[SLPVectorize] Basic ephemeral-value awareness

The SLP vectorizer should not vectorize ephemeral values. These are used to
express information to the optimizer, and vectorizing them does not lead to
faster code (because the ephemeral values are dropped prior to code generation,
vectorized or not), and obscures the information the instructions are
attempting to communicate (the logic that interprets the arguments to
@llvm.assume generically does not understand vectorized conditions).

Also, uses by ephemeral values are free (because they, and the necessary
extractelement instructions, will be dropped prior to code generation).

llvm-svn: 219816

1 files changed, 30 insertions, 3 deletions

diff --git a/llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp b/llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp
index 76b46779535..16ba3167e8b 100644
--- a/llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp
+++ b/llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp
@@ -21,6 +21,8 @@
 #include "llvm/ADT/SetVector.h"
 #include "llvm/ADT/Statistic.h"
 #include "llvm/Analysis/AliasAnalysis.h"
+#include "llvm/Analysis/AssumptionTracker.h"
+#include "llvm/Analysis/CodeMetrics.h"
 #include "llvm/Analysis/LoopInfo.h"
 #include "llvm/Analysis/ScalarEvolution.h"
 #include "llvm/Analysis/ScalarEvolutionExpressions.h"
@@ -396,10 +398,12 @@ public:
 
   BoUpSLP(Function *Func, ScalarEvolution *Se, const DataLayout *Dl,
           TargetTransformInfo *Tti, TargetLibraryInfo *TLi, AliasAnalysis *Aa,
-          LoopInfo *Li, DominatorTree *Dt)
+          LoopInfo *Li, DominatorTree *Dt, AssumptionTracker *AT)
       : NumLoadsWantToKeepOrder(0), NumLoadsWantToChangeOrder(0),
         F(Func), SE(Se), DL(Dl), TTI(Tti), TLI(TLi), AA(Aa), LI(Li), DT(Dt),
-        Builder(Se->getContext()) {}
+        Builder(Se->getContext()) {
+    CodeMetrics::collectEphemeralValues(F, AT, EphValues);
+  }
 
   /// \brief Vectorize the tree that starts with the elements in \p VL.
   /// Returns the vectorized root.
@@ -555,6 +559,9 @@ private:
   /// This list holds pairs of (Internal Scalar : External User).
   UserList ExternalUses;
 
+  /// Values used only by @llvm.assume calls.
+  SmallPtrSet<const Value *, 32> EphValues;
+
   /// Holds all of the instructions that we gathered.
   SetVector<Instruction *> GatherSeq;
   /// A list of blocks that we are going to CSE.
@@ -988,6 +995,16 @@ void BoUpSLP::buildTree_rec(ArrayRef<Value *> VL, unsigned Depth) {
   // We now know that this is a vector of instructions of the same type from
   // the same block.
 
+  // Don't vectorize ephemeral values.
+  for (unsigned i = 0, e = VL.size(); i != e; ++i) {
+    if (EphValues.count(VL[i])) {
+      DEBUG(dbgs() << "SLP: The instruction (" << *VL[i] <<
+            ") is ephemeral.\n");
+      newTreeEntry(VL, false);
+      return;
+    }
+  }
+
   // Check if this is a duplicate of another entry.
   if (ScalarToTreeEntry.count(VL[0])) {
     int Idx = ScalarToTreeEntry[VL[0]];
@@ -1710,6 +1727,12 @@ int BoUpSLP::getTreeCost() {
     if (!ExtractCostCalculated.insert(I->Scalar))
       continue;
 
+    // Uses by ephemeral values are free (because the ephemeral value will be
+    // removed prior to code generation, and so the extraction will be
+    // removed as well).
+    if (EphValues.count(I->User))
+      continue;
+
     VectorType *VecTy = VectorType::get(I->Scalar->getType(), BundleWidth);
     ExtractCost += TTI->getVectorInstrCost(Instruction::ExtractElement, VecTy,
                                            I->Lane);
@@ -2810,6 +2833,7 @@ struct SLPVectorizer : public FunctionPass {
   AliasAnalysis *AA;
   LoopInfo *LI;
   DominatorTree *DT;
+  AssumptionTracker *AT;
 
   bool runOnFunction(Function &F) override {
     if (skipOptnoneFunction(F))
@@ -2823,6 +2847,7 @@ struct SLPVectorizer : public FunctionPass {
     AA = &getAnalysis<AliasAnalysis>();
     LI = &getAnalysis<LoopInfo>();
     DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
+    AT = &getAnalysis<AssumptionTracker>();
 
     StoreRefs.clear();
     bool Changed = false;
@@ -2845,7 +2870,7 @@ struct SLPVectorizer : public FunctionPass {
 
     // Use the bottom up slp vectorizer to construct chains that start with
     // store instructions.
-    BoUpSLP R(&F, SE, DL, TTI, TLI, AA, LI, DT);
+    BoUpSLP R(&F, SE, DL, TTI, TLI, AA, LI, DT, AT);
 
     // Scan the blocks in the function in post order.
     for (po_iterator<BasicBlock*> it = po_begin(&F.getEntryBlock()),
@@ -2872,6 +2897,7 @@ struct SLPVectorizer : public FunctionPass {
 
   void getAnalysisUsage(AnalysisUsage &AU) const override {
     FunctionPass::getAnalysisUsage(AU);
+    AU.addRequired<AssumptionTracker>();
     AU.addRequired<ScalarEvolution>();
     AU.addRequired<AliasAnalysis>();
     AU.addRequired<TargetTransformInfo>();
@@ -3746,6 +3772,7 @@ static const char lv_name[] = "SLP Vectorizer";
 INITIALIZE_PASS_BEGIN(SLPVectorizer, SV_NAME, lv_name, false, false)
 INITIALIZE_AG_DEPENDENCY(AliasAnalysis)
 INITIALIZE_AG_DEPENDENCY(TargetTransformInfo)
+INITIALIZE_PASS_DEPENDENCY(AssumptionTracker)
 INITIALIZE_PASS_DEPENDENCY(ScalarEvolution)
 INITIALIZE_PASS_DEPENDENCY(LoopSimplify)
 INITIALIZE_PASS_END(SLPVectorizer, SV_NAME, lv_name, false, false)