[PATCH] D112573: [IndVarSimplify] Reduce nondeterministic behaviour in visitIVCast.

[Max Kazantsev via Phabricator via llvm-commits]

mkazantsev added a comment.

If I understand your proposal correctly, you propose to replace scheme "take signedness from 1st user which is non-deterministically selected" with scheme "if at least one user of widest type is signed cast, widen as signed; otherwize widen as unsigned"?

In general, it makes sense to me and it also seems to solve the non-determinism you are describing. The only concern I have is, you use signed widening as default (and unsigned is possible under stricter conditions). On the other hand, IV tries hard to turn all `icmp`'s to unsigned predicate form, preferring unsigned as default. This creates some discord between different parts of it.

Is it possible to use unsigned widening as break-even? In terms of your patch it would be replacing

  WI.IsSigned |= IsSigned;

with

  WI.IsSigned &= IsSigned;



================
Comment at: llvm/lib/Transforms/Scalar/IndVarSimplify.cpp:560
+  // unspecified order of a PHI nodes' users-iterator.
+  if (WI.IsSigned != IsSigned && IsSigned)
+    WI.IsSigned = true;
----------------
Do you mean `if (!WI.IsSigned && IsSigned)` ? I guess it's an equivalent check. And it is also equivalent to unconditional
```
WI.IsSigned |= IsSigned;
```



Repository:
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  https://reviews.llvm.org/D112573/new/

https://reviews.llvm.org/D112573