Re: Help with UDI instructions

[Steve Ellcey <sellcey at mips dot com>]

On Wed, 2013-07-17 at 20:49 +0100, Richard Sandiford wrote:
> 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

I am afraid I don't have any real information for you.  I didn't even
know these instructions existed until this email.  I talked to a few
people here and while they think a couple of customers used them they
don't have any details.  I did see a couple of documents over at
developer.mips.com about using CorExtend (the MIPS name for UDI's)
but I don't know if they bear any relationship to what people have
actually implemented on any chips.

Steve Ellcey
sellcey@mips.com