[llvm] [LV, VP]VP intrinsics support for the Loop Vectorizer + adding new tail-folding mode using EVL. (PR #76172)

[via llvm-commits]

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@@ -1521,6 +1525,36 @@ class LoopVectorizationCostModel {
     if (ForceTailFoldingStyle.getNumOccurrences()) {
       ChosenTailFoldingStyle.first = ChosenTailFoldingStyle.second =
           ForceTailFoldingStyle;
+      if (ChosenTailFoldingStyle.first == TailFoldingStyle::DataWithEVL) {
+        // FIXME: use actual opcode/data type for analysis here.
+        // FIXME: Investigate opportunity for fixed vector factor.
+        bool EVLIsLegal =
+            IsScalableVF && UserIC <= 1 &&
+            TTI.hasActiveVectorLength(0, nullptr, Align()) &&
+            !EnableVPlanNativePath &&
+            // FIXME: implement support for max safe dependency distance.
+            Legal->isSafeForAnyVectorWidth() &&
+            // FIXME: remove this once reductions are supported.
+            Legal->getReductionVars().empty() &&
+            // FIXME: remove this once vp_reverse is supported.
+            none_of(WideningDecisions,
+                    [](const std::pair<std::pair<Instruction *, ElementCount>,
+                                       std::pair<InstWidening, InstructionCost>>
+                           &Data) {
+                      return Data.second.first == CM_Widen_Reverse;
+                    });
+        if (!EVLIsLegal) {
+          ChosenTailFoldingStyle.first = ChosenTailFoldingStyle.second =
+              TailFoldingStyle::DataWithoutLaneMask;
+          LLVM_DEBUG(dbgs()
+                     << "LV: Preference for VP intrinsics indicated. Will "
+                        "not try to generate VP Intrinsics since "
+                     << (UserIC > 1
+                             ? "interleave count specified is greater than 1.\n"
+                             : "the target does not support vector length "
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ayalz wrote:

nit: the reason for failure, if not UserIC>1, is not necessarily lack of target support, i.e., TTI.hasActiveVectorLength(). Can state more generally but accurately, e.g., "due to non-interleaving reasons".

https://github.com/llvm/llvm-project/pull/76172