https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109230
--- Comment #14 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
I think series_p clearly documents that:
/* Return true if index OUT_BASE + I * OUT_STEP selects input
element IN_BASE + I * IN_STEP. For example, the call to test
whether a permute reverses a vector of N elements would be:
series_p (0, 1, N - 1, -1)
which would return true for { N - 1, N - 2, N - 3, ... }.
The calls to test for an interleaving of elements starting
at N1 and N2 would be:
series_p (0, 2, N1, 1) && series_p (1, 2, N2, 1).
which would return true for { N1, N2, N1 + 1, N2 + 1, ... }. */
bool
vec_perm_indices::series_p (unsigned int out_base, unsigned int out_step,
element_type in_base, element_type in_step) const
So, if you want to catch permutation { 0, nelts, 2, nelts + 2, 4, nelts + 4,
... }
then I think you need sel.series_p (0, 2, 0, 2) && sel.series_p (1, 2, nelts +
1, 2).
