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