[LV] Ignore some costs when loop gets fully unrolled

When VF equals the number of iterations, comparison instruction
and induction operation will be DCEed later. Ignoring the costs of these
instructions improves the cost model.

Author: igogo-x86

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -7246,6 +7246,26 @@ LoopVectorizationPlanner::precomputeCosts(VPlan &Plan, ElementCount VF,
         continue;
       IVInsts.push_back(CI);
     }
+
+    // If the given VF loop gets fully unrolled, ignore the costs of comparison
+    // and increment instruction, as they'll get simplified away
+    auto TC = CM.PSE.getSE()->getSmallConstantTripCount(OrigLoop);
+    auto *Cmp = OrigLoop->getLatchCmpInst();
+    if (Cmp && VF.isFixed() && VF.getFixedValue() == TC) {
+      CostCtx.SkipCostComputation.insert(Cmp);
+      for (Instruction *IVInst : IVInsts) {
+        bool IsSimplifiedAway = true;
+        for (auto *UIV : IVInst->users()) {
+          if (!Legal->isInductionVariable(UIV) && UIV != Cmp) {
+            IsSimplifiedAway = false;
+            break;
+          }
+        }
+        if (IsSimplifiedAway)
+          CostCtx.SkipCostComputation.insert(IVInst);
+      }
+    }
+
     for (Instruction *IVInst : IVInsts) {
       if (CostCtx.skipCostComputation(IVInst, VF.isVector()))
         continue;