[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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@@ -1172,6 +1172,45 @@ static VPActiveLaneMaskPHIRecipe *addVPLaneMaskPhiAndUpdateExitBranch(
   return LaneMaskPhi;
 }
 
+/// Replaces (ICMP_ULE, WideCanonicalIV, backedge-taken-count) pattern using
+/// the given idiom \p Idiom.
+static void replaceHeaderPredicateWithIdiom(
+    VPlan &Plan, VPValue &Idiom,
+    function_ref<bool(VPUser &, unsigned)> Cond = {}) {
+  auto *FoundWidenCanonicalIVUser =
+      find_if(Plan.getCanonicalIV()->users(),
+              [](VPUser *U) { return isa<VPWidenCanonicalIVRecipe>(U); });
+  if (FoundWidenCanonicalIVUser == Plan.getCanonicalIV()->users().end())
+    return;
+  auto *WideCanonicalIV =
+      cast<VPWidenCanonicalIVRecipe>(*FoundWidenCanonicalIVUser);
+  // Walk users of WideCanonicalIV and replace all compares of the form
+  // (ICMP_ULE, WideCanonicalIV, backedge-taken-count) with
+  // the given idiom VPValue.
+  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;
+
+    assert(CompareToReplace->getOperand(0) == WideCanonicalIV &&
+           "WidenCanonicalIV must be the first operand of the compare");
+    if (Cond) {
+      CompareToReplace->replaceUsesWithIf(&Idiom, Cond);
+      if (!CompareToReplace->getNumUsers())
+        CompareToReplace->eraseFromParent();
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ayalz wrote:

note: could have left erasing from parents to dce, simplifying the code a bit here, but pass is placed after optimize()...

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