[llvm-branch-commits] [llvm] [LV] Only create partial reductions when profitable. (PR #181706)

[Sander de Smalen via llvm-branch-commits]

================
@@ -6273,6 +6259,28 @@ void VPlanTransforms::createPartialReductions(VPlan &Plan,
     });
   };
 
+  auto IsProfitablePartialReductionChainForVF =
+      [&](ArrayRef<VPPartialReductionChain> Chain, ElementCount VF) -> bool {
+    InstructionCost PartialCost = 0, RegularCost = 0;
+
+    // The chain is a profitable partial reduction chain if
+    // the cost of handling the entire chain is cheaper when
+    // using partial reductions than when handling the entire
+    // chain using regular reductions.
+    for (const VPPartialReductionChain &Link : Chain) {
+      ExtendedReductionOperand ExtendedOp = Link.ExtendedOp;
+      PartialCost += getPartialReductionLinkCost(CostCtx, Link, VF);
+      RegularCost += Link.ReductionBinOp->computeCost(VF, CostCtx);
+      if (ExtendedOp.BinOp && ExtendedOp.BinOp != Link.ReductionBinOp)
+        RegularCost += ExtendedOp.BinOp->computeCost(VF, CostCtx);
+      if (ExtendedOp.CastRecipes[0])
+        RegularCost += ExtendedOp.CastRecipes[0]->computeCost(VF, CostCtx);
+      if (ExtendedOp.CastRecipes[1])
+        RegularCost += ExtendedOp.CastRecipes[1]->computeCost(VF, CostCtx);
+    }
+    return PartialCost.isValid() && PartialCost <= RegularCost;
----------------
sdesmalen-arm wrote:

That was an intentional change, if the cost is equal then a partial reduction would be more profitable because it reduces register pressure.

https://github.com/llvm/llvm-project/pull/181706