https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97832
--- Comment #3 from Michael_S <already5chosen at yahoo dot com> ---
(In reply to Richard Biener from comment #2)
> It's again reassociation making a mess out of the natural SLP opportunity
> (and thus SLP discovery fails miserably).
>
> One idea worth playing with would be to change reassociation to rank
> references
> from the same load group (as later vectorization would discover) the same.
>
> That said, further analysis and maybe a smaller testcase to look at is useful
> here. There is, after all, the opportunity to turn "bad" association at the
> source level to good for vectorization when -ffast-math is enabled as well.
It turned out, much simpler kernel suffers from the same problem.
void foo1x1(double* restrict y, const double* restrict x, int clen)
{
int xi = clen & 2;
double f_re = x[0+xi+0];
double f_im = x[4+xi+0];
int clen2 = (clen+xi) * 2;
#pragma GCC unroll 0
for (int c = 0; c < clen2; c += 8) {
// y[c] = y[c] - x[c]*conj(f);
#pragma GCC unroll 4
for (int k = 0; k < 4; ++k) {
double x_re = x[c+0+k];
double x_im = x[c+4+k];
double y_re = y[c+0+k];
double y_im = y[c+4+k];
y_re = y_re - x_re * f_re - x_im * f_im;;
y_im = y_im + x_re * f_im - x_im * f_re;
y[c+0+k] = y_re;
y[c+4+k] = y_im;
}
}
}
May be, it's possible to simplify further, but probably not by much.
