Rewrite all the 'PromoteLocallyUsedAlloca[s]' logic. With the power of

LargeBlockInfo, we can now dramatically simplify their implementation
and speed them up at the same time.  Now the code has time proportional
to the number of uses of the alloca, not the size of the block.

This also eliminates code that tried to batch up different allocas which
are used in the same blocks, and eliminates the 'retry list' logic which
was baroque and no unneccesary.  In addition to being a speedup for crazy
cases, this is also a nice cleanup:

PromoteMemoryToRegister.cpp |  270 +++++++++++++++-----------------------------
 1 file changed, 96 insertions(+), 174 deletions(-)



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@58229 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Transforms/Utils/PromoteMemoryToRegister.cpp

@@ -163,7 +163,6 @@ namespace {
     /// Allocas - The alloca instructions being promoted.
     ///
     std::vector<AllocaInst*> Allocas;
-    SmallVector<AllocaInst*, 16> &RetryList;
     DominatorTree &DT;
     DominanceFrontier &DF;
 
@@ -200,10 +199,9 @@ namespace {
     /// BBNumPreds - Lazily compute the number of predecessors a block has.
     DenseMap<const BasicBlock*, unsigned> BBNumPreds;
   public:
-    PromoteMem2Reg(const std::vector<AllocaInst*> &A,
+    PromoteMem2Reg(const std::vector<AllocaInst*> &A, DominatorTree &dt,
-                   SmallVector<AllocaInst*, 16> &Retry, DominatorTree &dt,
                    DominanceFrontier &df, AliasSetTracker *ast)
-      : Allocas(A), RetryList(Retry), DT(dt), DF(df), AST(ast) {}
+      : Allocas(A), DT(dt), DF(df), AST(ast) {}
 
     void run();
 
@@ -243,12 +241,9 @@ namespace {
     
     void RewriteSingleStoreAlloca(AllocaInst *AI, AllocaInfo &Info,
                                   LargeBlockInfo &LBI);
+    void PromoteSingleBlockAlloca(AllocaInst *AI, AllocaInfo &Info,
+                                  LargeBlockInfo &LBI);
 
-    bool PromoteLocallyUsedAlloca(BasicBlock *BB, AllocaInst *AI,
-                                  LargeBlockInfo &LBI);
-    void PromoteLocallyUsedAllocas(BasicBlock *BB,
-                                   const std::vector<AllocaInst*> &AIs,
-                                   LargeBlockInfo &LBI);
     
     void RenamePass(BasicBlock *BB, BasicBlock *Pred,
                     RenamePassData::ValVector &IncVals,
@@ -315,13 +310,6 @@ namespace {
 void PromoteMem2Reg::run() {
   Function &F = *DF.getRoot()->getParent();
 
-  // LocallyUsedAllocas - Keep track of all of the alloca instructions which are
-  // only used in a single basic block.  These instructions can be efficiently
-  // promoted by performing a single linear scan over that one block.  Since
-  // individual basic blocks are sometimes large, we group together all allocas
-  // that are live in a single basic block by the basic block they are live in.
-  std::map<BasicBlock*, std::vector<AllocaInst*> > LocallyUsedAllocas;
-
   if (AST) PointerAllocaValues.resize(Allocas.size());
 
   AllocaInfo Info;
@@ -376,12 +364,30 @@ void PromoteMem2Reg::run() {
     // If the alloca is only read and written in one basic block, just perform a
     // linear sweep over the block to eliminate it.
     if (Info.OnlyUsedInOneBlock) {
-      LocallyUsedAllocas[Info.OnlyBlock].push_back(AI);
+      PromoteSingleBlockAlloca(AI, Info, LBI);
       
-      // Remove the alloca from the Allocas list, since it will be processed.
+      // Finally, after the scan, check to see if the stores are all that is
+      // left.
+      if (Info.UsingBlocks.empty()) {
+        
+        // Remove the (now dead) stores and alloca.
+        while (!AI->use_empty()) {
+          StoreInst *SI = cast<StoreInst>(AI->use_back());
+          SI->eraseFromParent();
+          LBI.deleteValue(SI);
+        }
+        
+        if (AST) AST->deleteValue(AI);
+        AI->eraseFromParent();
+        LBI.deleteValue(AI);
+        
+        // The alloca has been processed, move on.
         RemoveFromAllocasList(AllocaNum);
+        
+        ++NumLocalPromoted;
         continue;
       }
+    }
     
     // If we haven't computed a numbering for the BB's in the function, do so
     // now.
@@ -406,26 +412,6 @@ void PromoteMem2Reg::run() {
     DetermineInsertionPoint(AI, AllocaNum, Info);
   }
 
-  // Process all allocas which are only used in a single basic block.
-  for (std::map<BasicBlock*, std::vector<AllocaInst*> >::iterator I =
-         LocallyUsedAllocas.begin(), E = LocallyUsedAllocas.end(); I != E; ++I){
-    const std::vector<AllocaInst*> &LocAllocas = I->second;
-    assert(!LocAllocas.empty() && "empty alloca list??");
-
-    // It's common for there to only be one alloca in the list.  Handle it
-    // efficiently.
-    if (LocAllocas.size() == 1) {
-      // If we can do the quick promotion pass, do so now.
-      if (PromoteLocallyUsedAlloca(I->first, LocAllocas[0], LBI))
-        RetryList.push_back(LocAllocas[0]);  // Failed, retry later.
-    } else {
-      // Locally promote anything possible.  Note that if this is unable to
-      // promote a particular alloca, it puts the alloca onto the Allocas vector
-      // for global processing.
-      PromoteLocallyUsedAllocas(I->first, LocAllocas, LBI);
-    }
-  }
-
   if (Allocas.empty())
     return; // All of the allocas must have been trivial!
 
@@ -752,7 +738,16 @@ void PromoteMem2Reg::RewriteSingleStoreAlloca(AllocaInst *AI,
 }
 
 
-/// PromoteLocallyUsedAlloca - Many allocas are only used within a single basic
+/// StoreIndexSearchPredicate - This is a helper predicate used to search by the
+/// first element of a pair.
+struct StoreIndexSearchPredicate {
+  bool operator()(const std::pair<unsigned, StoreInst*> &LHS,
+                  const std::pair<unsigned, StoreInst*> &RHS) {
+    return LHS.first < RHS.first;
+  }
+};
+
+/// PromoteSingleBlockAlloca - Many allocas are only used within a single basic
 /// block.  If this is the case, avoid traversing the CFG and inserting a lot of
 /// potentially useless PHI nodes by just performing a single linear pass over
 /// the basic block using the Alloca.
@@ -766,124 +761,72 @@ void PromoteMem2Reg::RewriteSingleStoreAlloca(AllocaInst *AI,
 ///
 /// ... so long as A is not used before undef is set.
 ///
-bool PromoteMem2Reg::PromoteLocallyUsedAlloca(BasicBlock *BB, AllocaInst *AI,
+void PromoteMem2Reg::PromoteSingleBlockAlloca(AllocaInst *AI, AllocaInfo &Info,
                                               LargeBlockInfo &LBI) {
-  assert(!AI->use_empty() && "There are no uses of the alloca!");
+  // The trickiest case to handle is when we have large blocks. Because of this,
+  // this code is optimized assuming that large blocks happen.  This does not
+  // significantly pessimize the small block case.  This uses LargeBlockInfo to
+  // make it efficient to get the index of various operations in the block.
   
-  // Handle degenerate cases quickly.
+  // Clear out UsingBlocks.  We will reconstruct it here if needed.
-  if (AI->hasOneUse()) {
+  Info.UsingBlocks.clear();
-    Instruction *U = cast<Instruction>(AI->use_back());
+  
-    if (LoadInst *LI = dyn_cast<LoadInst>(U)) {
+  // Walk the use-def list of the alloca, getting the locations of all stores.
-      // Must be a load of uninitialized value.
+  typedef SmallVector<std::pair<unsigned, StoreInst*>, 64> StoresByIndexTy;
-      LI->replaceAllUsesWith(UndefValue::get(AI->getAllocatedType()));
+  StoresByIndexTy StoresByIndex;
+  
+  for (Value::use_iterator UI = AI->use_begin(), E = AI->use_end();
+       UI != E; ++UI) 
+    if (StoreInst *SI = dyn_cast<StoreInst>(*UI))
+      StoresByIndex.push_back(std::make_pair(LBI.getInstructionIndex(SI), SI));
+
+  // If there are no stores to the alloca, just replace any loads with undef.
+  if (StoresByIndex.empty()) {
+    for (Value::use_iterator UI = AI->use_begin(), E = AI->use_end(); UI != E;) 
+      if (LoadInst *LI = dyn_cast<LoadInst>(*UI++)) {
+        LI->replaceAllUsesWith(UndefValue::get(LI->getType()));
         if (AST && isa<PointerType>(LI->getType()))
           AST->deleteValue(LI);
-    } else {
-      // Otherwise it must be a store which is never read.
-      assert(isa<StoreInst>(U));
-    }
-    LBI.deleteValue(U);
-    BB->getInstList().erase(U);
-  } else {
-    // Uses of the uninitialized memory location shall get undef.
-    Value *CurVal = 0;
-
-    for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ) {
-      Instruction *Inst = I++;
-      if (LoadInst *LI = dyn_cast<LoadInst>(Inst)) {
-        if (LI->getOperand(0) == AI) {
-          if (!CurVal) return true;  // Could not locally promote!
-
-          // Loads just returns the "current value"...
-          LI->replaceAllUsesWith(CurVal);
-          if (AST && isa<PointerType>(LI->getType()))
-            AST->deleteValue(LI);
-          BB->getInstList().erase(LI);
         LBI.deleteValue(LI);
+        LI->eraseFromParent();
       }
-      } else if (StoreInst *SI = dyn_cast<StoreInst>(Inst)) {
+    return;
-        if (SI->getOperand(1) == AI) {
-          // Store updates the "current value"...
-          CurVal = SI->getOperand(0);
-          BB->getInstList().erase(SI);
-          LBI.deleteValue(SI);
-        }
-      }
-    }
   }
   
-  // After traversing the basic block, there should be no more uses of the
+  // Sort the stores by their index, making it efficient to do a lookup with a
-  // alloca: remove it now.
+  // binary search.
-  assert(AI->use_empty() && "Uses of alloca from more than one BB??");
+  std::sort(StoresByIndex.begin(), StoresByIndex.end());
-  if (AST) AST->deleteValue(AI);
-  AI->eraseFromParent();
-  LBI.deleteValue(AI);
   
-  ++NumLocalPromoted;
+  // Walk all of the loads from this alloca, replacing them with the nearest
-  return false;
+  // store above them, if any.
+  for (Value::use_iterator UI = AI->use_begin(), E = AI->use_end(); UI != E;) {
+    LoadInst *LI = dyn_cast<LoadInst>(*UI++);
+    if (!LI) continue;
+    
+    unsigned LoadIdx = LBI.getInstructionIndex(LI);
+    
+    // Find the nearest store that has a lower than this load. 
+    StoresByIndexTy::iterator I = 
+      std::lower_bound(StoresByIndex.begin(), StoresByIndex.end(),
+                       std::pair<unsigned, StoreInst*>(LoadIdx, 0),
+                       StoreIndexSearchPredicate());
+    
+    // If there is no store before this load, then we can't promote this load.
+    if (I == StoresByIndex.begin()) {
+      // Can't handle this load, bail out.
+      Info.UsingBlocks.push_back(LI->getParent());
+      continue;
     }
       
-/// PromoteLocallyUsedAllocas - This method is just like
+    // Otherwise, there was a store before this load, the load takes its value.
-/// PromoteLocallyUsedAlloca, except that it processes multiple alloca
+    --I;
-/// instructions in parallel.  This is important in cases where we have large
+    LI->replaceAllUsesWith(I->second->getOperand(0));
-/// basic blocks, as we don't want to rescan the entire basic block for each
-/// alloca which is locally used in it (which might be a lot).
-void PromoteMem2Reg::
-PromoteLocallyUsedAllocas(BasicBlock *BB, const std::vector<AllocaInst*> &AIs,
-                          LargeBlockInfo &LBI) {
-  DenseMap<AllocaInst*, Value*> CurValues;
-  for (unsigned i = 0, e = AIs.size(); i != e; ++i)
-    CurValues[AIs[i]] = 0; // Insert with null value
-
-  for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ) {
-    Instruction *Inst = I++;
-    if (LoadInst *LI = dyn_cast<LoadInst>(Inst)) {
-      // Is this a load of an alloca we are tracking?
-      if (AllocaInst *AI = dyn_cast<AllocaInst>(LI->getOperand(0))) {
-        DenseMap<AllocaInst*, Value*>::iterator AIt = CurValues.find(AI);
-        if (AIt != CurValues.end()) {
-          // If loading an uninitialized value, allow the inter-block case to
-          // handle it.  Due to control flow, this might actually be ok.
-          if (AIt->second == 0) {  // Use of locally uninitialized value??
-            RetryList.push_back(AI);   // Retry elsewhere.
-            CurValues.erase(AIt);   // Stop tracking this here.
-            if (CurValues.empty()) return;
-          } else {
-            // Loads just returns the "current value"...
-            LI->replaceAllUsesWith(AIt->second);
     if (AST && isa<PointerType>(LI->getType()))
       AST->deleteValue(LI);
-            BB->getInstList().erase(LI);
+    LI->eraseFromParent();
     LBI.deleteValue(LI);
   }
 }
-      }
-    } else if (StoreInst *SI = dyn_cast<StoreInst>(Inst)) {
-      if (AllocaInst *AI = dyn_cast<AllocaInst>(SI->getOperand(1))) {
-        DenseMap<AllocaInst*, Value*>::iterator AIt = CurValues.find(AI);
-        if (AIt != CurValues.end()) {
-          // Store updates the "current value"...
-          AIt->second = SI->getOperand(0);
-          SI->eraseFromParent();
-          LBI.deleteValue(SI);
-        }
-      }
-    }
-  }
-  
-  // At the end of the block scan, all allocas in CurValues are dead.
-  for (DenseMap<AllocaInst*, Value*>::iterator I = CurValues.begin(),
-       E = CurValues.end(); I != E; ++I) {
-    AllocaInst *AI = I->first;
-    assert(AI->use_empty() && "Uses of alloca from more than one BB??");
-    if (AST) AST->deleteValue(AI);
-    AI->eraseFromParent();
-    LBI.deleteValue(AI);
-  }
-
-  NumLocalPromoted += CurValues.size();
-}
-
 
 
 // QueuePhiNode - queues a phi-node to be added to a basic-block for a specific
@@ -1044,26 +987,5 @@ void llvm::PromoteMemToReg(const std::vector<AllocaInst*> &Allocas,
   // If there is nothing to do, bail out...
   if (Allocas.empty()) return;
 
-  SmallVector<AllocaInst*, 16> RetryList;
+  PromoteMem2Reg(Allocas, DT, DF, AST).run();
-  PromoteMem2Reg(Allocas, RetryList, DT, DF, AST).run();
-
-  // PromoteMem2Reg may not have been able to promote all of the allocas in one
-  // pass, run it again if needed.
-  std::vector<AllocaInst*> NewAllocas;
-  while (!RetryList.empty()) {
-    // If we need to retry some allocas, this is due to there being no store
-    // before a read in a local block.  To counteract this, insert a store of
-    // undef into the alloca right after the alloca itself.
-    for (unsigned i = 0, e = RetryList.size(); i != e; ++i) {
-      BasicBlock::iterator BBI = RetryList[i];
-
-      new StoreInst(UndefValue::get(RetryList[i]->getAllocatedType()),
-                    RetryList[i], ++BBI);
-    }
-
-    NewAllocas.assign(RetryList.begin(), RetryList.end());
-    RetryList.clear();
-    PromoteMem2Reg(NewAllocas, RetryList, DT, DF, AST).run();
-    NewAllocas.clear();
-  }
 }