[LLVMdev] Multi-instruction patterns, tablegen and chains

Matthijs Kooijman matthijs at stdin.nl
Tue Nov 4 00:12:31 PST 2008

Hi Dan,

> Having tblgen pretend that the MOVE isn't the root seems a bit
> counter-intuitive though.
I didn't really mean making RD the root, but rather telling tablegen that RD
is the "primary" node, corresponding to the input pattern. This would allow
the properties of the input rd SDNode be properly transferred to the output RD
node instead of the MOVE node. I doubt this is a very elegant solution, nor
will it work in the general case, but this is what I meant anyway.

> I think it would be useful to have a way to explicitly tell
> tblgen which node(s) in the output pattern to hook up the chain(s)
> to. I believe this is related to a problem David Greene is seeing,
> where he has a case involving multiple chains.
> A fully general approach would be to make chain operands explicit
> in patterns, and to invent some syntax for identifying chain
> results. It might look something like this:
> def : Pat<(rd ch:$input_chain, imm:$addr)[$output_chain],
>            (MOVE (RD ch:$input_chain, imm:$addr)[$output_chain])>;
> This would be a pretty big change though.
So you're basically allowing for a way to specify outputs beyond the first
output in a dag? That does make sense, and might be useful for other cases to,

I won't be able to implement this, however, since I'm only prototyping just

> An alternative that's a bit less ambitious would be to have
> tablegen search the output pattern for nodes which are declared
> to support chain operands/results, and if it finds exactly one
> such node, use that node to hook up all the chain
> operands/results. If it finds more than one, it could issue
> an error. This would probably be simpler and less invasive,
> and probably enough for your specific example, though it
> wouldn't be as general-purpose.
Currently, I think that  the code supports having multiple nodes with chain
just fine (when I hook up a pattern to my RD instruction, I get an Emit
function that hooks up the chain to both RD and MOVE. I think it might even
join the two output chains together, not exactly sure about that. Not sure if
this is really what I want, but it would work.

So, the main thing needed is some way of telling that an instruction is
chained, other than giving it a pattern. Using hasCtrlDep for this makes
sense, but I'm afraid I won't be having the time nor expertise to implement
this... I'd be glad to test it though :-)


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