finish the first cut of a jump threading pass implementation.

llvm-svn: 50006

lib/Transforms/Scalar/JumpThreading.cpp

@@ -15,13 +15,16 @@
 #include "llvm/Transforms/Scalar.h"
 #include "llvm/IntrinsicInst.h"
 #include "llvm/Pass.h"
+#include "llvm/ADT/DenseMap.h"
 #include "llvm/ADT/Statistic.h"
+#include "llvm/Transforms/Utils/Local.h"
 #include "llvm/Support/CommandLine.h"
 #include "llvm/Support/Compiler.h"
 #include "llvm/Support/Debug.h"
 using namespace llvm;
 
-//STATISTIC(NumThreads, "Number of jumps threaded");
+STATISTIC(NumThreads, "Number of jumps threaded");
+STATISTIC(NumFolds,   "Number of terminators folded");
 
 static cl::opt<unsigned>
 Threshold("jump-threading-threshold", 
@@ -51,7 +54,8 @@ namespace {
     JumpThreading() : FunctionPass((intptr_t)&ID) {}
 
     bool runOnFunction(Function &F);
-    bool ThreadBlock(BasicBlock &BB);
+    bool ThreadBlock(BasicBlock *BB);
+    void ThreadEdge(BasicBlock *BB, BasicBlock *PredBB, BasicBlock *SuccBB);
   };
   char JumpThreading::ID = 0;
   RegisterPass<JumpThreading> X("jump-threading", "Jump Threading");
@@ -64,16 +68,24 @@ FunctionPass *llvm::createJumpThreadingPass() { return new JumpThreading(); }
 ///
 bool JumpThreading::runOnFunction(Function &F) {
   DOUT << "Jump threading on function '" << F.getNameStart() << "'\n";
-  bool Changed = false;
-  for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I)
-    Changed |= ThreadBlock(*I);
-  return Changed;
+  
+  bool AnotherIteration = true, EverChanged = false;
+  while (AnotherIteration) {
+    AnotherIteration = false;
+    bool Changed = false;
+    for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I)
+      while (ThreadBlock(I))
+        Changed = true;
+    AnotherIteration = Changed;
+    EverChanged |= Changed;
+  }
+  return EverChanged;
 }
 
 /// getJumpThreadDuplicationCost - Return the cost of duplicating this block to
 /// thread across it.
-static unsigned getJumpThreadDuplicationCost(const BasicBlock &BB) {
-  BasicBlock::const_iterator I = BB.begin();
+static unsigned getJumpThreadDuplicationCost(const BasicBlock *BB) {
+  BasicBlock::const_iterator I = BB->begin();
   /// Ignore PHI nodes, these will be flattened when duplication happens.
   while (isa<PHINode>(*I)) ++I;
 
@@ -114,31 +126,45 @@ static unsigned getJumpThreadDuplicationCost(const BasicBlock &BB) {
 
 /// ThreadBlock - If there are any predecessors whose control can be threaded
 /// through to a successor, transform them now.
-bool JumpThreading::ThreadBlock(BasicBlock &BB) {
-  // If there is only one predecessor or successor, then there is nothing to do.
-  if (BB.getTerminator()->getNumSuccessors() == 1 || BB.getSinglePredecessor())
-    return false;
-  
+bool JumpThreading::ThreadBlock(BasicBlock *BB) {
   // See if this block ends with a branch of switch.  If so, see if the
   // condition is a phi node.  If so, and if an entry of the phi node is a
   // constant, we can thread the block.
   Value *Condition;
-  if (BranchInst *BI = dyn_cast<BranchInst>(BB.getTerminator()))
+  if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator())) {
+    // Can't thread an unconditional jump.
+    if (BI->isUnconditional()) return false;
     Condition = BI->getCondition();
-  else if (SwitchInst *SI = dyn_cast<SwitchInst>(BB.getTerminator()))
+  } else if (SwitchInst *SI = dyn_cast<SwitchInst>(BB->getTerminator()))
     Condition = SI->getCondition();
   else
     return false; // Must be an invoke.
+  
+  // If the terminator of this block is branching on a constant, simplify the
+  // terminator to an unconditional branch.  This can occur do to threading in
+  // other blocks.
+  if (isa<ConstantInt>(Condition)) {
+    DOUT << "  In block '" << BB->getNameStart()
+         << "' folding terminator: " << *BB->getTerminator();
+    ++NumFolds;
+    ConstantFoldTerminator(BB);
+    return true;
+  }
+  
+  // If there is only a single predecessor of this block, nothing to fold.
+  if (BB->getSinglePredecessor())
+    return false;
 
   // See if this is a phi node in the current block.
   PHINode *PN = dyn_cast<PHINode>(Condition);
-  if (!PN || PN->getParent() != &BB) return false;
+  if (!PN || PN->getParent() != BB) return false;
   
   // See if the phi node has any constant values.  If so, we can determine where
   // the corresponding predecessor will branch.
   unsigned PredNo = ~0U;
+  ConstantInt *PredCst = 0;
   for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
-    if (isa<ConstantInt>(PN->getIncomingValue(i))) {
+    if ((PredCst = dyn_cast<ConstantInt>(PN->getIncomingValue(i)))) {
       PredNo = i;
       break;
     }
@@ -152,13 +178,107 @@ bool JumpThreading::ThreadBlock(BasicBlock &BB) {
   // See if the cost of duplicating this block is low enough.
   unsigned JumpThreadCost = getJumpThreadDuplicationCost(BB);
   if (JumpThreadCost > Threshold) {
-    DOUT << "  Not threading BB '" << BB.getNameStart()
+    DOUT << "  Not threading BB '" << BB->getNameStart()
          << "' - Cost is too high: " << JumpThreadCost << "\n";
     return false;
   }
-
-  DOUT << "  Threading BB '" << BB.getNameStart() << "'.  Cost is: "
-       << JumpThreadCost << "\n";
   
-  return false;
+  // If so, we can actually do this threading.  Figure out which predecessor and
+  // which successor we are threading for.
+  BasicBlock *PredBB = PN->getIncomingBlock(PredNo);
+  BasicBlock *SuccBB;
+  if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator()))
+    SuccBB = BI->getSuccessor(PredCst == ConstantInt::getFalse());
+  else {
+    SwitchInst *SI = cast<SwitchInst>(BB->getTerminator());
+    SuccBB = SI->getSuccessor(SI->findCaseValue(PredCst));
+  }
+  
+  // TODO: If there are multiple preds with the same incoming value for the PHI,
+  // factor them together so we get one thread block for the whole group.  This
+  // is important for things like "phi i1 [true, true, false, true, x]" where
+  // we only need to clone the block for the true blocks once.
+  
+  DOUT << "  Threading edge from '" << PredBB->getNameStart() << "' to '"
+       << SuccBB->getNameStart() << "' with cost: " << JumpThreadCost
+       << ", across block:\n    "
+       << *BB;
+       
+  ThreadEdge(BB, PredBB, SuccBB);
+  ++NumThreads;
+  return true;
+}
+
+/// ThreadEdge - We have decided that it is safe and profitable to thread an
+/// edge from PredBB to SuccBB across BB.  Transform the IR to reflect this
+/// change.
+void JumpThreading::ThreadEdge(BasicBlock *BB, BasicBlock *PredBB, 
+                               BasicBlock *SuccBB) {
+
+  // Jump Threading can not update SSA properties correctly if the values
+  // defined in the duplicated block are used outside of the block itself.  For
+  // this reason, we spill all values that are used outside of BB to the stack.
+  for (BasicBlock::iterator I = BB->begin(); I != BB->end(); ++I)
+    if (I->isUsedOutsideOfBlock(BB)) {
+      // We found a use of I outside of BB.  Create a new stack slot to
+      // break this inter-block usage pattern.
+      DemoteRegToStack(*I);
+    }
+ 
+  // We are going to have to map operands from the original BB block to the new
+  // copy of the block 'NewBB'.  If there are PHI nodes in BB, evaluate them to
+  // account for entry from PredBB.
+  DenseMap<Instruction*, Value*> ValueMapping;
+  
+  BasicBlock *NewBB =
+    BasicBlock::Create(BB->getName()+".thread", BB->getParent(), BB);
+  NewBB->moveAfter(PredBB);
+  
+  BasicBlock::iterator BI = BB->begin();
+  for (; PHINode *PN = dyn_cast<PHINode>(BI); ++BI)
+    ValueMapping[PN] = PN->getIncomingValueForBlock(PredBB);
+  
+  // Clone the non-phi instructions of BB into NewBB, keeping track of the
+  // mapping and using it to remap operands in the cloned instructions.
+  for (; !isa<TerminatorInst>(BI); ++BI) {
+    Instruction *New = BI->clone();
+    New->setName(BI->getNameStart());
+    NewBB->getInstList().push_back(New);
+    ValueMapping[BI] = New;
+   
+    // Remap operands to patch up intra-block references.
+    for (unsigned i = 0, e = New->getNumOperands(); i != e; ++i)
+      if (Instruction *Inst = dyn_cast<Instruction>(New->getOperand(i)))
+        if (Value *Remapped = ValueMapping[Inst])
+          New->setOperand(i, Remapped);
+  }
+  
+  // We didn't copy the terminator from BB over to NewBB, because there is now
+  // an unconditional jump to SuccBB.  Insert the unconditional jump.
+  BranchInst::Create(SuccBB, NewBB);
+  
+  // Check to see if SuccBB has PHI nodes. If so, we need to add entries to the
+  // PHI nodes for NewBB now.
+  for (BasicBlock::iterator PNI = SuccBB->begin(); isa<PHINode>(PNI); ++PNI) {
+    PHINode *PN = cast<PHINode>(PNI);
+    // Ok, we have a PHI node.  Figure out what the incoming value was for the
+    // DestBlock.
+    Value *IV = PN->getIncomingValueForBlock(BB);
+    
+    // Remap the value if necessary.
+    if (Instruction *Inst = dyn_cast<Instruction>(IV))
+      if (Value *MappedIV = ValueMapping[Inst])
+        IV = MappedIV;
+    PN->addIncoming(IV, NewBB);
+  }
+  
+  // Finally, NewBB is good to go.  Update the terminator of PredBB to jump to
+  // NewBB instead of BB.  This eliminates predecessors from BB, which requires
+  // us to simplify any PHI nodes in BB.
+  TerminatorInst *PredTerm = PredBB->getTerminator();
+  for (unsigned i = 0, e = PredTerm->getNumSuccessors(); i != e; ++i)
+    if (PredTerm->getSuccessor(i) == BB) {
+      BB->removePredecessor(PredBB);
+      PredTerm->setSuccessor(i, NewBB);
+    }
 }