[PATCH] D28393: [SCEV] Make howFarToZero produce a smaller max backedge-taken count

[Eli Friedman via Phabricator via llvm-commits]

efriedma added inline comments.


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Comment at: lib/Analysis/ScalarEvolution.cpp:7217
+    const SCEV *DistancePlusOne = getAddExpr(Distance, getOne(Distance->getType()));
+    if (isLoopEntryGuardedByCond(L, ICmpInst::ICMP_NE, DistancePlusOne, Zero)) {
+      ConstantRange CR = getUnsignedRange(DistancePlusOne);
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sbaranga wrote:
> This looks correct to me, but I think it could use a comment.
> 
> Do we only need to match against Distance + 1 because this is what we typically get from rotating the loop? Otherwise I think this could be generalized to Distance + constant, but we would need to do some work to find the value of the constant.
The transform is correct because we prove "Distance + 1" doesn't overflow.  I'll add a comment explaining that.

You could generalize this to "Distance + Constant", but it's not clear what sort of pattern we would be looking for.  (Also, at that point, it would probably be better to just write a context-sensitive version of getUnsignedRange().)


Repository:
  rL LLVM

https://reviews.llvm.org/D28393