[llvm-dev] Specifying DAG patterns in the instruction

[Dylan McKay via llvm-dev]

Try visualising the DAG like this.

```
                       ---- GPR:$rA
                      /
set GPR:$rd  ---- add
                      \
                       ---- GPR:$rB
```

Each instruction forms a DAG with its operands being subnodes.

The core instruction selection logic just looks for the pattern `(set
GPR:$rd, (add GPR:$rA, GPR:$rB))`. It then becomes a simple substitution.
This is a DAG because there all nodes are directed and acyclic.


On Thu, Jan 28, 2016 at 7:33 PM, Rail Shafigulin via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> I'm confused about how to specify DAG patterns for a given instruction
>
> Here is an example for my target
>
> class ALU1_RR<bits<4> subOp, string asmstr, SDNode OpNode>
>   : ALU_RR<subOp, asmstr,
>            [(set GPR:$rD, (OpNode (i32 GPR:$rA), (i32 GPR:$rB)))]>;
>
> def ADD  : ALU1_RR<0x0, "l.add", add>;
>
> The set operation simply creates a list. The add operation creates a
> union. So at the end  [(set GPR:$rD, (OpNode (i32 GPR:$rA), (i32
> GPR:$rB)))] becomes a just a regular set. How come this is a DAG? I feel
> like I'm missing something. Would anyone be able to help with the
> explanation?
>
> --
> Rail Shafigulin
> Software Engineer
> Esencia Technologies
>
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>
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