[llvm] [VPlan] First step towards VPlan cost modeling. (PR #92555)

[via llvm-commits]

================
@@ -7300,6 +7295,166 @@ 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::cost(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 of both induction increment instructions that are
+  // represented by recipes and those that are not, to avoid distinguishing
+  // between them here, and skip all recipes that represent induction increments
+  // (the former case) later on, if they exist, to avoid counting them twice.
+  // TODO: Switch to more accurate costing based on VPlan.
+  for (const auto &[IV, _] : Legal->getInductionVars()) {
+    Instruction *IVInc = cast<Instruction>(
+        IV->getIncomingValueForBlock(OrigLoop->getLoopLatch()));
+    assert(!CostCtx.SkipCostComputation.contains(IVInc) &&
+           "Same IV increment for multiple inductions?");
+    CostCtx.SkipCostComputation.insert(IVInc);
+    InstructionCost InductionCost = CostCtx.getLegacyCost(IVInc, VF);
+    LLVM_DEBUG({
+      dbgs() << "Cost of " << InductionCost << " for VF " << VF
+             << ":\n induction increment " << *IVInc << "\n";
+      IVInc->dump();
+    });
+    Cost += InductionCost;
+  }
+
+  /// Compute the cost of all exiting conditions of the loop using the legacy
+  /// cost model. This is to match the legacy behavior, which adds the cost of
+  /// all exit conditions. Note that this over-estimates the cost, as there will
+  /// be a single condition to control the vector loop.
+  SmallVector<BasicBlock *> Exiting;
+  CM.TheLoop->getExitingBlocks(Exiting);
+  // Add the cost of all exit conditions.
+  for (BasicBlock *EB : Exiting) {
+    auto *Term = dyn_cast<BranchInst>(EB->getTerminator());
+    if (!Term)
+      continue;
+    if (auto *CondI = dyn_cast<Instruction>(Term->getOperand(0))) {
+      assert(!CostCtx.SkipCostComputation.contains(CondI) &&
+             "Condition alrady skipped?");
----------------
ayalz wrote:

```suggestion
             "Condition already skipped?");
```

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