[llvm-dev] Re-using a node in a DAG output pattern in TableGen

[Jason Eckhardt via llvm-dev]

You might give OutPatFrag a try:

// OutPatFrag is a pattern fragment that is used as part of an output pattern
// (not an input pattern). These do not have predicates or transforms, but are
// used to avoid repeated subexpressions in output patterns.
class OutPatFrag<dag ops, dag frag>
 : PatFrag<ops, frag, [{}], NOOP_SDNodeXForm>;

________________________________
From: llvm-dev <llvm-dev-bounces at lists.llvm.org> on behalf of Cyril Six via llvm-dev <llvm-dev at lists.llvm.org>
Sent: Monday, November 8, 2021 6:37 AM
To: llvm-dev <llvm-dev at lists.llvm.org>
Subject: [llvm-dev] Re-using a node in a DAG output pattern in TableGen

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Hello,

I currently have an instruction selection pattern that looks like this :

def : Pat<myinputop type:$v0 type:$v1,
          (COMPNHQ
            (ADDHQrr
              (SLLHQSri $v0, 8),
              (SLLHQSri $v1, 8),
            ),
            (SLLHQSri $v0, 8),
            comparison_ltu
          )>;
This output pattern is supposed to be a DAG: the node (SLLHQSri $v0, 8) is used two times.

Experimentally, TableGen seems to recognize it as such (the node `(SLLHQSri $v0, 8)` will not be duplicated).

I am wondering if there is a way to avoid having to repeat (SLLHQSri $v0, 8) in the syntax of TableGen, something like:
def : Pat<inputpattern type:$v0 type:$v1,
          (COMPNHQ
            (ADDHQrr
              (SLLHQSri $v0, 8):$myvar,
              (SLLHQSri $v1, 8),
            ),
            $myvar,
            comparison_ltu
          )>;
If I try this, I get the error "Node 'myvar' in output pattern but not input pattern".

Does anyone know if there is a way to name nodes from an output pattern to produce a non-tree output DAG?

Do you know of any existing pattern featuring output DAG that are not trees?

Thanks

Cyril Six
Compiler Engineer • Kalray
Phone:
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