[llvm] [LoopVectorizer][AArch64] Move getMinTripCountTailFoldingThreshold later. (PR #132170)

[David Green via llvm-commits]

================
@@ -4105,13 +4102,40 @@ LoopVectorizationCostModel::computeMaxVF(ElementCount UserVF, unsigned UserIC) {
     const SCEV *Rem = SE->getURemExpr(
         SE->applyLoopGuards(ExitCount, TheLoop),
         SE->getConstant(BackedgeTakenCount->getType(), MaxVFtimesIC));
-    if (Rem->isZero()) {
+    return Rem->isZero();
+  };
+
+  if (MaxPowerOf2RuntimeVF && *MaxPowerOf2RuntimeVF > 0) {
+    assert((UserVF.isNonZero() || isPowerOf2_32(*MaxPowerOf2RuntimeVF)) &&
+           "MaxFixedVF must be a power of 2");
+    if (IsKnownModTripCountZero(*MaxPowerOf2RuntimeVF)) {
       // Accept MaxFixedVF if we do not have a tail.
       LLVM_DEBUG(dbgs() << "LV: No tail will remain for any chosen VF.\n");
       return MaxFactors;
     }
   }
 
+  if (MaxTC && MaxTC <= TTI.getMinTripCountTailFoldingThreshold()) {
+    if (MaxPowerOf2RuntimeVF && *MaxPowerOf2RuntimeVF > 0) {
+      // If we have a low-trip-count, and the fixed-width VF is known to divide
+      // the trip count but the scalable factor does not, use the fixed-width
+      // factor in preference to allow the generation of a non-predicated loop.
+      if (ScalarEpilogueStatus == CM_ScalarEpilogueNotAllowedLowTripLoop &&
+          IsKnownModTripCountZero(MaxFactors.FixedVF.getFixedValue())) {
+        LLVM_DEBUG(dbgs() << "LV: Picking a fixed-width so that no tail will "
+                             "remain for any chosen VF.\n");
+        MaxFactors.ScalableVF = ElementCount::getScalable(0);
+        return MaxFactors;
+      }
+    }
+
----------------
davemgreen wrote:

It looks like it gets emitted in one of the parent functions. None of the other returns from this function use 
emitRemarkWithHints, which was the reason I didn't more it over.

reportVectorizationFailure will emit the -Rpass=analysis output. The -Rpass=missed remark will be emitted by the VF now being scalar
```
remark: the cost-model indicates that vectorization is not beneficial [-Rpass-missed=loop-vectorize]
```

https://github.com/llvm/llvm-project/pull/132170