[PATCH] D73181: [SCEV] Do not use backedge SCEV of PHI if its input is another PHI

[Denis Antrushin via Phabricator via llvm-commits]

dantrushin marked an inline comment as done.
dantrushin added inline comments.


================
Comment at: llvm/lib/Analysis/ScalarEvolution.cpp:8247
+            if (!isa<PHINode>(BackedgeVal) &&
+                IsAvailableOnEntry(LI, DT, OnBackedge, PN->getParent()))
               return OnBackedge;
----------------
sanjoy.google wrote:
> dantrushin wrote:
> > Bailing out here when any PHI is seen, because I did not find convinient API to get SCEV at previous iteration.
> > Is there one?
> I don't think you need to go through SCEV here.  You can just pick the `llvm::Value` for the PHI for the backedge and get the SCEV for that.
Excuse me, I don't understand (or, rather my comment was unclear).

Existing code already does that. In terms of test case below, `PN` is PHI `%local_3_4` 
Its backedge `Value` is `%local_6_6` which is another PHI from the same block.
But using `%local_6_6`'s SCEV as value for `%local_3_4` is wrong, because `%local_3_4` uses value of `%local_6_6` 
from previous iteration
So my fix is not to perform this optimization when PHI's input is another PHI

Or you mean something else?


Repository:
  rG LLVM Github Monorepo

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  https://reviews.llvm.org/D73181/new/

https://reviews.llvm.org/D73181