[llvm] [VPlan] Split out optimizeEVLMasks. NFC (PR #174925)

[Ramkumar Ramachandra via llvm-commits]

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@@ -2995,8 +2979,42 @@ static VPRecipeBase *optimizeMaskToEVL(VPValue *HeaderMask,
   return nullptr;
 }
 
-/// Replace recipes with their EVL variants.
-static void transformRecipestoEVLRecipes(VPlan &Plan, VPValue &EVL) {
+/// Optimize away any EVL-based header masks to VP intrinsic based recipes.
+/// The transforms here need to preserve the original semantics.
+void VPlanTransforms::optimizeEVLMasks(VPlan &Plan) {
+  // Find the EVL-based header mask if it exists: icmp ult step-vector, EVL
+  VPInstruction *HeaderMask = nullptr;
+  for (VPRecipeBase &R : *Plan.getVectorLoopRegion()->getEntryBasicBlock()) {
+    if (match(&R, m_ICmp(m_VPInstruction<VPInstruction::StepVector>(),
+                         m_EVL(m_VPValue())))) {
+      HeaderMask = cast<VPInstruction>(&R);
+      break;
+    }
+  }
+  if (!HeaderMask)
+    return;
+
+  VPTypeAnalysis TypeInfo(Plan);
+  VPValue *EVL = HeaderMask->getOperand(1);
+
+  for (VPUser *U : collectUsersRecursively(HeaderMask)) {
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artagnon wrote:

collectUsersRecursively returns an rvalue from `takeVector`, which is fine if the users are read-only, but we eraseFromParent -- I think you need an lvalue.

https://github.com/llvm/llvm-project/pull/174925