[PATCH] D79001: [ARM][MVE] Tail-predication: support nested loops with dependent iterators.

[Sam Parker via Phabricator via llvm-commits]

samparker added a comment.

I'm not convinced that this is the route to take, I think we should be questioning whether it's necessary to back substitute the NumElements through MatchElemCountLoopSetup. If we don't have to do that, then I think we can use SCEV arithmetic, instead of pattern matching, to get our count:

  --- a/llvm/lib/Target/ARM/MVETailPredication.cpp
  +++ b/llvm/lib/Target/ARM/MVETailPredication.cpp
  @@ -494,16 +494,12 @@ bool MVETailPredication::ComputeRuntimeElements(TripCountPattern &TCP) {
       } else
         return nullptr;
   
  -    if (auto *RoundUp = dyn_cast<SCEVAddExpr>(S->getLHS())) {
  -      if (auto *Const = dyn_cast<SCEVConstant>(RoundUp->getOperand(0))) {
  -        if (Const->getAPInt() != (VF->getValue() - 1))
  -          return nullptr;
  -      } else
  -        return nullptr;
  -
  -      return RoundUp->getOperand(1);
  -    }
  -    return nullptr;
  +    auto *RoundUp = S->getLHS();
  +    auto VFMinusOne =
  +      SE->getAddExpr(S->getRHS(),
  +                     SE->getNegativeSCEV(SE->getOne(S->getType())));
  +    auto UndoRound = SE->getAddExpr(RoundUp, SE->getNegativeSCEV(VFMinusOne));
  +    return UndoRound;
     };
   
     // TODO: Can we use SCEV helpers, such as findArrayDimensions, and friends to
  @@ -534,9 +530,6 @@ bool MVETailPredication::ComputeRuntimeElements(TripCountPattern &TCP) {
     SCEVExpander Expander(*SE, DL, "elements");
     TCP.NumElements = Expander.expandCodeFor(Elems, Elems->getType(), InsertPt);
   
  -  if (!MatchElemCountLoopSetup(L, TCP.Shuffle, TCP.NumElements))
  -    return false;
  -
     return true;
   }


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  https://reviews.llvm.org/D79001/new/

https://reviews.llvm.org/D79001