[LLVMdev] Isel DAG documentation?

[David Given]

I'm having a great deal of trouble figuring out how to write instruction
patterns which actually match the DAG produced by the compiler. I can't
seem to find any documentation on both what the various nodes represent
or on what the syntax accepted by TableGen is. The backends I have
access to all seem to do this in different (and obscure) ways. And when
things go wrong the compiler seems to dump the DAG tree in a entirely
different syntax to that used by TableGen.

Can anyone point me at any decent resources as to what all this stuff
actually does? And how to figure out what's going wrong when it
inevitably does go wrong?

So far all I've found is include/llvm/Target/TargetSelectionDAG.td,
which at least lists them but doesn't say what they do, and the C++
implementations in SelectionDAGNodes.h, which is entirely undocumented...

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