specify dag pattern for an instructions

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1

I’d like to apologize in advance if the question seems dumb, but I’ve never specified a DAG pattern for an instruction a don’t know how it looks in LLVM.

My target has an instruction which is similar to X86 horizontal add. The instruction, let’s call is sumreduce, sums up all the elements in a vector and stores a result in GPR. What I’m trying to do is to replace a specific DAG pattern (it is given below) with this instruction (DAG node).

Here is the pattern I need to replace:

r0 r1
\ /
add r2
\ /
add r3
\ /
add

r4

with
r4 = sumreduce r0 r1 r2 r3

For now assume that the vector has 4 elements in it and all of them are i32.

I would appreciate any pointers on how to do it. If someone could point me to an example code that converts a DAG pattern to a DAG node I would greatly appreciate it.

2

It sounds like you want to specify a target-specific DAG combine. I’m sure most targets have many examples of this, but I’m familiar with PPC, so you can have a look at the node ISD::SIGN_EXTEND in lib/Target/PowerPC/PPCISelLowering.cpp. We specify in the target constructor that there’s a DAG combine for the node (setTargetDAGCombine(ISD::SIGN_EXTEND);). Then we specify what to do with it in PPCTargetLowering::PerformDAGCombine. Of course, the semantics are very different from what you’re trying to do, but I think the general approach is what you’re looking for (i.e. note that there’s a target-specific DAG combine, then specify the details of the combine).

I hope this helps and that I haven’t misunderstood your question.

Nemanja