[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
+/// ...
+/// %NextEVL = EXPLICIT-VECTOR-LENGTH + %P, %EVL
+/// ...
+///
+void VPlanTransforms::addExplicitVectorLength(VPlan &Plan) {
+  VPBasicBlock *Header = Plan.getVectorLoopRegion()->getEntryBasicBlock();
+  auto *CanonicalIVPHI = Plan.getCanonicalIV();
+  VPValue *StartV = CanonicalIVPHI->getStartValue();
+
+  // Walk users of WideCanonicalIV and replace all compares of the form
+  // (ICMP_ULE, WideCanonicalIV, backedge-taken-count) with an
+  // all-true-mask.
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ayalz wrote:

> Does the EVLPhi->OpVPEVL->Next cycle have any users but itself? Such cycles may be subject to DCE'd.

Ah, this is fine, missed that "Replace all uses of VPCanonicalIVPHIRecipe by VPEVLBasedIVPHIRecipe except for the canonical IV increment" part.

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