LoopVectorizer: Refactor the code that checks if it is safe to predicate blocks.

In this code we keep track of pointers that we are allowed to read from, if they are accessed by non-predicated blocks.
We use this list to allow vectorization of conditional loads in predicated blocks because we know that these addresses don't segfault.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@185214 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/Transforms/Vectorize/LoopVectorize.cpp b/lib/Transforms/Vectorize/LoopVectorize.cpp
index d4be08d..6f52074 100644 (file)
--- a/lib/Transforms/Vectorize/LoopVectorize.cpp
+++ b/lib/Transforms/Vectorize/LoopVectorize.cpp
@@ -354,79 +354,6 @@ static void setDebugLocFromInst(IRBuilder<> &B, const Value *Ptr) {
     B.SetCurrentDebugLocation(DebugLoc());
 }
 
-/// \brief Check if conditionally executed loads are hoistable.
-///
-/// This class has two functions: isHoistableLoad and canHoistAllLoads.
-/// isHoistableLoad should be called on all load instructions that are executed
-/// conditionally. After all conditional loads are processed, the client should
-/// call canHoistAllLoads to determine if all of the conditional executed loads
-/// have an unconditional memory access to the same memory address in the loop.
-class LoadHoisting {
-  typedef SmallPtrSet<Value *, 8> MemorySet;
-
-  Loop *TheLoop;
-  DominatorTree *DT;
-  MemorySet CondLoadAddrSet;
-
-public:
-  LoadHoisting(Loop *L, DominatorTree *D) : TheLoop(L), DT(D) {}
-
-  /// \brief Check if the instruction is a load with a identifiable address.
-  bool isHoistableLoad(Instruction *L);
-
-  /// \brief Check if all of the conditional loads are hoistable because there
-  /// exists an unconditional memory access to the same address in the loop.
-  bool canHoistAllLoads();
-};
-
-bool LoadHoisting::isHoistableLoad(Instruction *L) {
-  LoadInst *LI = dyn_cast<LoadInst>(L);
-  if (!LI)
-    return false;
-
-  CondLoadAddrSet.insert(LI->getPointerOperand());
-  return true;
-}
-
-static void addMemAccesses(BasicBlock *BB, SmallPtrSet<Value *, 8> &Set) {
-  for (BasicBlock::iterator BI = BB->begin(), BE = BB->end(); BI != BE; ++BI) {
-    if (LoadInst *LI = dyn_cast<LoadInst>(BI)) // Try a load.
-      Set.insert(LI->getPointerOperand());
-    else if (StoreInst *SI = dyn_cast<StoreInst>(BI)) // Try a store.
-      Set.insert(SI->getPointerOperand());
-  }
-}
-
-bool LoadHoisting::canHoistAllLoads() {
-  // No conditional loads.
-  if (CondLoadAddrSet.empty())
-    return true;
-
-  MemorySet UncondMemAccesses;
-  std::vector<BasicBlock*> &LoopBlocks = TheLoop->getBlocksVector();
-  BasicBlock *LoopLatch = TheLoop->getLoopLatch();
-
-  // Iterate over the unconditional blocks and collect memory access addresses.
-  for (unsigned i = 0, e = LoopBlocks.size(); i < e; ++i) {
-    BasicBlock *BB = LoopBlocks[i];
-
-    // Ignore conditional blocks.
-    if (BB != LoopLatch && !DT->dominates(BB, LoopLatch))
-      continue;
-
-    addMemAccesses(BB, UncondMemAccesses);
-  }
-
-  // And make sure there is a matching unconditional access for every
-  // conditional load.
-  for (MemorySet::iterator MI = CondLoadAddrSet.begin(),
-       ME = CondLoadAddrSet.end(); MI != ME; ++MI)
-    if (!UncondMemAccesses.count(*MI))
-      return false;
-
-  return true;
-}
-
 /// LoopVectorizationLegality checks if it is legal to vectorize a loop, and
 /// to what vectorization factor.
 /// This class does not look at the profitability of vectorization, only the
@@ -446,7 +373,7 @@ public:
                             DominatorTree *DT, TargetLibraryInfo *TLI)
       : TheLoop(L), SE(SE), DL(DL), DT(DT), TLI(TLI),
         Induction(0), WidestIndTy(0), HasFunNoNaNAttr(false),
-        MaxSafeDepDistBytes(-1U), LoadSpeculation(L, DT) {}
+        MaxSafeDepDistBytes(-1U) {}
 
   /// This enum represents the kinds of reductions that we support.
   enum ReductionKind {
@@ -638,8 +565,9 @@ private:
   void collectLoopUniforms();
 
   /// Return true if all of the instructions in the block can be speculatively
-  /// executed.
-  bool blockCanBePredicated(BasicBlock *BB);
+  /// executed. \p SafePtrs is a list of addresses that are known to be legal
+  /// and we know that we can read from them without segfault.
+  bool blockCanBePredicated(BasicBlock *BB, SmallPtrSet<Value *, 8>& SafePtrs);
 
   /// Returns True, if 'Phi' is the kind of reduction variable for type
   /// 'Kind'. If this is a reduction variable, it adds it to ReductionList.
@@ -697,9 +625,6 @@ private:
   bool HasFunNoNaNAttr;
 
   unsigned MaxSafeDepDistBytes;
-
-  /// Utility to determine whether loads can be speculated.
-  LoadHoisting LoadSpeculation;
 };
 
 /// LoopVectorizationCostModel - estimates the expected speedups due to
@@ -2611,6 +2536,24 @@ bool LoopVectorizationLegality::canVectorizeWithIfConvert() {
   assert(TheLoop->getNumBlocks() > 1 && "Single block loops are vectorizable");
   std::vector<BasicBlock*> &LoopBlocks = TheLoop->getBlocksVector();
 
+  // A list of pointers that we can safely read and write to.
+  SmallPtrSet<Value *, 8> SafePointes;
+
+  // Collect safe addresses.
+  for (unsigned i = 0, e = LoopBlocks.size(); i < e; ++i) {
+    BasicBlock *BB = LoopBlocks[i];
+
+    if (blockNeedsPredication(BB))
+      continue;
+
+    for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) {
+      if (LoadInst *LI = dyn_cast<LoadInst>(I))
+        SafePointes.insert(LI->getPointerOperand());
+      else if (StoreInst *SI = dyn_cast<StoreInst>(I))
+        SafePointes.insert(SI->getPointerOperand());
+    }
+  }
+
   // Collect the blocks that need predication.
   for (unsigned i = 0, e = LoopBlocks.size(); i < e; ++i) {
     BasicBlock *BB = LoopBlocks[i];
@@ -2620,14 +2563,10 @@ bool LoopVectorizationLegality::canVectorizeWithIfConvert() {
       return false;
 
     // We must be able to predicate all blocks that need to be predicated.
-    if (blockNeedsPredication(BB) && !blockCanBePredicated(BB))
+    if (blockNeedsPredication(BB) && !blockCanBePredicated(BB, SafePointes))
       return false;
   }
 
-  // Check that we can actually speculate the hoistable loads.
-  if (!LoadSpeculation.canHoistAllLoads())
-    return false;
-
   // We can if-convert this loop.
   return true;
 }
@@ -4056,11 +3995,15 @@ bool LoopVectorizationLegality::blockNeedsPredication(BasicBlock *BB)  {
   return !DT->dominates(BB, Latch);
 }
 
-bool LoopVectorizationLegality::blockCanBePredicated(BasicBlock *BB) {
+bool LoopVectorizationLegality::blockCanBePredicated(BasicBlock *BB,
+                                            SmallPtrSet<Value *, 8>& SafePtrs) {
   for (BasicBlock::iterator it = BB->begin(), e = BB->end(); it != e; ++it) {
     // We might be able to hoist the load.
-    if (it->mayReadFromMemory() && !LoadSpeculation.isHoistableLoad(it))
-      return false;
+    if (it->mayReadFromMemory()) {
+      LoadInst *LI = dyn_cast<LoadInst>(it);
+      if (!LI || !SafePtrs.count(LI->getPointerOperand()))
+        return false;
+    }
 
     // We don't predicate stores at the moment.
     if (it->mayWriteToMemory() || it->mayThrow())