Ian Lance Taylor <i...@google.com> writes: > Mat Hostetter <mhostet...@tilera.com> writes: > >> Since the high bits are already zero, that would be less efficient on >> most platforms, so guarding it with something like this would probably >> be smarter: >> >> if (targetm.mode_rep_extended (mode, GET_MODE(x)) == SIGN_EXTEND) >> return simplify_gen_unary (TRUNCATE, mode, x, GET_MODE (x)); >> >> I'm happy to believe I'm doing something wrong in my back end, but I'm >> not sure what that would be. I could also believe these are obscure >> edge cases no one cared about before. Any tips would be appreciated. > > Interesting. I think you are in obscure edge case territory. Your > suggestion makes sense to me, and in fact it should probably be put > into gen_lowpart_common.
FWIW, I disagree. Firstly, mode_rep_extended is a special case of !TRULY_NOOP_TRUNCATION so the above check should use that. Secondly, in MIPS we call gen_lowpart to convert DI to SI when we know it's safe. In this case we always want a subreg not a truncate for better code. So I don't think gen_lowpart_common is the right place to fix this. I think the right fix is to call convert_to_mode or convert_move in the expansion code which ensure the proper truncation. Adam
