[PATCH] D19682: [LSR] Skip transformation if the cost of no LSR is cheaper than the best solution selected

[Jun Bum Lim via llvm-commits]

junbuml created this revision.
junbuml added reviewers: gberry, qcolombet, mcrosier.
junbuml added a subscriber: llvm-commits.
Herald added subscribers: mzolotukhin, mcrosier.


I'm posting this to get any early feedback. Please let me know if this make sense.
    
TODO: tests should be added.

http://reviews.llvm.org/D19682

Files:
  lib/Transforms/Scalar/LoopStrengthReduce.cpp

Index: lib/Transforms/Scalar/LoopStrengthReduce.cpp
===================================================================
--- lib/Transforms/Scalar/LoopStrengthReduce.cpp
+++ lib/Transforms/Scalar/LoopStrengthReduce.cpp
@@ -1759,6 +1759,8 @@
                     DenseSet<const SCEV *> &VisitedRegs) const;
   void Solve(SmallVectorImpl<const Formula *> &Solution) const;
 
+  bool IsNothingCheaperThanBestSolution(Cost BestCost) const;
+
   BasicBlock::iterator
     HoistInsertPosition(BasicBlock::iterator IP,
                         const SmallVectorImpl<Instruction *> &Inputs) const;
@@ -4211,6 +4213,25 @@
   NarrowSearchSpaceByPickingWinnerRegs();
 }
 
+bool LSRInstance::IsNothingCheaperThanBestSolution(Cost BestCost) const {
+  DenseSet<const SCEV *> VisitedRegs;
+  SmallPtrSet<const SCEV *, 16> Regs;
+  Cost BaseCost;
+
+  for (const LSRFixup &LF : Fixups) {
+    const SCEV *S = SE.getSCEV(LF.OperandValToReplace);
+    Formula F;
+    F.initialMatch(S, L, SE);
+
+    const LSRUse &LU = Uses[LF.LUIdx];
+    BaseCost.RateFormula(TTI, F, Regs, VisitedRegs, L, LU.Offsets, SE, DT, LU);
+    if (BestCost < BaseCost)
+      return false;
+  }
+  return (BaseCost < BestCost);
+}
+
+
 /// This is the recursive solver.
 void LSRInstance::SolveRecurse(SmallVectorImpl<const Formula *> &Solution,
                                Cost &SolutionCost,
@@ -4306,6 +4327,10 @@
   if (Solution.empty()) {
     DEBUG(dbgs() << "\nNo Satisfactory Solution\n");
     return;
+  } else if (IsNothingCheaperThanBestSolution(SolutionCost)) {
+    DEBUG(dbgs() << "\nThe best solution is not cheaper than no LSR\n");
+    Solution.clear();
+    return;
   }
 
   // Ok, we've now made all our decisions.


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