[LLVMdev] Why change "sub x, 5" to "add x, -5" ?

[Krzysztof Parzyszek]

On 7/10/2015 2:22 AM, Frédéric Heitzmann wrote:
>
>
> Unaware of your proposal, I used the TargetLowering::PerformDAGCombine hook.
> Is there a reason to prefere PreprocessISelDAG ?

Target-specific PerformDAGCombine only runs when the standard combiner 
doesn't do anything.  Also, various legalization steps can potentially 
undo what it did.  PreprocessISelDAG runs always and there is nothing to 
get in its way.

-Krzysztof

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