https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105591
--- Comment #12 from Hongtao.liu <crazylht at gmail dot com> --- (In reply to Jakub Jelinek from comment #11) > 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. Makes sense, thanks for the explanation.
