On Wed, Nov 21, 2018 at 08:52:21AM -0800, Michael Eager wrote: > On 11/21/2018 08:33 AM, Segher Boessenkool wrote: > >On Tue, Nov 20, 2018 at 10:07:35AM -0800, Michael Eager wrote: > >>The internal RTL should not be dictating what the target arch can or > >>cannot implement. Reload should insert any needed conversions, > >>especially ones which narrow the size. > > > >Well, that depends. A zero_extract of mem is only defined for byte_mode, > >just like SET is only defined for VOIDmode (on the SET itself, not its > >args). Because this is guaranteed, nothing in GCC ever needs to check > >this. That is the theory of course; in reality quite a few targets have > >used other modes for the mem in a zero_extract, and this seems to have > >mostly worked. > > This restriction on zero_extract MEM args (and only MEM) seems to be > completely arbitrary. What is it about the operation of extracting a > bit field which makes it dependent on the memory access size? If the mode is required to be byte_mode, then all code dealing with it can assume it to be byte_mode. Without first having to check it, and without having to handle other modes. > The value of SET is VOIDmode, in that it has no value. Not sure what > your point is here. See above. > >As another example, closer by, an extract length of 0 is not allowed > >either, for zero_extract. And this *did* cause problems recently. > > This is a restriction which does make sense. It isn't clear what > the value of a zero length field is, or how to represent it. If > something is undefined, then there is a strong argument for making is > invalid. (Are there architectures which have instructions which extract > a zero length bit field? I doubt it.) It doesn't matter if you (or I, or anyone) think it makes sense; the rules are the rules. If you want to change the rules, then post a patch. > >Why was it documented as requiring byte mode? Was this changed, just > >the documentation was not updated? > > Ancient history. As Jeff said, perhaps an architectural requirement of > VAX or m68k. This wasn't changed, as far as I'm aware. So a mem in a *_extract is still required to be byte_mode you say? I don't think we can change this in the documentation without reviewing all code that deals with *_extract to see if this will work :-/ Segher
