a];
u[334 * a] *= 2.0;
}
*x = c;
}
int
main ()
{
int d, e;
for (d = 0; d < 333 * 333; d++)
u[d] = 499.0;
for (d = 0; d < 333; d++)
u[d * 334] = (d + 2);
foo (&e);
if (e != 333 * (2 + 334) / 2 * 333)
__builtin_abort ();
return 0;
}
BTW, I'm really surprised we vectorize this even without -Ofast, it is a double
reduction, therefore reducing it causes different floating point operations
between the vectorized and non-vectorized cases.
