[llvm] [VPlan] First step towards VPlan cost modeling (LegacyCM in CostCtx) (PR #92555)

[via llvm-commits]

================
@@ -7391,6 +7384,124 @@ LoopVectorizationPlanner::plan(ElementCount UserVF, unsigned UserIC) {
   return VF;
 }
 
+InstructionCost VPCostContext::getLegacyCost(Instruction *UI,
+                                             ElementCount VF) const {
+  return CM.getInstructionCost(UI, VF).first;
+}
+
+bool VPCostContext::skipCostComputation(Instruction *UI, bool IsVector) const {
+  return (IsVector && CM.VecValuesToIgnore.contains(UI)) ||
+         SkipCostComputation.contains(UI);
+}
+
+InstructionCost LoopVectorizationPlanner::computeCost(VPlan &Plan,
+                                                      ElementCount VF) const {
+  InstructionCost Cost = 0;
+  LLVMContext &LLVMCtx = OrigLoop->getHeader()->getContext();
+  VPCostContext CostCtx(CM.TTI, Legal->getWidestInductionType(), LLVMCtx, CM);
+
+  // Cost modeling for inductions is inaccurate in the legacy cost model
+  // compared to the recipes that are generated. To match here initially during
+  // VPlan cost model bring up directly use the induction costs from the legacy
+  // cost model. Note that we do this as pre-processing; the VPlan may not have
+  // any recipes associated with the original induction increment instruction.
+  // We precompute the cost for both cases, and always skip recipes for
+  // induction increments later on, if they exist.
+  // TODO: Switch to more accurate costing based on VPlan.
+  for (const auto &[IV, _] : Legal->getInductionVars()) {
+    Instruction *IVInc = cast<Instruction>(
+        IV->getIncomingValueForBlock(OrigLoop->getLoopLatch()));
+    InstructionCost InductionCost = CM.getInstructionCost(IVInc, VF).first;
+    LLVM_DEBUG({
+      dbgs() << "Cost of " << InductionCost << " for VF " << VF
+             << ":\n induction increment " << *IVInc << "\n";
+      IVInc->dump();
+    });
+    Cost += InductionCost;
+    assert(!CostCtx.SkipCostComputation.contains(IVInc) &&
+           "Same IV increment for multiple inductions?");
+    CostCtx.SkipCostComputation.insert(IVInc);
----------------
ayalz wrote:

```suggestion
    assert(!CostCtx.SkipCostComputation.contains(IVInc) &&
           "Same IV increment for multiple inductions?");
    CostCtx.SkipCostComputation.insert(IVInc);
    LLVM_DEBUG({
      dbgs() << "Cost of " << InductionCost << " for VF " << VF
             << ":\n induction increment " << *IVInc << "\n";
      IVInc->dump();
    });
    Cost += InductionCost;
```
consistent with the order of asserting/marking-dumping-accumulating the costs of reductions below; there they depend on having a cost, here it is independent of cost.

https://github.com/llvm/llvm-project/pull/92555