[PATCH] D92177: [NFC][InstructionCost] Refactor LoopVectorizationCostModel::selectVectorizationFactor

[David Sherwood via Phabricator via llvm-commits]

david-arm created this revision.
david-arm added reviewers: sdesmalen, CarolineConcatto, ctetreau, dmgreen.
Herald added subscribers: llvm-commits, hiraditya.
Herald added a project: LLVM.
david-arm requested review of this revision.

This patch is the first of two patches that are designed to migrate
costs in LoopVectorize.cpp to use the new InstructionCost class.
This refactoring is necessary preparation for the second part, which
switches from using float cost to InstructionCost. The tuple is
introduced here because the InstructionCost could be invalid and
so it's not possible to maintain a running cost value in the loop
that divides the cost by the width.

See this patch for the introduction of the type: https://reviews.llvm.org/D91174
See this thread for context: http://lists.llvm.org/pipermail/llvm-dev/2020-November/146408.html


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D92177

Files:
  llvm/lib/Transforms/Vectorize/LoopVectorize.cpp


Index: llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
===================================================================
--- llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
+++ llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
@@ -5408,54 +5408,57 @@
 LoopVectorizationCostModel::selectVectorizationFactor(ElementCount MaxVF) {
   assert(!MaxVF.isScalable() && "scalable vectors not yet supported");
 
-  float Cost = expectedCost(ElementCount::getFixed(1)).first;
-  const float ScalarCost = Cost;
-  unsigned Width = 1;
-  LLVM_DEBUG(dbgs() << "LV: Scalar loop costs: " << (int)ScalarCost << ".\n");
+  unsigned ExpectedCost = expectedCost(ElementCount::getFixed(1)).first;
+  LLVM_DEBUG(dbgs() << "LV: Scalar loop costs: " << ExpectedCost << ".\n");
+
+  std::pair<float, unsigned> MinCost = {ExpectedCost, 1};
+  const float ScalarCost = ExpectedCost;
 
   bool ForceVectorization = Hints->getForce() == LoopVectorizeHints::FK_Enabled;
   if (ForceVectorization && MaxVF.isVector()) {
     // Ignore scalar width, because the user explicitly wants vectorization.
     // Initialize cost to max so that VF = 2 is, at least, chosen during cost
     // evaluation.
-    Cost = std::numeric_limits<float>::max();
+    MinCost.first = std::numeric_limits<float>::max();
   }
 
+  auto isLowerVectorCost = [](std::pair<float, unsigned> LHS,
+                              std::pair<float, unsigned> RHS) {
+    return (LHS.first / LHS.second) < (RHS.first / RHS.second);
+  };
+
   for (unsigned i = 2; i <= MaxVF.getFixedValue(); i *= 2) {
     // Notice that the vector loop needs to be executed less times, so
     // we need to divide the cost of the vector loops by the width of
     // the vector elements.
     VectorizationCostTy C = expectedCost(ElementCount::getFixed(i));
-    float VectorCost = C.first / (float)i;
+    std::pair<float, unsigned> VectorCost = {C.first, i};
     LLVM_DEBUG(dbgs() << "LV: Vector loop of width " << i
-                      << " costs: " << (int)VectorCost << ".\n");
+                      << " costs: " << (int)(C.first / i) << ".\n");
     if (!C.second && !ForceVectorization) {
       LLVM_DEBUG(
           dbgs() << "LV: Not considering vector loop of width " << i
                  << " because it will not generate any vector instructions.\n");
       continue;
     }
-    if (VectorCost < Cost) {
-      Cost = VectorCost;
-      Width = i;
-    }
+    if (isLowerVectorCost(VectorCost, MinCost))
+      MinCost = VectorCost;
   }
 
   if (!EnableCondStoresVectorization && NumPredStores) {
     reportVectorizationFailure("There are conditional stores.",
         "store that is conditionally executed prevents vectorization",
         "ConditionalStore", ORE, TheLoop);
-    Width = 1;
-    Cost = ScalarCost;
+    MinCost = {ScalarCost, 1};
   }
 
-  LLVM_DEBUG(if (ForceVectorization && Width > 1 && Cost >= ScalarCost) dbgs()
+  LLVM_DEBUG(if (ForceVectorization && MinCost.second > 1 &&
+                 isLowerVectorCost({ScalarCost, 1}, MinCost)) dbgs()
              << "LV: Vectorization seems to be not beneficial, "
              << "but was forced by a user.\n");
-  LLVM_DEBUG(dbgs() << "LV: Selecting VF: " << Width << ".\n");
-  VectorizationFactor Factor = {ElementCount::getFixed(Width),
-                                (unsigned)(Width * Cost)};
-  return Factor;
+  LLVM_DEBUG(dbgs() << "LV: Selecting VF: " << MinCost.second << ".\n");
+  ElementCount EC = ElementCount::getFixed(MinCost.second);
+  return {EC, (unsigned)MinCost.first};
 }
 
 std::pair<unsigned, unsigned>


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