[llvm] [VPlan] Add VPlan-based addMinIterCheck, replace ILV for non-epilogue. (PR #153643)

[via llvm-commits]

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@@ -671,6 +671,90 @@ void VPlanTransforms::attachCheckBlock(VPlan &Plan, Value *Cond,
   }
 }
 
+void VPlanTransforms::addMinimumIterationCheck(
+    VPlan &Plan, ElementCount VF, unsigned UF,
+    ElementCount MinProfitableTripCount, bool RequiresScalarEpilogue,
+    bool TailFolded, bool CheckNeededWithTailFolding, Loop *OrigLoop,
+    const uint32_t *MinItersBypassWeights, DebugLoc DL, ScalarEvolution &SE) {
+  // Generate code to check if the loop's trip count is less than VF * UF, or
+  // equal to it in case a scalar epilogue is required; this implies that the
+  // vector trip count is zero. This check also covers the case where adding one
+  // to the backedge-taken count overflowed leading to an incorrect trip count
+  // of zero. In this case we will also jump to the scalar loop.
+  CmpInst::Predicate CmpPred =
+      RequiresScalarEpilogue ? ICmpInst::ICMP_ULE : ICmpInst::ICMP_ULT;
+  // If tail is to be folded, vector loop takes care of all iterations.
+  VPValue *TripCountVPV = Plan.getTripCount();
+  const SCEV *TripCount = vputils::getSCEVExprForVPValue(TripCountVPV, SE);
+  Type *TripCountTy = TripCount->getType();
+  auto CreateMinTripCount = [&]() -> const SCEV * {
+    // Create or get max(MinProfitableTripCount, UF * VF) and return it.
+    const SCEV *VFxUF =
+        SE.getElementCount(TripCountTy, (VF * UF), SCEV::FlagNUW);
+    const SCEV *MinProfitableTripCountSCEV =
+        SE.getElementCount(TripCountTy, MinProfitableTripCount, SCEV::FlagNUW);
+    const SCEV *Max = SE.getUMaxExpr(MinProfitableTripCountSCEV, VFxUF);
+    if (!VF.isScalable())
+      return Max;
+
+    if (UF * VF.getKnownMinValue() >=
+        MinProfitableTripCount.getKnownMinValue()) {
----------------
ayalz wrote:

```suggestion
    if (VF.isScalable() && UF * VF.getKnownMinValue() >=
        MinProfitableTripCount.getKnownMinValue()) {
```
? Could be hoisted above computing Max and MinProfitableTripCountSCEV.

https://github.com/llvm/llvm-project/pull/153643