[llvm] [LV][VPlan] Add initial support for CSA vectorization (PR #106560)

[Michael Maitland via llvm-commits]

================
@@ -7550,9 +7555,17 @@ InstructionCost LoopVectorizationPlanner::cost(VPlan &Plan,
 /// not have corresponding recipes in \p Plan and are not marked to be ignored
 /// in \p CostCtx. This means the VPlan contains simplification that the legacy
 /// cost-model did not account for.
-static bool planContainsAdditionalSimplifications(VPlan &Plan,
-                                                  VPCostContext &CostCtx,
-                                                  Loop *TheLoop) {
+static bool
+planContainsAdditionalSimplifications(VPlan &Plan, VPCostContext &CostCtx,
+                                      Loop *TheLoop,
+                                      LoopVectorizationLegality &Legal) {
+  // CSA cost is more complicated since there is significant overhead in the
+  // preheader and middle block. It also contains recipes that are not backed by
+  // underlying instructions in the original loop. This makes it difficult to
+  // model in the legacy cost model.
+  if (!Legal.getCSAs().empty())
----------------
michaelmaitland wrote:

What is the reason to walk all recipes in the plan looking for CSA recipes when we can do this check in O(1) like this?

https://github.com/llvm/llvm-project/pull/106560