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?
The value of SET is VOIDmode, in that it has no value. Not sure what
your point is here.
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.)
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.
--
Michael Eager eager@xxxxxxxxxx
1960 Park Blvd., Palo Alto, CA 94306
