[llvm] [VPlan] Update getBestPlan to return VF, use also for epilogue vec. (PR #98821)

[via llvm-commits]

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@@ -10015,21 +10012,25 @@ bool LoopVectorizePass::processLoop(Loop *L) {
     } else {
       // If we decided that it is *legal* to vectorize the loop, then do it.
 
+      ElementCount BestVF = LVP.getBestVF();
+      LLVM_DEBUG(dbgs() << "VF picked by VPlan cost model: " << BestVF << "\n");
+      assert(VF.Width == BestVF &&
+             "VPlan cost model and legacy cost model disagreed");
+      VPlan &BestPlan = LVP.getBestPlanFor(BestVF);
       // Consider vectorizing the epilogue too if it's profitable.
       VectorizationFactor EpilogueVF =
-          LVP.selectEpilogueVectorizationFactor(VF.Width, IC);
+          LVP.selectEpilogueVectorizationFactor(BestVF, IC);
       if (EpilogueVF.Width.isVector()) {
 
         // The first pass vectorizes the main loop and creates a scalar epilogue
         // to be vectorized by executing the plan (potentially with a different
         // factor) again shortly afterwards.
-        EpilogueLoopVectorizationInfo EPI(VF.Width, IC, EpilogueVF.Width, 1);
+        EpilogueLoopVectorizationInfo EPI(BestVF, IC, EpilogueVF.Width, 1);
         EpilogueVectorizerMainLoop MainILV(L, PSE, LI, DT, TLI, TTI, AC, ORE,
                                            EPI, &LVL, &CM, BFI, PSI, Checks);
 
         assert(EPI.MainLoopVF == VF.Width && "VFs must match");
----------------
ayalz wrote:

```suggestion
        assert(EPI.MainLoopVF == BestVF && "VFs must match");
```
or dropped - given that this holds by construction of EPI?

Maybe worth revisiting the use of `EPI` throughout, separately, as it leads to an inflation of VF's, a redundant setting of EpilogueUF asserted to be 1, a confusing resetting of `EPI.MainLoopVF = EPI.EpilogueVF;` below.

https://github.com/llvm/llvm-project/pull/98821