[LLVMdev] How to specify patterns for instructions with accumulator in selection DAG?

[Dongrui She]

Hi,

I am wondering how to specify the selection DAG patterns for instructions
that use accumulator.
For example multiply-accumulate instruction with one destination operand and
two source operands:
mac $dst, $src1, $src2 ;; $dst += $src1*$src2

Seems that it has a cycle in the pattern. So how do I specify it in the DAG?
There are a few instructions in the ARM backend like this one, but the
patterns are left blank.
-- 

Regards,

Dongrui
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