https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105591
--- Comment #11 from Jakub Jelinek <jakub at gcc dot gnu.org> --- Because VEC_PERM_EXPR doesn't require the mask argument to be constant (and neither does __builtin_shuffle, unlike e.g. __builtin_shufflevector). If the mask argument remains non-constant until end of compilation, the modulo N or 2*N needs to be done at runtime (of course unless used hw instruction performs something like that itself). And that is the reason why it is defined this way. During compilation there are way too many spots where a constant could be propagated into a formerly non-constant operand of the VEC_PERM_EXPR, and no guarantee that all such propagations (it isn't in a single spot in a single pass, it is really many) will do some extra code to canonicalize it. It can be, but we can't guarantee it. For __builtin_shufflevector, we supposedly want to introduce some VEC_PERM_EXPR variant which would only allow constant mask argument and which would have different behavior, -1 standing for I don't care rather than -1 % N or -1 % (2*N). If we introduce something like that, we could certainly require that the new expr's operand is only -1..N-1 or -1..2*N-1 and could have then say match.pd that turns a VEC_PERM_EXPR with a constant argument into the new expr with canonical argument.
