Help with UDI instructions

[Richard Sandiford <rdsandiford at googlemail dot com>]

Steve, this is going to seem like a bit of a pointless distraction, sorry,
but do you have any internal info about how the "udi..." instructions are
being used in practice?  Since the whole point is that the instructions are
_user_-defined, I realise the answer's probably "no" by design.

The instructions were added in:

    http://article.gmane.org/gmane.comp.gnu.binutils/27744/match=udi0

I was just wondering about the way that they all have WR_d|RD_s|RD_t
(write to the rd register, read from the rs and rt registers).
The use of these flags is pretty limited, so it probably isn't going to
matter in practice.  But the ISA manual doesn't AIUI force a particular
format for the middle part of user SPECIAL2 opcodes, and things like
Octeon's CINS write to rt instead of rd, so I was wondering whether
it would be safer to assume that "udi..." modifies all GPRs (except $0,
of course).  Anyone agree or disagree?

The only reason I'm asking is that I'm trying to make the opcode descriptions
list the read and write operands by operand number rather than field type,
and this is one of the few cases where that isn't possible.  We could have
a special flag for the equivalent of "modifies rd, rs and rt" instead,
if assuming all GPRs seems too heavyweight.

Thanks,
Richard