[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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@@ -1201,20 +1240,75 @@ void VPlanTransforms::addActiveLaneMask(
   // Walk users of WideCanonicalIV and replace all compares of the form
   // (ICMP_ULE, WideCanonicalIV, backedge-taken-count) with an
   // active-lane-mask.
-  VPValue *BTC = Plan.getOrCreateBackedgeTakenCount();
-  for (VPUser *U : SmallVector<VPUser *>(WideCanonicalIV->users())) {
-    auto *CompareToReplace = dyn_cast<VPInstruction>(U);
-    if (!CompareToReplace ||
-        CompareToReplace->getOpcode() != Instruction::ICmp ||
-        CompareToReplace->getPredicate() != CmpInst::ICMP_ULE ||
-        CompareToReplace->getOperand(1) != BTC)
-      continue;
+  replaceHeaderPredicateWithIdiom(Plan, *LaneMask);
+}
 
-    assert(CompareToReplace->getOperand(0) == WideCanonicalIV &&
-           "WidenCanonicalIV must be the first operand of the compare");
-    CompareToReplace->replaceAllUsesWith(LaneMask);
-    CompareToReplace->eraseFromParent();
+/// Add a VPEVLBasedIVPHIRecipe and related recipes to \p Plan and
+/// replaces all uses except the canonical IV increment of
+/// VPCanonicalIVPHIRecipe with a VPEVLBasedIVPHIRecipe. VPCanonicalIVPHIRecipe
+/// is used only for loop iterations counting after this transformation.
+///
+/// The function uses the following definitions:
+///  %StartV is the canonical induction start value.
+///
+/// The function adds the following recipes:
+///
+/// vector.ph:
+/// ...
+///
+/// vector.body:
+/// ...
+/// %P = EXPLICIT-VECTOR-LENGTH-BASED-IV-PHI [ %StartV, %vector.ph ], [
+/// %NextEVL, %vector.body ] %EVL = EXPLICIT-VECTOR-LENGTH %P, original TC
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ayalz wrote:

newline in the middle?

add the optional cast?

use names consistently in code and example: EVLPhi, VPEVL, NextEVLIV = EVLPhi + VPEVL

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