Implement an efficient and fully general SSA update mechanism that

works on unstructured CFGs.  This implements PR217, our oldest open PR.

llvm-svn: 83705

include/llvm/Transforms/Utils/SSAUpdater.h

@@ -0,0 +1,71 @@
+//===-- SSAUpdater.h - Unstructured SSA Update Tool -------------*- C++ -*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This file declares the SSAUpdater class.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_TRANSFORMS_UTILS_SSAUPDATER_H
+#define LLVM_TRANSFORMS_UTILS_SSAUPDATER_H
+
+namespace llvm {
+  class Value;
+  class BasicBlock;
+  class Use;
+  
+/// SSAUpdater - This class updates SSA form for a set of values defined in
+/// multiple blocks.  This is used when code duplication or another unstructured
+/// transformation wants to rewrite a set of uses of one value with uses of a
+/// set of values.
+class SSAUpdater {
+  /// AvailableVals - This keeps track of which value to use on a per-block
+  /// basis.  When we insert PHI nodes, we keep track of them here.  We use
+  /// WeakVH's for the value of the map because we RAUW PHI nodes when we
+  /// eliminate them, and want the WeakVH to track this.
+  //typedef DenseMap<BasicBlock*, TrackingVH<Value> > AvailableValsTy;
+  void *AV;
+  
+  /// PrototypeValue is an arbitrary representative value, which we derive names
+  /// and a type for PHI nodes.
+  Value *PrototypeValue;
+  
+  /// IncomingPredInfo - We use this as scratch space when doing our recursive
+  /// walk.  This should only be used in GetValueInBlockInternal, normally it
+  /// should be empty.
+  //std::vector<std::pair<BasicBlock*, TrackingVH<Value> > > IncomingPredInfo;
+  void *IPI;
+public:
+  SSAUpdater();
+  ~SSAUpdater();
+  
+  /// Initialize - Reset this object to get ready for a new set of SSA
+  /// updates.  ProtoValue is the value used to name PHI nodes.
+  void Initialize(Value *ProtoValue);
+  
+  /// AddAvailableValue - Indicate that a rewritten value is available in the
+  /// specified block with the specified value.
+  void AddAvailableValue(BasicBlock *BB, Value *V);
+  
+  /// GetValueInBlock - Construct SSA form, materializing a value in the
+  /// specified block.
+  Value *GetValueInBlock(BasicBlock *BB);
+  
+  /// RewriteUse - Rewrite a use of the symbolic value.  This handles PHI nodes,
+  /// which use their value in the corresponding predecessor.
+  void RewriteUse(Use &U);
+  
+private:
+  Value *GetValueInBlockInternal(BasicBlock *BB);
+  void operator=(const SSAUpdater&); // DO NOT IMPLEMENT
+  SSAUpdater(const SSAUpdater&);     // DO NOT IMPLEMENT
+};
+
+} // End llvm namespace
+
+#endif

lib/Transforms/Utils/CMakeLists.txt

@@ -19,6 +19,7 @@ add_llvm_library(LLVMTransformUtils
   LowerSwitch.cpp
   Mem2Reg.cpp
   PromoteMemoryToRegister.cpp
+  SSAUpdater.cpp
   SSI.cpp
   SimplifyCFG.cpp
   UnifyFunctionExitNodes.cpp

lib/Transforms/Utils/SSAUpdater.cpp

@@ -0,0 +1,232 @@
+//===- SSAUpdater.cpp - Unstructured SSA Update Tool ----------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This file implements the SSAUpdater class.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Transforms/Utils/SSAUpdater.h"
+#include "llvm/Instructions.h"
+#include "llvm/ADT/DenseMap.h"
+#include "llvm/Support/CFG.h"
+#include "llvm/Support/Debug.h"
+#include "llvm/Support/ValueHandle.h"
+#include "llvm/Support/raw_ostream.h"
+using namespace llvm;
+
+typedef DenseMap<BasicBlock*, TrackingVH<Value> > AvailableValsTy;
+typedef std::vector<std::pair<BasicBlock*, TrackingVH<Value> > >
+                IncomingPredInfoTy;
+
+static AvailableValsTy &getAvailableVals(void *AV) {
+  return *static_cast<AvailableValsTy*>(AV);
+}
+
+static IncomingPredInfoTy &getIncomingPredInfo(void *IPI) {
+  return *static_cast<IncomingPredInfoTy*>(IPI);
+}
+
+
+SSAUpdater::SSAUpdater() : AV(0), PrototypeValue(0), IPI(0) {}
+
+SSAUpdater::~SSAUpdater() {
+  delete &getAvailableVals(AV);
+  delete &getIncomingPredInfo(IPI);
+}
+
+/// Initialize - Reset this object to get ready for a new set of SSA
+/// updates.  ProtoValue is the value used to name PHI nodes.
+void SSAUpdater::Initialize(Value *ProtoValue) {
+  if (AV == 0)
+    AV = new AvailableValsTy();
+  else
+    getAvailableVals(AV).clear();
+  
+  if (IPI == 0)
+    IPI = new IncomingPredInfoTy();
+  else
+    getIncomingPredInfo(IPI).clear();
+  PrototypeValue = ProtoValue;
+}
+
+/// AddAvailableValue - Indicate that a rewritten value is available in the
+/// specified block with the specified value.
+void SSAUpdater::AddAvailableValue(BasicBlock *BB, Value *V) {
+  assert(PrototypeValue != 0 && "Need to initialize SSAUpdater");
+  assert(PrototypeValue->getType() == V->getType() &&
+         "All rewritten values must have the same type");
+  getAvailableVals(AV)[BB] = V;
+}
+
+/// GetValueInBlock - Construct SSA form, materializing a value in the
+/// specified block.
+Value *SSAUpdater::GetValueInBlock(BasicBlock *BB) {
+  assert(getIncomingPredInfo(IPI).empty() && "Unexpected Internal State");
+  Value *Res = GetValueInBlockInternal(BB);
+  assert(getIncomingPredInfo(IPI).empty() && "Unexpected Internal State");
+  return Res;
+}
+
+/// RewriteUse - Rewrite a use of the symbolic value.  This handles PHI nodes,
+/// which use their value in the corresponding predecessor.
+void SSAUpdater::RewriteUse(Use &U) {
+  Instruction *User = cast<Instruction>(U.getUser());
+  BasicBlock *UseBB = User->getParent();
+  if (PHINode *UserPN = dyn_cast<PHINode>(User))
+    UseBB = UserPN->getIncomingBlock(U);
+  
+  U.set(GetValueInBlock(UseBB));
+}
+
+
+/// GetValueInBlock - Check to see if AvailableVals has an entry for the
+/// specified BB and if so, return it.  If not, construct SSA form by walking
+/// predecessors inserting PHI nodes as needed until we get to a block where the
+/// value is available.
+///
+Value *SSAUpdater::GetValueInBlockInternal(BasicBlock *BB) {
+  AvailableValsTy &AvailableVals = getAvailableVals(AV);
+  
+  // Query AvailableVals by doing an insertion of null.
+  std::pair<AvailableValsTy::iterator, bool> InsertRes =
+  AvailableVals.insert(std::make_pair(BB, WeakVH()));
+  
+  // Handle the case when the insertion fails because we have already seen BB.
+  if (!InsertRes.second) {
+    // If the insertion failed, there are two cases.  The first case is that the
+    // value is already available for the specified block.  If we get this, just
+    // return the value.
+    if (InsertRes.first->second != 0)
+      return InsertRes.first->second;
+    
+    // Otherwise, if the value we find is null, then this is the value is not
+    // known but it is being computed elsewhere in our recursion.  This means
+    // that we have a cycle.  Handle this by inserting a PHI node and returning
+    // it.  When we get back to the first instance of the recursion we will fill
+    // in the PHI node.
+    return InsertRes.first->second =
+    PHINode::Create(PrototypeValue->getType(), PrototypeValue->getName(),
+                    &BB->front());
+  }
+  
+  // Okay, the value isn't in the map and we just inserted a null in the entry
+  // to indicate that we're processing the block.  Since we have no idea what
+  // value is in this block, we have to recurse through our predecessors.
+  //
+  // While we're walking our predecessors, we keep track of them in a vector,
+  // then insert a PHI node in the end if we actually need one.  We could use a
+  // smallvector here, but that would take a lot of stack space for every level
+  // of the recursion, just use IncomingPredInfo as an explicit stack.
+  IncomingPredInfoTy &IncomingPredInfo = getIncomingPredInfo(IPI);
+  unsigned FirstPredInfoEntry = IncomingPredInfo.size();
+  
+  // As we're walking the predecessors, keep track of whether they are all
+  // producing the same value.  If so, this value will capture it, if not, it
+  // will get reset to null.  We distinguish the no-predecessor case explicitly
+  // below.
+  TrackingVH<Value> SingularValue;
+  
+  // We can get our predecessor info by walking the pred_iterator list, but it
+  // is relatively slow.  If we already have PHI nodes in this block, walk one
+  // of them to get the predecessor list instead.
+  if (PHINode *SomePhi = dyn_cast<PHINode>(BB->begin())) {
+    for (unsigned i = 0, e = SomePhi->getNumIncomingValues(); i != e; ++i) {
+      BasicBlock *PredBB = SomePhi->getIncomingBlock(i);
+      Value *PredVal = GetValueInBlockInternal(PredBB);
+      IncomingPredInfo.push_back(std::make_pair(PredBB, PredVal));
+      
+      // Compute SingularValue.
+      if (i == 0)
+        SingularValue = PredVal;
+      else if (PredVal != SingularValue)
+        SingularValue = 0;
+    }
+  } else {
+    bool isFirstPred = true;
+    for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) {
+      BasicBlock *PredBB = *PI;
+      Value *PredVal = GetValueInBlockInternal(PredBB);
+      IncomingPredInfo.push_back(std::make_pair(PredBB, PredVal));
+      
+      // Compute SingularValue.
+      if (isFirstPred) {
+        SingularValue = PredVal;
+        isFirstPred = false;
+      } else if (PredVal != SingularValue)
+        SingularValue = 0;
+    }
+  }
+  
+  // If there are no predecessors, then we must have found an unreachable block
+  // just return 'undef'.  Since there are no predecessors, InsertRes must not
+  // be invalidated.
+  if (IncomingPredInfo.size() == FirstPredInfoEntry)
+    return InsertRes.first->second = UndefValue::get(PrototypeValue->getType());
+  
+  /// Look up BB's entry in AvailableVals.  'InsertRes' may be invalidated.  If
+  /// this block is involved in a loop, a no-entry PHI node will have been
+  /// inserted as InsertedVal.  Otherwise, we'll still have the null we inserted
+  /// above.
+  TrackingVH<Value> &InsertedVal = AvailableVals[BB];
+  
+  // If all the predecessor values are the same then we don't need to insert a
+  // PHI.  This is the simple and common case.
+  if (SingularValue) {
+    // If a PHI node got inserted, replace it with the singlar value and delete
+    // it.
+    if (InsertedVal) {
+      PHINode *OldVal = cast<PHINode>(InsertedVal);
+      // Be careful about dead loops.  These RAUW's also update InsertedVal.
+      if (InsertedVal != SingularValue)
+        OldVal->replaceAllUsesWith(SingularValue);
+      else
+        OldVal->replaceAllUsesWith(UndefValue::get(InsertedVal->getType()));
+      OldVal->eraseFromParent();
+    } else {
+      InsertedVal = SingularValue;
+    }
+    
+    // Drop the entries we added in IncomingPredInfo to restore the stack.
+    IncomingPredInfo.erase(IncomingPredInfo.begin()+FirstPredInfoEntry,
+                           IncomingPredInfo.end());
+    return InsertedVal;
+  }
+  
+  // Otherwise, we do need a PHI: insert one now if we don't already have one.
+  if (InsertedVal == 0)
+    InsertedVal = PHINode::Create(PrototypeValue->getType(),
+                                  PrototypeValue->getName(), &BB->front());
+  
+  PHINode *InsertedPHI = cast<PHINode>(InsertedVal);
+  InsertedPHI->reserveOperandSpace(IncomingPredInfo.size()-FirstPredInfoEntry);
+  
+  // Fill in all the predecessors of the PHI.
+  for (std::vector<std::pair<BasicBlock*, TrackingVH<Value> > >::iterator I =
+       IncomingPredInfo.begin()+FirstPredInfoEntry, E = IncomingPredInfo.end();
+       I != E; ++I)
+    InsertedPHI->addIncoming(I->second, I->first);
+  
+  // Drop the entries we added in IncomingPredInfo to restore the stack.
+  IncomingPredInfo.erase(IncomingPredInfo.begin()+FirstPredInfoEntry,
+                         IncomingPredInfo.end());
+  
+  // See if the PHI node can be merged to a single value.  This can happen in
+  // loop cases when we get a PHI of itself and one other value.
+  if (Value *ConstVal = InsertedPHI->hasConstantValue()) {
+    InsertedPHI->replaceAllUsesWith(ConstVal);
+    InsertedPHI->eraseFromParent();
+    InsertedVal = ConstVal;
+  } else {
+    DEBUG(errs() << "  Inserted PHI: " << *InsertedPHI << "\n");
+  }
+  
+  return InsertedVal;
+}
+
+