aboutsummaryrefslogtreecommitdiffstats
path: root/lib/Analysis/SparsePropagation.cpp
blob: edd82f5fe296081f635da4713ac27b0043f14d59 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
//===- SparsePropagation.cpp - Sparse Conditional Property Propagation ----===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file implements an abstract sparse conditional propagation algorithm,
// modeled after SCCP, but with a customizable lattice function.
//
//===----------------------------------------------------------------------===//

#include "llvm/Analysis/SparsePropagation.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/Instructions.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
using namespace llvm;

#define DEBUG_TYPE "sparseprop"

//===----------------------------------------------------------------------===//
//                  AbstractLatticeFunction Implementation
//===----------------------------------------------------------------------===//

AbstractLatticeFunction::~AbstractLatticeFunction() {}

/// PrintValue - Render the specified lattice value to the specified stream.
void AbstractLatticeFunction::PrintValue(LatticeVal V, raw_ostream &OS) {
  if (V == UndefVal)
    OS << "undefined";
  else if (V == OverdefinedVal)
    OS << "overdefined";
  else if (V == UntrackedVal)
    OS << "untracked";
  else
    OS << "unknown lattice value";
}

//===----------------------------------------------------------------------===//
//                          SparseSolver Implementation
//===----------------------------------------------------------------------===//

/// getOrInitValueState - Return the LatticeVal object that corresponds to the
/// value, initializing the value's state if it hasn't been entered into the
/// map yet.   This function is necessary because not all values should start
/// out in the underdefined state... Arguments should be overdefined, and
/// constants should be marked as constants.
///
SparseSolver::LatticeVal SparseSolver::getOrInitValueState(Value *V) {
  DenseMap<Value*, LatticeVal>::iterator I = ValueState.find(V);
  if (I != ValueState.end()) return I->second;  // Common case, in the map
  
  LatticeVal LV;
  if (LatticeFunc->IsUntrackedValue(V))
    return LatticeFunc->getUntrackedVal();
  else if (Constant *C = dyn_cast<Constant>(V))
    LV = LatticeFunc->ComputeConstant(C);
  else if (Argument *A = dyn_cast<Argument>(V))
    LV = LatticeFunc->ComputeArgument(A);
  else if (!isa<Instruction>(V))
    // All other non-instructions are overdefined.
    LV = LatticeFunc->getOverdefinedVal();
  else
    // All instructions are underdefined by default.
    LV = LatticeFunc->getUndefVal();
  
  // If this value is untracked, don't add it to the map.
  if (LV == LatticeFunc->getUntrackedVal())
    return LV;
  return ValueState[V] = LV;
}

/// UpdateState - When the state for some instruction is potentially updated,
/// this function notices and adds I to the worklist if needed.
void SparseSolver::UpdateState(Instruction &Inst, LatticeVal V) {
  DenseMap<Value*, LatticeVal>::iterator I = ValueState.find(&Inst);
  if (I != ValueState.end() && I->second == V)
    return;  // No change.
  
  // An update.  Visit uses of I.
  ValueState[&Inst] = V;
  InstWorkList.push_back(&Inst);
}

/// MarkBlockExecutable - This method can be used by clients to mark all of
/// the blocks that are known to be intrinsically live in the processed unit.
void SparseSolver::MarkBlockExecutable(BasicBlock *BB) {
  DEBUG(dbgs() << "Marking Block Executable: " << BB->getName() << "\n");
  BBExecutable.insert(BB);   // Basic block is executable!
  BBWorkList.push_back(BB);  // Add the block to the work list!
}

/// markEdgeExecutable - Mark a basic block as executable, adding it to the BB
/// work list if it is not already executable...
void SparseSolver::markEdgeExecutable(BasicBlock *Source, BasicBlock *Dest) {
  if (!KnownFeasibleEdges.insert(Edge(Source, Dest)).second)
    return;  // This edge is already known to be executable!
  
  DEBUG(dbgs() << "Marking Edge Executable: " << Source->getName()
        << " -> " << Dest->getName() << "\n");

  if (BBExecutable.count(Dest)) {
    // The destination is already executable, but we just made an edge
    // feasible that wasn't before.  Revisit the PHI nodes in the block
    // because they have potentially new operands.
    for (BasicBlock::iterator I = Dest->begin(); isa<PHINode>(I); ++I)
      visitPHINode(*cast<PHINode>(I));
    
  } else {
    MarkBlockExecutable(Dest);
  }
}


/// getFeasibleSuccessors - Return a vector of booleans to indicate which
/// successors are reachable from a given terminator instruction.
void SparseSolver::getFeasibleSuccessors(TerminatorInst &TI,
                                         SmallVectorImpl<bool> &Succs,
                                         bool AggressiveUndef) {
  Succs.resize(TI.getNumSuccessors());
  if (TI.getNumSuccessors() == 0) return;
  
  if (BranchInst *BI = dyn_cast<BranchInst>(&TI)) {
    if (BI->isUnconditional()) {
      Succs[0] = true;
      return;
    }
    
    LatticeVal BCValue;
    if (AggressiveUndef)
      BCValue = getOrInitValueState(BI->getCondition());
    else
      BCValue = getLatticeState(BI->getCondition());
    
    if (BCValue == LatticeFunc->getOverdefinedVal() ||
        BCValue == LatticeFunc->getUntrackedVal()) {
      // Overdefined condition variables can branch either way.
      Succs[0] = Succs[1] = true;
      return;
    }

    // If undefined, neither is feasible yet.
    if (BCValue == LatticeFunc->getUndefVal())
      return;

    Constant *C = LatticeFunc->GetConstant(BCValue, BI->getCondition(), *this);
    if (!C || !isa<ConstantInt>(C)) {
      // Non-constant values can go either way.
      Succs[0] = Succs[1] = true;
      return;
    }

    // Constant condition variables mean the branch can only go a single way
    Succs[C->isNullValue()] = true;
    return;
  }
  
  if (isa<InvokeInst>(TI)) {
    // Invoke instructions successors are always executable.
    // TODO: Could ask the lattice function if the value can throw.
    Succs[0] = Succs[1] = true;
    return;
  }
  
  if (isa<IndirectBrInst>(TI)) {
    Succs.assign(Succs.size(), true);
    return;
  }
  
  SwitchInst &SI = cast<SwitchInst>(TI);
  LatticeVal SCValue;
  if (AggressiveUndef)
    SCValue = getOrInitValueState(SI.getCondition());
  else
    SCValue = getLatticeState(SI.getCondition());
  
  if (SCValue == LatticeFunc->getOverdefinedVal() ||
      SCValue == LatticeFunc->getUntrackedVal()) {
    // All destinations are executable!
    Succs.assign(TI.getNumSuccessors(), true);
    return;
  }
  
  // If undefined, neither is feasible yet.
  if (SCValue == LatticeFunc->getUndefVal())
    return;
  
  Constant *C = LatticeFunc->GetConstant(SCValue, SI.getCondition(), *this);
  if (!C || !isa<ConstantInt>(C)) {
    // All destinations are executable!
    Succs.assign(TI.getNumSuccessors(), true);
    return;
  }
  SwitchInst::CaseIt Case = SI.findCaseValue(cast<ConstantInt>(C));
  Succs[Case.getSuccessorIndex()] = true;
}


/// isEdgeFeasible - Return true if the control flow edge from the 'From'
/// basic block to the 'To' basic block is currently feasible...
bool SparseSolver::isEdgeFeasible(BasicBlock *From, BasicBlock *To,
                                  bool AggressiveUndef) {
  SmallVector<bool, 16> SuccFeasible;
  TerminatorInst *TI = From->getTerminator();
  getFeasibleSuccessors(*TI, SuccFeasible, AggressiveUndef);
  
  for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i)
    if (TI->getSuccessor(i) == To && SuccFeasible[i])
      return true;
  
  return false;
}

void SparseSolver::visitTerminatorInst(TerminatorInst &TI) {
  SmallVector<bool, 16> SuccFeasible;
  getFeasibleSuccessors(TI, SuccFeasible, true);
  
  BasicBlock *BB = TI.getParent();
  
  // Mark all feasible successors executable...
  for (unsigned i = 0, e = SuccFeasible.size(); i != e; ++i)
    if (SuccFeasible[i])
      markEdgeExecutable(BB, TI.getSuccessor(i));
}

void SparseSolver::visitPHINode(PHINode &PN) {
  // The lattice function may store more information on a PHINode than could be
  // computed from its incoming values.  For example, SSI form stores its sigma
  // functions as PHINodes with a single incoming value.
  if (LatticeFunc->IsSpecialCasedPHI(&PN)) {
    LatticeVal IV = LatticeFunc->ComputeInstructionState(PN, *this);
    if (IV != LatticeFunc->getUntrackedVal())
      UpdateState(PN, IV);
    return;
  }

  LatticeVal PNIV = getOrInitValueState(&PN);
  LatticeVal Overdefined = LatticeFunc->getOverdefinedVal();
  
  // If this value is already overdefined (common) just return.
  if (PNIV == Overdefined || PNIV == LatticeFunc->getUntrackedVal())
    return;  // Quick exit
  
  // Super-extra-high-degree PHI nodes are unlikely to ever be interesting,
  // and slow us down a lot.  Just mark them overdefined.
  if (PN.getNumIncomingValues() > 64) {
    UpdateState(PN, Overdefined);
    return;
  }
  
  // Look at all of the executable operands of the PHI node.  If any of them
  // are overdefined, the PHI becomes overdefined as well.  Otherwise, ask the
  // transfer function to give us the merge of the incoming values.
  for (unsigned i = 0, e = PN.getNumIncomingValues(); i != e; ++i) {
    // If the edge is not yet known to be feasible, it doesn't impact the PHI.
    if (!isEdgeFeasible(PN.getIncomingBlock(i), PN.getParent(), true))
      continue;
    
    // Merge in this value.
    LatticeVal OpVal = getOrInitValueState(PN.getIncomingValue(i));
    if (OpVal != PNIV)
      PNIV = LatticeFunc->MergeValues(PNIV, OpVal);
    
    if (PNIV == Overdefined)
      break;  // Rest of input values don't matter.
  }

  // Update the PHI with the compute value, which is the merge of the inputs.
  UpdateState(PN, PNIV);
}


void SparseSolver::visitInst(Instruction &I) {
  // PHIs are handled by the propagation logic, they are never passed into the
  // transfer functions.
  if (PHINode *PN = dyn_cast<PHINode>(&I))
    return visitPHINode(*PN);
  
  // Otherwise, ask the transfer function what the result is.  If this is
  // something that we care about, remember it.
  LatticeVal IV = LatticeFunc->ComputeInstructionState(I, *this);
  if (IV != LatticeFunc->getUntrackedVal())
    UpdateState(I, IV);
  
  if (TerminatorInst *TI = dyn_cast<TerminatorInst>(&I))
    visitTerminatorInst(*TI);
}

void SparseSolver::Solve(Function &F) {
  MarkBlockExecutable(&F.getEntryBlock());
  
  // Process the work lists until they are empty!
  while (!BBWorkList.empty() || !InstWorkList.empty()) {
    // Process the instruction work list.
    while (!InstWorkList.empty()) {
      Instruction *I = InstWorkList.back();
      InstWorkList.pop_back();

      DEBUG(dbgs() << "\nPopped off I-WL: " << *I << "\n");

      // "I" got into the work list because it made a transition.  See if any
      // users are both live and in need of updating.
      for (User *U : I->users()) {
        Instruction *UI = cast<Instruction>(U);
        if (BBExecutable.count(UI->getParent()))   // Inst is executable?
          visitInst(*UI);
      }
    }

    // Process the basic block work list.
    while (!BBWorkList.empty()) {
      BasicBlock *BB = BBWorkList.back();
      BBWorkList.pop_back();

      DEBUG(dbgs() << "\nPopped off BBWL: " << *BB);

      // Notify all instructions in this basic block that they are newly
      // executable.
      for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I)
        visitInst(*I);
    }
  }
}

void SparseSolver::Print(Function &F, raw_ostream &OS) const {
  OS << "\nFUNCTION: " << F.getName() << "\n";
  for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB) {
    if (!BBExecutable.count(BB))
      OS << "INFEASIBLE: ";
    OS << "\t";
    if (BB->hasName())
      OS << BB->getName() << ":\n";
    else
      OS << "; anon bb\n";
    for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) {
      LatticeFunc->PrintValue(getLatticeState(I), OS);
      OS << *I << "\n";
    }
    
    OS << "\n";
  }
}