[PATCH] D86696: [Attributor][WIP] Introduce Loop AA

[Luofan Chen via Phabricator via llvm-commits]

bbn added inline comments.


================
Comment at: llvm/lib/Transforms/IPO/AttributorAttributes.cpp:7943
+        if (auto *BI = dyn_cast<BranchInst>(&I)) {
+          auto *Op = BI->getOperand(0);
+          const auto &OpAA =
----------------
baziotis wrote:
> uenoku wrote:
> > baziotis wrote:
> > > For that to make sense, you have to verify that the branch `isConditional()`. Also, note that loops
> > > may be [[ https://llvm.org/docs/LoopTerminology.html#rotated-loops | rotated ]]. In that case, the condition is not in the header block, but in the latch.
> > > 
> > > Generally, loops can get quite complicated and so, as Kuter said, you probably want to use
> > > SCEV (for example, backedge taken count etc.) I'll have to think that again because SCEV
> > > won't benefit from Attributor, meaning, at a high-level, using constant range AA when it makes
> > > sense seems reasonable.
> > FWIW, AAConstantRange actually uses SCEV internally but I agree that using AAConstantRange might make things complicated as a starting point. So you can use SCEV first. 
> Yes, but the point is that using constant range directly makes things difficult because we have to re-deduce information and loop structure that SCEV is supposed to handle (e.g. the code above is doing something similar to backedge taken count).
Thanks for the idea.
1. I think even if the loop is rotated, we can use such method to determine whether a loop
    is endless or not, right?
2. I think SCEV could be a good idea, but I cannot find a reasonable test for this. The range I
    get is always "full-set", can you give an example for this? Thanks


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