[PATCH] D35741: Add MemorySSA alternative to AliasSetTracker in LICM.

[Alina Sbirlea via llvm-commits]

>
> Do you have preliminary numbers on how much faster MSSA- AST is than the
> regular
> AST?  For instance, what's the performance difference between repeatedly
> constructing the AST vs. constructing the MSSA-AST over some
> representative IR?
>
> In other words, can we get an upper bound on the speedup without doing all
> the
> work needed to teach all the loop passes to preserve MSSA?  If this upper
> bound
> is near-0, then we can cut our losses and avoid doing a bunch of
> unnecessary
> work.


On unit tests / regression tests, the overhead is too small for both, not
representative.
On the pathological case, both need to be capped. I'll get some preliminary
numbers on the testcase in PR28670, and the larger version I extracted
then, but this will only point at what the cap should be, so again,
probably not representative?

I would hope that the MSSA-ASTs will not be the final solution, since the
purpose here is to address PR31883.
Davide, do you have additional testcases that I can look into? (the ones
from the profile in PR31883?)
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