[LLVMdev] Isel DAG documentation?

[Krzysztof Parzyszek]

You can use -mllvm -debug-only=isel to see the DAG during the lowering 
process, and it will look mostly like what you see in the error messages.
You can also add statements that print the SD nodes to the code that 
builds the DAG, or the DAG combiner.

I'd take a working backend, and watch what happens when I modify the .td 
files.  Pick one that seems the easiest to understand, or simply ask 
about the obscurity when something looks strange.

Your experiences with this are probably not uncommon...

-Krzysztof


On 3/7/2014 6:28 PM, David Given wrote:
> 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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