[llvm] r296535 - Fix PR 24415 (at least), by making our post-dominator tree behavior sane.

[Daniel Berlin via llvm-commits]

>
>
>>> Most of the text above is just to give some background to others. I do
>>> not expect you to answer in such detail. The only question I have if you
>>> can give a specific example that would break with the alternative
>>> solution I propose and which does not require strong post-dominance
>>> (which is likely hard to guarantee)?
>>>
>> Yes.
> ADCE breaks *right* now without the hack it has to try to work around this
> (which is not correct in all cases, FWIW).
> This is directly because of the path property. It also can break in some
> cases because it's not sufficient to check just part of the path for the
> discontinuity you introduce above.
>
> SSUPRE will break  because it relies on the path property to optimize
> stores properly.
> In fact, i'm pretty much not aware of a single optimization relying on
> post-dom info that will not break. I'm also positive i can't work around it.
>
>
As another analysis example, this will break the linear time reverse IDF
calculation. It already does (and adce tries to work around that now), but
it will so even more :)

It relies on the path property for correctness of the computation.
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