[llvm] [LV][EVL] Support fixed-order recurrence idiom with EVL tail folding. (PR #124093)

[Alexey Bataev via llvm-commits]

================
@@ -1727,6 +1783,30 @@ static void transformRecipestoEVLRecipes(VPlan &Plan, VPValue &EVL) {
   VPTypeAnalysis TypeInfo(CanonicalIVType);
   LLVMContext &Ctx = CanonicalIVType->getContext();
   VPValue *AllOneMask = Plan.getOrAddLiveIn(ConstantInt::getTrue(Ctx));
+  VPRegionBlock *LoopRegion = Plan.getVectorLoopRegion();
+  VPBasicBlock *Header = LoopRegion->getEntryBasicBlock();
+
+  // Create a scalar phi to track the previous EVL if fixed-order recurrence is
+  // contained.
+  VPScalarPHIRecipe *PrevEVL = nullptr;
+  bool ContainsFORs =
+      any_of(Header->phis(), IsaPred<VPFirstOrderRecurrencePHIRecipe>);
+  if (ContainsFORs) {
+    // TODO: Use VPInstruction::ExplicitVectorLength to get maximum EVL.
----------------
alexey-bataev wrote:

I think Mel means the case, where VF is larger than the total trip count. In this case we can precalculate Explicit VL and use it instead of RuntimeVF. It may help the performance

https://github.com/llvm/llvm-project/pull/124093