initial selection DAG

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1

Hi,

I wonder how the initial selection DAG is built in the backends.

From working backends I get:

----8<-------
Initial selection DAG: BB#0 'main:'
SelectionDAG has 18 nodes:
----8<-------

From my (not working) backend I get:

----8<-------
Initial selection DAG: BB#0 'main:'
SelectionDAG has 15 nodes:
----8<-------
I miss three nodes and I wonder what do I have to implement to get at least the right initial selection DAG?

Do I have to define all instructions in the td files first?

Cheers,
Nico

2

Hi Nico,

I miss three nodes and I wonder what do I have to implement to get at least the right initial selection DAG?

Most of the work is done by the files in lib/CodeGen/SelectionDAG
(first SelectionDAGBuilder.cpp, then the Legalizers and DAGCombiner) .
However, various target-specific hooks are used during the process
(the biggest handling function-calls and formal parameters). These
hooks and the data that effect them are defined in
lib/Target/XXX/XXXISelLowering.cpp

What should the nodes you're missing be doing? It's difficult to say
quite what part of ISelLowering you need to look at without knowing
that.

Do I have to define all instructions in the td files first?

No. In principle you should be able to get a fully correct selection
dag without defining a single instruction. It wouldn't be much use for
most targets, but it would exist.

Tim.

3

Hi,

sorry for my late reply. You were right. Something was missed in the xxxLowering.cpp.

Thank you!

Kind regards,
Nico

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