llvm dev - Oct 2016 - Target DAG-combine vs. Target custom lowering

Hi,

I am new to LLVM. I read the the Codegen documentation, and browsed thru
the code of some of the backends. One thing i didn't understand was when
should a transformation happen in the TargetLowering::PerformDAGCombine and
not in the custom lowering phase.

Thanks, Servius
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On 10/13/2016 1:56 PM, Servius Womack via llvm-dev
wrote:
> Hi,
>
> I am new to LLVM. I read the the Codegen documentation, and browsed thru
> the code of some of the backends. One thing i didn't understand was
when
> should a transformation happen in the TargetLowering::PerformDAGCombine
> and not in the custom lowering phase.
Lowering is really "legalization", so rewriting some construct in a
form
that is legal on your target. This only happens once. Combining is 
mostly an optimization, and it happens several times, including before 
and after legalization.

-Krzysztof

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