make_extraction

[Richard Henderson <rth at cygnus dot com>]

Can someone tell me what 

  /* Unless we are allowed to span bytes, reject this if we would be
     spanning bytes or if the position is not a constant and the length
     is not 1.  In all other cases, we would only be going outside
     out object in cases when an original shift would have been
     undefined.  */
  if (! spans_byte
      && ((pos_rtx == 0 && pos + len > GET_MODE_BITSIZE (is_mode))
          || (pos_rtx != 0 && len != 1)))
    return 0;

is attempting to do?  I'm trying to figure out why

  (and:DI (lshiftrt:DI reg1 reg2) (const_int 255))

is not a valid extraction.  Seems to me that it should be fine -- if
reg2 is out of range we get undefined results doing things the long
way anyway.

I hate to clutter up the md file with special patterns that combine
should have been doing the right thing with in the first place...


r~