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

[Frédéric Heitzmann]

2015-07-08 17:58 GMT+02:00 escha <escha at apple.com>:

> [...]
>
> If you want to “revert" this sort of thing, you can do it at Select() time
> or PreprocessISelDAG(), which is what I did on an out-of-tree backend to
> turn add X, -C into sub X, C on selection time. This still lets all the
> intermediate optimizations take advantage of the canonicalization.
>
> —escha


Unaware of your proposal, I used the TargetLowering::PerformDAGCombine hook.
Is there a reason to prefere PreprocessISelDAG ?

--
Fred
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