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      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 #include "llvm/Analysis/SparsePropagation.h"
     16 #include "llvm/IR/Constants.h"
     17 #include "llvm/IR/Function.h"
     18 #include "llvm/IR/Instructions.h"
     19 #include "llvm/Support/Debug.h"
     20 #include "llvm/Support/raw_ostream.h"
     21 using namespace llvm;
     22 
     23 #define DEBUG_TYPE "sparseprop"
     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, raw_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(dbgs() << "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(dbgs() << "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 || !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->isNullValue()] = 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   if (isa<IndirectBrInst>(TI)) {
    170     Succs.assign(Succs.size(), true);
    171     return;
    172   }
    173 
    174   SwitchInst &SI = cast<SwitchInst>(TI);
    175   LatticeVal SCValue;
    176   if (AggressiveUndef)
    177     SCValue = getOrInitValueState(SI.getCondition());
    178   else
    179     SCValue = getLatticeState(SI.getCondition());
    180 
    181   if (SCValue == LatticeFunc->getOverdefinedVal() ||
    182       SCValue == LatticeFunc->getUntrackedVal()) {
    183     // All destinations are executable!
    184     Succs.assign(TI.getNumSuccessors(), true);
    185     return;
    186   }
    187 
    188   // If undefined, neither is feasible yet.
    189   if (SCValue == LatticeFunc->getUndefVal())
    190     return;
    191 
    192   Constant *C = LatticeFunc->GetConstant(SCValue, SI.getCondition(), *this);
    193   if (!C || !isa<ConstantInt>(C)) {
    194     // All destinations are executable!
    195     Succs.assign(TI.getNumSuccessors(), true);
    196     return;
    197   }
    198   SwitchInst::CaseIt Case = SI.findCaseValue(cast<ConstantInt>(C));
    199   Succs[Case.getSuccessorIndex()] = true;
    200 }
    201 
    202 
    203 /// isEdgeFeasible - Return true if the control flow edge from the 'From'
    204 /// basic block to the 'To' basic block is currently feasible...
    205 bool SparseSolver::isEdgeFeasible(BasicBlock *From, BasicBlock *To,
    206                                   bool AggressiveUndef) {
    207   SmallVector<bool, 16> SuccFeasible;
    208   TerminatorInst *TI = From->getTerminator();
    209   getFeasibleSuccessors(*TI, SuccFeasible, AggressiveUndef);
    210 
    211   for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i)
    212     if (TI->getSuccessor(i) == To && SuccFeasible[i])
    213       return true;
    214 
    215   return false;
    216 }
    217 
    218 void SparseSolver::visitTerminatorInst(TerminatorInst &TI) {
    219   SmallVector<bool, 16> SuccFeasible;
    220   getFeasibleSuccessors(TI, SuccFeasible, true);
    221 
    222   BasicBlock *BB = TI.getParent();
    223 
    224   // Mark all feasible successors executable...
    225   for (unsigned i = 0, e = SuccFeasible.size(); i != e; ++i)
    226     if (SuccFeasible[i])
    227       markEdgeExecutable(BB, TI.getSuccessor(i));
    228 }
    229 
    230 void SparseSolver::visitPHINode(PHINode &PN) {
    231   // The lattice function may store more information on a PHINode than could be
    232   // computed from its incoming values.  For example, SSI form stores its sigma
    233   // functions as PHINodes with a single incoming value.
    234   if (LatticeFunc->IsSpecialCasedPHI(&PN)) {
    235     LatticeVal IV = LatticeFunc->ComputeInstructionState(PN, *this);
    236     if (IV != LatticeFunc->getUntrackedVal())
    237       UpdateState(PN, IV);
    238     return;
    239   }
    240 
    241   LatticeVal PNIV = getOrInitValueState(&PN);
    242   LatticeVal Overdefined = LatticeFunc->getOverdefinedVal();
    243 
    244   // If this value is already overdefined (common) just return.
    245   if (PNIV == Overdefined || PNIV == LatticeFunc->getUntrackedVal())
    246     return;  // Quick exit
    247 
    248   // Super-extra-high-degree PHI nodes are unlikely to ever be interesting,
    249   // and slow us down a lot.  Just mark them overdefined.
    250   if (PN.getNumIncomingValues() > 64) {
    251     UpdateState(PN, Overdefined);
    252     return;
    253   }
    254 
    255   // Look at all of the executable operands of the PHI node.  If any of them
    256   // are overdefined, the PHI becomes overdefined as well.  Otherwise, ask the
    257   // transfer function to give us the merge of the incoming values.
    258   for (unsigned i = 0, e = PN.getNumIncomingValues(); i != e; ++i) {
    259     // If the edge is not yet known to be feasible, it doesn't impact the PHI.
    260     if (!isEdgeFeasible(PN.getIncomingBlock(i), PN.getParent(), true))
    261       continue;
    262 
    263     // Merge in this value.
    264     LatticeVal OpVal = getOrInitValueState(PN.getIncomingValue(i));
    265     if (OpVal != PNIV)
    266       PNIV = LatticeFunc->MergeValues(PNIV, OpVal);
    267 
    268     if (PNIV == Overdefined)
    269       break;  // Rest of input values don't matter.
    270   }
    271 
    272   // Update the PHI with the compute value, which is the merge of the inputs.
    273   UpdateState(PN, PNIV);
    274 }
    275 
    276 
    277 void SparseSolver::visitInst(Instruction &I) {
    278   // PHIs are handled by the propagation logic, they are never passed into the
    279   // transfer functions.
    280   if (PHINode *PN = dyn_cast<PHINode>(&I))
    281     return visitPHINode(*PN);
    282 
    283   // Otherwise, ask the transfer function what the result is.  If this is
    284   // something that we care about, remember it.
    285   LatticeVal IV = LatticeFunc->ComputeInstructionState(I, *this);
    286   if (IV != LatticeFunc->getUntrackedVal())
    287     UpdateState(I, IV);
    288 
    289   if (TerminatorInst *TI = dyn_cast<TerminatorInst>(&I))
    290     visitTerminatorInst(*TI);
    291 }
    292 
    293 void SparseSolver::Solve(Function &F) {
    294   MarkBlockExecutable(&F.getEntryBlock());
    295 
    296   // Process the work lists until they are empty!
    297   while (!BBWorkList.empty() || !InstWorkList.empty()) {
    298     // Process the instruction work list.
    299     while (!InstWorkList.empty()) {
    300       Instruction *I = InstWorkList.back();
    301       InstWorkList.pop_back();
    302 
    303       DEBUG(dbgs() << "\nPopped off I-WL: " << *I << "\n");
    304 
    305       // "I" got into the work list because it made a transition.  See if any
    306       // users are both live and in need of updating.
    307       for (User *U : I->users()) {
    308         Instruction *UI = cast<Instruction>(U);
    309         if (BBExecutable.count(UI->getParent()))   // Inst is executable?
    310           visitInst(*UI);
    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.getName() << "\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->getName() << ":\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 
    348