[llvm] [LV] Add tests for excluded cases in epiloge tail-folding (PR #217364)

[Florian Hahn via llvm-commits]

================
@@ -7148,23 +7150,55 @@ getEpilogueTailLowering(const LoopVectorizationCostModel &MainCM, const Loop *L,
   if (!EnableEpilogueVectorization) {
     reportVectorizationInfo(
         "Options conflict, epilogue vectorization is disallowed while "
-        "epilogue tail-folding allowed!\n",
+        "epilogue tail-folding allowed!",
         "UnsupportedEpilogueTailFoldingPolicy", ORE, L);
     return CM_EpilogueAllowed;
   }
 
+  if (!Hints.getWidth() || !hasForcedEpilogueVF()) {
+    reportVectorizationInfo("For now, epilogue tail-folding can't be "
+                            "applied without forced main/epilogue loop VF",
+                            "UnsupportedEpilogueTailFoldingPolicy", ORE, L);
+    return CM_EpilogueAllowed;
+  }
+
+  if (ElementCount::isKnownLE(Hints.getWidth(), EpilogueVectorizationForceVF)) {
+    reportVectorizationInfo("For now, epilogue tail-folding can't be "
+                            "applied when mainVF <= epilogueVF",
+                            "UnsupportedEpilogueTailFoldingPolicy", ORE, L);
+    return CM_EpilogueAllowed;
+  }
+
+  if (!L->isInnermost()) {
+    reportVectorizationInfo(
+        "Epilogue tail-folding is not supported for outer loop",
+        "InvalidTailFoldedEpilogue", ORE, L);
+    return CM_EpilogueAllowed;
+  }
+
   // If scalar epilogue is explicitly required, we can't apply TF.
   if (MainCM.requiresScalarEpilogue(/*IsVectorizing*/ true)) {
-    LLVM_DEBUG(dbgs() << "LV: Epilogue tail-folding can't be applied because "
-                         "scalar epilogue is required\n"
-                         "LV: Fall back to a normal epilogue\n");
+    reportVectorizationInfo(
+        "Epilogue tail-folding can't be applied because scalar epilogue is "
+        "required. Fall back to a normal epilogue",
+        "InvalidTailFoldedEpilogue", ORE, L);
     return CM_EpilogueAllowed;
   }
 
   // If having epilogue is NOT allowed, then no epilogue to apply TF for.
   if (!MainCM.isEpilogueAllowed()) {
-    LLVM_DEBUG(dbgs() << "LV: No epilogue to apply tail-folding for.\n"
-                         "LV: Fall back to a normal epilogue\n");
+    reportVectorizationInfo(
+        "Not applying tail-folding to the epilogue, since tail-folding is "
+        "already requested for the main vector loop.",
----------------
fhahn wrote:

I think there are some reasons no epilogue is allowed other than `CM_EpilogueNotNeededFoldTail`, which are unrelated to tail-folding (like ` CM_EpilogueNotAllowedOptSize` and `CM_EpilogueNotAllowedLowTripLoop`)

Maybe it would be worth to keep the wording closer to the original `no epilogue allowed`?

https://github.com/llvm/llvm-project/pull/217364