lib/Analysis/SparsePropagation.cpp

   1 //===- SparsePropagation.cpp - Sparse Conditional Property Propagation ----===//
   2 //
   3 //                     The LLVM Compiler Infrastructure
   4 //
   5 // This file is distributed under the University of Illinois Open Source
   6 // License. See LICENSE.TXT for details.
   7 //
   8 //===----------------------------------------------------------------------===//
   9 //
  10 // This file implements an abstract sparse conditional propagation algorithm,
  11 // modeled after SCCP, but with a customizable lattice function.
  12 //
  13 //===----------------------------------------------------------------------===//
  14
  15 #define DEBUG_TYPE "sparseprop"
  16 #include "llvm/Analysis/SparsePropagation.h"
  17 #include "llvm/Constants.h"
  18 #include "llvm/Function.h"
  19 #include "llvm/Instructions.h"
  20 #include "llvm/Support/Debug.h"
  21 #include "llvm/Support/raw_ostream.h"
  22 using namespace llvm;
  23
  24 //===----------------------------------------------------------------------===//
  25 //                  AbstractLatticeFunction Implementation
  26 //===----------------------------------------------------------------------===//
  27
  28 AbstractLatticeFunction::~AbstractLatticeFunction() {}
  29
  30 /// PrintValue - Render the specified lattice value to the specified stream.
  31 void AbstractLatticeFunction::PrintValue(LatticeVal V, raw_ostream &OS) {
  32   if (V == UndefVal)
  33     OS << "undefined";
  34   else if (V == OverdefinedVal)
  35     OS << "overdefined";
  36   else if (V == UntrackedVal)
  37     OS << "untracked";
  38   else
  39     OS << "unknown lattice value";
  40 }
  41
  42 //===----------------------------------------------------------------------===//
  43 //                          SparseSolver Implementation
  44 //===----------------------------------------------------------------------===//
  45
  46 /// getOrInitValueState - Return the LatticeVal object that corresponds to the
  47 /// value, initializing the value's state if it hasn't been entered into the
  48 /// map yet.   This function is necessary because not all values should start
  49 /// out in the underdefined state... Arguments should be overdefined, and
  50 /// constants should be marked as constants.
  51 ///
  52 SparseSolver::LatticeVal SparseSolver::getOrInitValueState(Value *V) {
  53   DenseMap<Value*, LatticeVal>::iterator I = ValueState.find(V);
  54   if (I != ValueState.end()) return I->second;  // Common case, in the map
  55
  56   LatticeVal LV;
  57   if (LatticeFunc->IsUntrackedValue(V))
  58     return LatticeFunc->getUntrackedVal();
  59   else if (Constant *C = dyn_cast<Constant>(V))
  60     LV = LatticeFunc->ComputeConstant(C);
  61   else if (Argument *A = dyn_cast<Argument>(V))
  62     LV = LatticeFunc->ComputeArgument(A);
  63   else if (!isa<Instruction>(V))
  64     // All other non-instructions are overdefined.
  65     LV = LatticeFunc->getOverdefinedVal();
  66   else
  67     // All instructions are underdefined by default.
  68     LV = LatticeFunc->getUndefVal();
  69
  70   // If this value is untracked, don't add it to the map.
  71   if (LV == LatticeFunc->getUntrackedVal())
  72     return LV;
  73   return ValueState[V] = LV;
  74 }
  75
  76 /// UpdateState - When the state for some instruction is potentially updated,
  77 /// this function notices and adds I to the worklist if needed.
  78 void SparseSolver::UpdateState(Instruction &Inst, LatticeVal V) {
  79   DenseMap<Value*, LatticeVal>::iterator I = ValueState.find(&Inst);
  80   if (I != ValueState.end() && I->second == V)
  81     return;  // No change.
  82
  83   // An update.  Visit uses of I.
  84   ValueState[&Inst] = V;
  85   InstWorkList.push_back(&Inst);
  86 }
  87
  88 /// MarkBlockExecutable - This method can be used by clients to mark all of
  89 /// the blocks that are known to be intrinsically live in the processed unit.
  90 void SparseSolver::MarkBlockExecutable(BasicBlock *BB) {
  91   DEBUG(dbgs() << "Marking Block Executable: " << BB->getName() << "\n");
  92   BBExecutable.insert(BB);   // Basic block is executable!
  93   BBWorkList.push_back(BB);  // Add the block to the work list!
  94 }
  95
  96 /// markEdgeExecutable - Mark a basic block as executable, adding it to the BB
  97 /// work list if it is not already executable...
  98 void SparseSolver::markEdgeExecutable(BasicBlock *Source, BasicBlock *Dest) {
  99   if (!KnownFeasibleEdges.insert(Edge(Source, Dest)).second)
 100     return;  // This edge is already known to be executable!
 101
 102   DEBUG(dbgs() << "Marking Edge Executable: " << Source->getName()
 103         << " -> " << Dest->getName() << "\n");
 104
 105   if (BBExecutable.count(Dest)) {
 106     // The destination is already executable, but we just made an edge
 107     // feasible that wasn't before.  Revisit the PHI nodes in the block
 108     // because they have potentially new operands.
 109     for (BasicBlock::iterator I = Dest->begin(); isa<PHINode>(I); ++I)
 110       visitPHINode(*cast<PHINode>(I));
 111
 112   } else {
 113     MarkBlockExecutable(Dest);
 114   }
 115 }
 116
 117
 118 /// getFeasibleSuccessors - Return a vector of booleans to indicate which
 119 /// successors are reachable from a given terminator instruction.
 120 void SparseSolver::getFeasibleSuccessors(TerminatorInst &TI,
 121                                          SmallVectorImpl<bool> &Succs,
 122                                          bool AggressiveUndef) {
 123   Succs.resize(TI.getNumSuccessors());
 124   if (TI.getNumSuccessors() == 0) return;
 125
 126   if (BranchInst *BI = dyn_cast<BranchInst>(&TI)) {
 127     if (BI->isUnconditional()) {
 128       Succs[0] = true;
 129       return;
 130     }
 131
 132     LatticeVal BCValue;
 133     if (AggressiveUndef)
 134       BCValue = getOrInitValueState(BI->getCondition());
 135     else
 136       BCValue = getLatticeState(BI->getCondition());
 137
 138     if (BCValue == LatticeFunc->getOverdefinedVal() ||
 139         BCValue == LatticeFunc->getUntrackedVal()) {
 140       // Overdefined condition variables can branch either way.
 141       Succs[0] = Succs[1] = true;
 142       return;
 143     }
 144
 145     // If undefined, neither is feasible yet.
 146     if (BCValue == LatticeFunc->getUndefVal())
 147       return;
 148
 149     Constant *C = LatticeFunc->GetConstant(BCValue, BI->getCondition(), *this);
 150     if (C == 0 || !isa<ConstantInt>(C)) {
 151       // Non-constant values can go either way.
 152       Succs[0] = Succs[1] = true;
 153       return;
 154     }
 155
 156     // Constant condition variables mean the branch can only go a single way
 157     Succs[C->isNullValue()] = true;
 158     return;
 159   }
 160
 161   if (isa<InvokeInst>(TI)) {
 162     // Invoke instructions successors are always executable.
 163     // TODO: Could ask the lattice function if the value can throw.
 164     Succs[0] = Succs[1] = true;
 165     return;
 166   }
 167
 168   if (isa<IndirectBrInst>(TI)) {
 169     Succs.assign(Succs.size(), true);
 170     return;
 171   }
 172
 173   SwitchInst &SI = cast<SwitchInst>(TI);
 174   LatticeVal SCValue;
 175   if (AggressiveUndef)
 176     SCValue = getOrInitValueState(SI.getCondition());
 177   else
 178     SCValue = getLatticeState(SI.getCondition());
 179
 180   if (SCValue == LatticeFunc->getOverdefinedVal() ||
 181       SCValue == LatticeFunc->getUntrackedVal()) {
 182     // All destinations are executable!
 183     Succs.assign(TI.getNumSuccessors(), true);
 184     return;
 185   }
 186
 187   // If undefined, neither is feasible yet.
 188   if (SCValue == LatticeFunc->getUndefVal())
 189     return;
 190
 191   Constant *C = LatticeFunc->GetConstant(SCValue, SI.getCondition(), *this);
 192   if (C == 0 || !isa<ConstantInt>(C)) {
 193     // All destinations are executable!
 194     Succs.assign(TI.getNumSuccessors(), true);
 195     return;
 196   }
 197
 198   Succs[SI.findCaseValue(cast<ConstantInt>(C))] = true;
 199 }
 200
 201
 202 /// isEdgeFeasible - Return true if the control flow edge from the 'From'
 203 /// basic block to the 'To' basic block is currently feasible...
 204 bool SparseSolver::isEdgeFeasible(BasicBlock *From, BasicBlock *To,
 205                                   bool AggressiveUndef) {
 206   SmallVector<bool, 16> SuccFeasible;
 207   TerminatorInst *TI = From->getTerminator();
 208   getFeasibleSuccessors(*TI, SuccFeasible, AggressiveUndef);
 209
 210   for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i)
 211     if (TI->getSuccessor(i) == To && SuccFeasible[i])
 212       return true;
 213
 214   return false;
 215 }
 216
 217 void SparseSolver::visitTerminatorInst(TerminatorInst &TI) {
 218   SmallVector<bool, 16> SuccFeasible;
 219   getFeasibleSuccessors(TI, SuccFeasible, true);
 220
 221   BasicBlock *BB = TI.getParent();
 222
 223   // Mark all feasible successors executable...
 224   for (unsigned i = 0, e = SuccFeasible.size(); i != e; ++i)
 225     if (SuccFeasible[i])
 226       markEdgeExecutable(BB, TI.getSuccessor(i));
 227 }
 228
 229 void SparseSolver::visitPHINode(PHINode &PN) {
 230   // The lattice function may store more information on a PHINode than could be
 231   // computed from its incoming values.  For example, SSI form stores its sigma
 232   // functions as PHINodes with a single incoming value.
 233   if (LatticeFunc->IsSpecialCasedPHI(&PN)) {
 234     LatticeVal IV = LatticeFunc->ComputeInstructionState(PN, *this);
 235     if (IV != LatticeFunc->getUntrackedVal())
 236       UpdateState(PN, IV);
 237     return;
 238   }
 239
 240   LatticeVal PNIV = getOrInitValueState(&PN);
 241   LatticeVal Overdefined = LatticeFunc->getOverdefinedVal();
 242
 243   // If this value is already overdefined (common) just return.
 244   if (PNIV == Overdefined || PNIV == LatticeFunc->getUntrackedVal())
 245     return;  // Quick exit
 246
 247   // Super-extra-high-degree PHI nodes are unlikely to ever be interesting,
 248   // and slow us down a lot.  Just mark them overdefined.
 249   if (PN.getNumIncomingValues() > 64) {
 250     UpdateState(PN, Overdefined);
 251     return;
 252   }
 253
 254   // Look at all of the executable operands of the PHI node.  If any of them
 255   // are overdefined, the PHI becomes overdefined as well.  Otherwise, ask the
 256   // transfer function to give us the merge of the incoming values.
 257   for (unsigned i = 0, e = PN.getNumIncomingValues(); i != e; ++i) {
 258     // If the edge is not yet known to be feasible, it doesn't impact the PHI.
 259     if (!isEdgeFeasible(PN.getIncomingBlock(i), PN.getParent(), true))
 260       continue;
 261
 262     // Merge in this value.
 263     LatticeVal OpVal = getOrInitValueState(PN.getIncomingValue(i));
 264     if (OpVal != PNIV)
 265       PNIV = LatticeFunc->MergeValues(PNIV, OpVal);
 266
 267     if (PNIV == Overdefined)
 268       break;  // Rest of input values don't matter.
 269   }
 270
 271   // Update the PHI with the compute value, which is the merge of the inputs.
 272   UpdateState(PN, PNIV);
 273 }
 274
 275
 276 void SparseSolver::visitInst(Instruction &I) {
 277   // PHIs are handled by the propagation logic, they are never passed into the
 278   // transfer functions.
 279   if (PHINode *PN = dyn_cast<PHINode>(&I))
 280     return visitPHINode(*PN);
 281
 282   // Otherwise, ask the transfer function what the result is.  If this is
 283   // something that we care about, remember it.
 284   LatticeVal IV = LatticeFunc->ComputeInstructionState(I, *this);
 285   if (IV != LatticeFunc->getUntrackedVal())
 286     UpdateState(I, IV);
 287
 288   if (TerminatorInst *TI = dyn_cast<TerminatorInst>(&I))
 289     visitTerminatorInst(*TI);
 290 }
 291
 292 void SparseSolver::Solve(Function &F) {
 293   MarkBlockExecutable(&F.getEntryBlock());
 294
 295   // Process the work lists until they are empty!
 296   while (!BBWorkList.empty() || !InstWorkList.empty()) {
 297     // Process the instruction work list.
 298     while (!InstWorkList.empty()) {
 299       Instruction *I = InstWorkList.back();
 300       InstWorkList.pop_back();
 301
 302       DEBUG(dbgs() << "\nPopped off I-WL: " << *I << "\n");
 303
 304       // "I" got into the work list because it made a transition.  See if any
 305       // users are both live and in need of updating.
 306       for (Value::use_iterator UI = I->use_begin(), E = I->use_end();
 307            UI != E; ++UI) {
 308         Instruction *U = cast<Instruction>(*UI);
 309         if (BBExecutable.count(U->getParent()))   // Inst is executable?
 310           visitInst(*U);
 311       }
 312     }
 313
 314     // Process the basic block work list.
 315     while (!BBWorkList.empty()) {
 316       BasicBlock *BB = BBWorkList.back();
 317       BBWorkList.pop_back();
 318
 319       DEBUG(dbgs() << "\nPopped off BBWL: " << *BB);
 320
 321       // Notify all instructions in this basic block that they are newly
 322       // executable.
 323       for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I)
 324         visitInst(*I);
 325     }
 326   }
 327 }
 328
 329 void SparseSolver::Print(Function &F, raw_ostream &OS) const {
 330   OS << "\nFUNCTION: " << F.getNameStr() << "\n";
 331   for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB) {
 332     if (!BBExecutable.count(BB))
 333       OS << "INFEASIBLE: ";
 334     OS << "\t";
 335     if (BB->hasName())
 336       OS << BB->getNameStr() << ":\n";
 337     else
 338       OS << "; anon bb\n";
 339     for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) {
 340       LatticeFunc->PrintValue(getLatticeState(I), OS);
 341       OS << *I << "\n";
 342     }
 343
 344     OS << "\n";
 345   }
 346 }
 347