[llvm] [LV] Add -fold-epilogue-tail option for tail-folded epilogue (PR #190697)

[David Sherwood via llvm-commits]

================
@@ -7557,6 +7583,43 @@ getEpilogueLowering(Function *F, Loop *L, LoopVectorizeHints &Hints,
   return CM_EpilogueAllowed;
 }
 
+/// Determine how to lower the epilogue for the vector epilogue loop.
+/// Check if there are any conflicts that prevent tail-folding the epilogue.
+/// \return CM_EpilogueNotNeededFoldEpilogueTail if epilogue tail-folding
+/// is possible, otherwise CM_EpilogueAllowed.
+static EpilogueLowering
+getEpilogueTailLowering(const LoopVectorizationCostModel &CM) {
+  // Epilogue TF is only enabled when explicitly requested via command line.
+  if (TailFoldingPolicy != TailFoldingPolicyTy::PreferFoldEpilogueTail)
+    // fallback to a normal epilogue.
+    return CM_EpilogueAllowed;
+
+  if (!EnableEpilogueVectorization) {
+    LLVM_DEBUG(dbgs() << "LV: Options conflict, epilogue vectorization is "
+                         "disallowed while epilogue tail-folding allowed!\n");
+    LLVM_DEBUG(dbgs() << "LV: Fallback to a normal epilogue\n");
+    return CM_EpilogueAllowed;
+  }
+
+  // If scalar epilogue is explicitly required, we can't apply TF.
+  if (CM.requiresScalarEpilogue(/*IsVectorizing*/ true)) {
+    LLVM_DEBUG(dbgs() << "LV: Epilogue tail-folding can't be applied because "
+                         "scalar epilogue is required\n");
+    LLVM_DEBUG(dbgs() << "LV: Fallback to a normal epilogue\n");
----------------
david-arm wrote:

Might be good to combine the two LLVM_DEBUG statements into one, i.e. something like

```
  LLVM_DEBUG(dbgs() << "LV: Epilogue tail-folding can't be applied because "
                       "scalar epilogue is required\n"
                    << "LV: Fallback to a normal epilogue\n");
```

and same for the debug above?

https://github.com/llvm/llvm-project/pull/190697