[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.
+  Value *TrueMask =
+      ConstantInt::getTrue(CanonicalIVPHI->getScalarType()->getContext());
+  VPValue *VPTrueMask = Plan.getVPValueOrAddLiveIn(TrueMask);
+  replaceHeaderPredicateWithIdiom(Plan, *VPTrueMask, [](VPUser &U, unsigned) {
+    return isa<VPWidenMemoryInstructionRecipe>(U);
+  });
+  // Now create the ExplicitVectorLengthPhi recipe in the main loop.
+  auto *EVLPhi = new VPEVLBasedIVPHIRecipe(StartV, DebugLoc());
+  EVLPhi->insertAfter(CanonicalIVPHI);
+  auto *VPEVL = new VPInstruction(VPInstruction::ExplicitVectorLength,
+                                  {EVLPhi, Plan.getTripCount()});
+  VPEVL->insertBefore(*Header, Header->getFirstNonPhi());
+
+  auto *CanonicalIVIncrement =
+      cast<VPInstruction>(CanonicalIVPHI->getBackedgeValue());
+  VPSingleDefRecipe *OpVPEVL = VPEVL;
+  if (CanonicalIVPHI->getScalarType() !=
+      IntegerType::get(CanonicalIVPHI->getScalarType()->getContext(),
+                       /*NumBits=*/32)) {
+    OpVPEVL = new VPScalarCastRecipe(Instruction::ZExt, OpVPEVL,
+                                     CanonicalIVPHI->getScalarType());
+    OpVPEVL->insertBefore(CanonicalIVIncrement);
----------------
ayalz wrote:

Why depend on CanonicalIVIncrement, can simply place the cast and the bump after VPEVL in the header?

Canonical IV must have a type no smaller than i32?

Simply ask for the size of Canonical IV's scalar type, than compare if equal to i32?

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