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

[Florian Hahn via llvm-commits]

================
@@ -7396,6 +7397,118 @@ LoopVectorizationPlanner::plan(ElementCount UserVF, unsigned UserIC) {
   return VF;
 }
 
+InstructionCost VPCostContext::getLegacyCost(Instruction *UI, ElementCount VF) {
+  return CM.getInstructionCost(UI, VF).first;
+}
+
+bool VPCostContext::skipForCostComputation(Instruction *UI) const {
+  return CM.VecValuesToIgnore.contains(UI) || SkipCostComputation.contains(UI);
+}
+
+InstructionCost LoopVectorizationPlanner::computeCost(VPlan &Plan,
+                                                      ElementCount VF) const {
+  InstructionCost Cost = 0;
+  LLVMContext &Ctx = OrigLoop->getHeader()->getContext();
+  VPCostContext CostCtx(CM.TTI, Legal->getWidestInductionType(), Ctx, 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 and skip induction recipes. Note that we do this as
+  // pre-processing; the VPlan may not have any recipes associated with the
+  // original induction increment instruction.
+  // 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;
+    CostCtx.SkipCostComputation.insert(IVInc);
+  }
+
+  // The legacy cost model has special logic to compute the cost of in-loop
+  // reductions, which may be smaller than the sum of all instructions involved
+  // in the reduction. Pre-compute the cost for now.
+  // TODO: Switch to costing based on VPlan once the logic has been ported.
+  for (const auto &[RedPhi, RdxDesc] : Legal->getReductionVars()) {
+    if (!CM.isInLoopReduction(RedPhi))
+      continue;
+
+    SmallVector<Instruction *, 4> ReductionOperations =
+        RdxDesc.getReductionOpChain(RedPhi, OrigLoop);
+    // Also include the operands of instructions in the chain, as the cost-model
+    // may mark extends as free.
+    for (unsigned I = 0, E = ReductionOperations.size(); I != E; ++I) {
+      for (Value *Op : ReductionOperations[I]->operands()) {
+        if (auto *I = dyn_cast<Instruction>(Op))
+          ReductionOperations.push_back(I);
+      }
+    }
+    for (Instruction *I : ReductionOperations) {
+      auto ReductionCost = CM.getReductionPatternCost(
+          I, VF, ToVectorTy(I->getType(), VF), TTI::TCK_RecipThroughput);
+      if (!ReductionCost)
+        continue;
+
+      if (!CostCtx.SkipCostComputation.insert(I).second)
+        continue;
----------------
fhahn wrote:

We need to first check if we have a reduction pattern cost before inserting into the set, as otherwise we may insert instructions without a reduction pattern cost.

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