[llvm] [LV] Ignore some costs when loop gets fully unrolled (PR #106699)

[David Sherwood via llvm-commits]

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@@ -7221,6 +7235,20 @@ LoopVectorizationPlanner::precomputeCosts(VPlan &Plan, ElementCount VF,
         continue;
       IVInsts.push_back(CI);
     }
+
+    // If with the given VF loop gets fully unrolled, ignore the costs of
+    // comparison and induction instructions, as they'll get simplified away
+    auto TC = CM.PSE.getSE()->getSmallConstantTripCount(OrigLoop);
+    auto *Cmp = OrigLoop->getLatchCmpInst();
+    if (Cmp && TC == VF.getKnownMinValue()) {
+      CostCtx.SkipCostComputation.insert(Cmp);
+      for (const auto &[IV, IndDesc] : Legal->getInductionVars()) {
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david-arm wrote:

Again, same comment as above - there could be outside users of the induction variables and the increment will remain.

https://github.com/llvm/llvm-project/pull/106699