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

[via llvm-commits]

================
@@ -747,6 +747,66 @@ void VPRegionBlock::execute(VPTransformState *State) {
   State->Instance.reset();
 }
 
+InstructionCost VPBasicBlock::cost(ElementCount VF, VPCostContext &Ctx) {
+  InstructionCost Cost = 0;
+  for (VPRecipeBase &R : Recipes)
+    Cost += R.cost(VF, Ctx);
+  return Cost;
+}
+
+InstructionCost VPRegionBlock::cost(ElementCount VF, VPCostContext &Ctx) {
+  InstructionCost Cost = 0;
+  if (!isReplicator()) {
+    for (VPBlockBase *Block : vp_depth_first_shallow(getEntry()))
+      Cost += Block->cost(VF, Ctx);
+
+    // Add the cost for the backedge.
+    Cost += 1;
+    return Cost;
+  }
+
+  // Compute the cost of a replicate region. Replicating isn't supported for
+  // scalable vectors, return an invalid cost for them.
+  // TODO: Discard scalable VPlans with replicate recipes earlier after
+  // construction.
+  if (VF.isScalable())
+    return InstructionCost::getInvalid();
+
+  // First compute the cost of the conditionally executed recipes, followed by
+  // account for the branching cost, except if the mask is a header mask or
+  // uniform condition.
+  using namespace llvm::VPlanPatternMatch;
+  VPBasicBlock *Then = cast<VPBasicBlock>(getEntry()->getSuccessors()[0]);
+  Cost += Then->cost(VF, Ctx);
+
+  // Note the cost estimates below closely match the current legacy cost model.
+  auto *BOM = cast<VPBranchOnMaskRecipe>(&getEntryBasicBlock()->front());
+  VPValue *Cond = BOM->getOperand(0);
+
+  // Check if Cond is a uniform compare or a header mask and don't account for
+  // branching costs. A uniform condition corresponding to a single branch per
+  // VF, and the header mask will always be true except in the last iteration.
+  if (vputils::isUniformBoolean(Cond) ||
+      vputils::isHeaderMask(Cond, *getPlan()))
+    return Cost;
+
+  // For the scalar case, we may not always execute the original predicated
+  // block, Thus, scale the block's cost by the probability of executing it.
+  if (VF.isScalar())
+    return Cost / getReciprocalPredBlockProb();
----------------
ayalz wrote:

```suggestion
    return ThenCost / getReciprocalPredBlockProb();
```


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