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On Jul 10, 2015, at 12:22 AM, Frédéric Heitzmann <frederic.heitzmann@gmail.com> wrote:

2015-07-08 17:58 GMT+02:00 escha <escha@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 ?

If you write a target-specific combine that conflicts with a canonicalization, it’ll infinite loop because it’ll oscillate between the two forms.

—escha