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

[via llvm-commits]

================
@@ -7391,6 +7387,126 @@ 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 = CM.getInstructionCost(IVInc, VF).first;
+    LLVM_DEBUG({
+      dbgs() << "Cost of " << InductionCost << " for VF " << VF
+             << ":\n induction increment " << *IVInc << "\n";
+      IVInc->dump();
+    });
+    Cost += InductionCost;
+  }
+
+  // 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;
+
+    const auto &ChainOps = RdxDesc.getReductionOpChain(RedPhi, OrigLoop);
+    SetVector<Instruction *> ChainOpsAndOperands(ChainOps.begin(),
+                                                 ChainOps.end());
+    // Also include the operands of instructions in the chain, as the cost-model
+    // may mark extends as free.
+    for (auto *ChainOp : ChainOps) {
+      for (Value *Op : ChainOp->operands()) {
+        if (auto *I = dyn_cast<Instruction>(Op))
+          ChainOpsAndOperands.insert(I);
+      }
+    }
+
+    // Pre-compute the cost for I, if it has a reduction pattern cost.
+    for (Instruction *I : ChainOpsAndOperands) {
+      auto ReductionCost = CM.getReductionPatternCost(
+          I, VF, ToVectorTy(I->getType(), VF), TTI::TCK_RecipThroughput);
+      if (!ReductionCost)
+        continue;
+
+      assert(!CostCtx.SkipCostComputation.contains(I) &&
+             "reduction op visited multiple times");
+      CostCtx.SkipCostComputation.insert(I);
+      LLVM_DEBUG(dbgs() << "Cost of " << ReductionCost << " for VF " << VF
+                        << ":\n in-loop reduction " << *I << "\n");
+      Cost += *ReductionCost;
+    }
+  }
+
----------------
ayalz wrote:

Worth emphasizing that
```suggestion
  // Now compute and add the VPlan-based cost.
```

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