Add a new LoadAndStorePromoter class, which implements the general

"promote a bunch of load and stores" logic, allowing the code to
be shared and reused.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@123456 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Transforms/Utils/SSAUpdater.h

@@ -108,6 +108,38 @@ private:
   void operator=(const SSAUpdater&); // DO NOT IMPLEMENT
   SSAUpdater(const SSAUpdater&);     // DO NOT IMPLEMENT
 };
+  
+/// LoadAndStorePromoter - This little helper class provides a convenient way to
+/// promote a collection of loads and stores into SSA Form using the SSAUpdater.
+/// This handles complexities that SSAUpdater doesn't, such as multiple loads
+/// and stores in one block.
+///
+/// Clients of this class are expected to subclass this and implement the
+/// virtual methods.
+///
+class LoadAndStorePromoter {
+public:
+  LoadAndStorePromoter() {}
+  virtual ~LoadAndStorePromoter() {}
+  
+  /// run - This does the promotion.  Insts is a list of loads and stores to
+  /// promote, and Name is the basename for the PHIs to insert.  After this is
+  /// complete, the loads and stores are removed from the code.
+  void run(StringRef Name, const SmallVectorImpl<Instruction*> &Insts,
+           SSAUpdater *SSA = 0);
+  
+  
+  /// Return true if the specified instruction is in the Inst list (which was
+  /// passed into the run method).  Clients should implement this with a more
+  /// efficient version if possible.
+  virtual bool isInstInList(Instruction *I,
+                            const SmallVectorImpl<Instruction*> &Insts) const {
+    for (unsigned i = 0, e = Insts.size(); i != e; ++i)
+      if (Insts[i] == I)
+        return true;
+    return false;
+  }
+};
 
 } // End llvm namespace
 

lib/Transforms/Utils/SSAUpdater.cpp

@@ -343,3 +343,157 @@ Value *SSAUpdater::GetValueAtEndOfBlockInternal(BasicBlock *BB) {
   SSAUpdaterImpl<SSAUpdater> Impl(this, &AvailableVals, InsertedPHIs);
   return Impl.GetValue(BB);
 }
+
+//===----------------------------------------------------------------------===//
+// LoadAndStorePromoter Implementation
+//===----------------------------------------------------------------------===//
+
+void LoadAndStorePromoter::run(StringRef BaseName,
+                               const SmallVectorImpl<Instruction*> &Insts,
+                               SSAUpdater *SSA) {
+  if (Insts.empty()) return;
+  
+  // If no SSAUpdater was provided, use a default one.  This allows the client
+  // to capture inserted PHI nodes etc if they want.
+  SSAUpdater DefaultSSA;
+  if (SSA == 0) SSA = &DefaultSSA;
+  
+  const Type *ValTy;
+  if (LoadInst *LI = dyn_cast<LoadInst>(Insts[0]))
+    ValTy = LI->getType();
+  else
+    ValTy = cast<StoreInst>(Insts[0])->getOperand(0)->getType();
+    
+  SSA->Initialize(ValTy, BaseName);
+  
+  // First step: bucket up uses of the alloca by the block they occur in.
+  // This is important because we have to handle multiple defs/uses in a block
+  // ourselves: SSAUpdater is purely for cross-block references.
+  // FIXME: Want a TinyVector<Instruction*> since there is often 0/1 element.
+  DenseMap<BasicBlock*, std::vector<Instruction*> > UsesByBlock;
+  
+  for (unsigned i = 0, e = Insts.size(); i != e; ++i) {
+    Instruction *User = Insts[i];
+    UsesByBlock[User->getParent()].push_back(User);
+  }
+  
+  // Okay, now we can iterate over all the blocks in the function with uses,
+  // processing them.  Keep track of which loads are loading a live-in value.
+  // Walk the uses in the use-list order to be determinstic.
+  SmallVector<LoadInst*, 32> LiveInLoads;
+  DenseMap<Value*, Value*> ReplacedLoads;
+  
+  for (unsigned i = 0, e = Insts.size(); i != e; ++i) {
+    Instruction *User = Insts[i];
+    BasicBlock *BB = User->getParent();
+    std::vector<Instruction*> &BlockUses = UsesByBlock[BB];
+    
+    // If this block has already been processed, ignore this repeat use.
+    if (BlockUses.empty()) continue;
+    
+    // Okay, this is the first use in the block.  If this block just has a
+    // single user in it, we can rewrite it trivially.
+    if (BlockUses.size() == 1) {
+      // If it is a store, it is a trivial def of the value in the block.
+      if (StoreInst *SI = dyn_cast<StoreInst>(User))
+        SSA->AddAvailableValue(BB, SI->getOperand(0));
+      else 
+        // Otherwise it is a load, queue it to rewrite as a live-in load.
+        LiveInLoads.push_back(cast<LoadInst>(User));
+      BlockUses.clear();
+      continue;
+    }
+    
+    // Otherwise, check to see if this block is all loads.
+    bool HasStore = false;
+    for (unsigned i = 0, e = BlockUses.size(); i != e; ++i) {
+      if (isa<StoreInst>(BlockUses[i])) {
+        HasStore = true;
+        break;
+      }
+    }
+    
+    // If so, we can queue them all as live in loads.  We don't have an
+    // efficient way to tell which on is first in the block and don't want to
+    // scan large blocks, so just add all loads as live ins.
+    if (!HasStore) {
+      for (unsigned i = 0, e = BlockUses.size(); i != e; ++i)
+        LiveInLoads.push_back(cast<LoadInst>(BlockUses[i]));
+      BlockUses.clear();
+      continue;
+    }
+    
+    // Otherwise, we have mixed loads and stores (or just a bunch of stores).
+    // Since SSAUpdater is purely for cross-block values, we need to determine
+    // the order of these instructions in the block.  If the first use in the
+    // block is a load, then it uses the live in value.  The last store defines
+    // the live out value.  We handle this by doing a linear scan of the block.
+    Value *StoredValue = 0;
+    for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E; ++II) {
+      if (LoadInst *L = dyn_cast<LoadInst>(II)) {
+        // If this is a load from an unrelated pointer, ignore it.
+        if (!isInstInList(L, Insts)) continue;
+        
+        // If we haven't seen a store yet, this is a live in use, otherwise
+        // use the stored value.
+        if (StoredValue) {
+          L->replaceAllUsesWith(StoredValue);
+          ReplacedLoads[L] = StoredValue;
+        } else {
+          LiveInLoads.push_back(L);
+        }
+        continue;
+      }
+      
+      if (StoreInst *S = dyn_cast<StoreInst>(II)) {
+        // If this is a store to an unrelated pointer, ignore it.
+        if (!isInstInList(S, Insts)) continue;
+        
+        // Remember that this is the active value in the block.
+        StoredValue = S->getOperand(0);
+      }
+    }
+    
+    // The last stored value that happened is the live-out for the block.
+    assert(StoredValue && "Already checked that there is a store in block");
+    SSA->AddAvailableValue(BB, StoredValue);
+    BlockUses.clear();
+  }
+  
+  // Okay, now we rewrite all loads that use live-in values in the loop,
+  // inserting PHI nodes as necessary.
+  for (unsigned i = 0, e = LiveInLoads.size(); i != e; ++i) {
+    LoadInst *ALoad = LiveInLoads[i];
+    Value *NewVal = SSA->GetValueInMiddleOfBlock(ALoad->getParent());
+    ALoad->replaceAllUsesWith(NewVal);
+    ReplacedLoads[ALoad] = NewVal;
+  }
+  
+  // Now that everything is rewritten, delete the old instructions from the
+  // function.  They should all be dead now.
+  for (unsigned i = 0, e = Insts.size(); i != e; ++i) {
+    Instruction *User = Insts[i];
+    
+    // If this is a load that still has uses, then the load must have been added
+    // as a live value in the SSAUpdate data structure for a block (e.g. because
+    // the loaded value was stored later).  In this case, we need to recursively
+    // propagate the updates until we get to the real value.
+    if (!User->use_empty()) {
+      Value *NewVal = ReplacedLoads[User];
+      assert(NewVal && "not a replaced load?");
+      
+      // Propagate down to the ultimate replacee.  The intermediately loads
+      // could theoretically already have been deleted, so we don't want to
+      // dereference the Value*'s.
+      DenseMap<Value*, Value*>::iterator RLI = ReplacedLoads.find(NewVal);
+      while (RLI != ReplacedLoads.end()) {
+        NewVal = RLI->second;
+        RLI = ReplacedLoads.find(NewVal);
+      }
+      
+      User->replaceAllUsesWith(NewVal);
+    }
+    
+    User->eraseFromParent();
+  }
+}