[llvm] [LV] Simplify epilogue VF selection by comparing against MaxTripCount (PR #186185)

[David Sherwood via llvm-commits]

================
@@ -4313,55 +4313,26 @@ std::unique_ptr<VPlan> LoopVectorizationPlanner::selectBestEpiloguePlan(
   if (RemainingIterations->isZero())
     return nullptr;
 
-  if (MainLoopVF.isFixed()) {
-    MaxTripCount = MainLoopVF.getFixedValue() * IC - 1;
-    if (SE.isKnownPredicate(CmpInst::ICMP_ULT, RemainingIterations,
-                            SE.getConstant(TCType, MaxTripCount))) {
-      MaxTripCount = SE.getUnsignedRangeMax(RemainingIterations).getZExtValue();
-    }
-    LLVM_DEBUG(dbgs() << "LEV: Maximum Trip Count for Epilogue: "
-                      << MaxTripCount << "\n");
-  }
+  unsigned MaxTripCount = EstimatedRuntimeVF.getFixedValue() * IC - 1;
+  if (SE.isKnownPredicate(CmpInst::ICMP_ULT, RemainingIterations,
----------------
david-arm wrote:

What happens when `RemainingIterations` is scalable? In this case the expression will involve `vscale` and we'll be comparing against a constant with the new code. If `RemainingIterations` is scalable then it also means `MainLoopVF ` is scalable. I think you either have to:

1. Use calculations depending upon whether `RemainingIterations` is scalable or not. For example, you can use calculations on `RemainingIterations` and `MainLoopVF` directly keeping the comparison in the vscale space. Or
2. Estimate the remaining iterations using VScaleForTuning.

https://github.com/llvm/llvm-project/pull/186185