SparsePropagation.cpp revision b3056faa5554ded7ac1ac5865d10ef5839fb77d3 
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"
22using namespace llvm;
23
24//===----------------------------------------------------------------------===//
25//                  AbstractLatticeFunction Implementation
26//===----------------------------------------------------------------------===//
27
28AbstractLatticeFunction::~AbstractLatticeFunction() {}
29
30/// PrintValue - Render the specified lattice value to the specified stream.
31void AbstractLatticeFunction::PrintValue(LatticeVal V, std::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///
52SparseSolver::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.
78void 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.
90void SparseSolver::MarkBlockExecutable(BasicBlock *BB) {
91  DOUT << "Marking Block Executable: " << BB->getNameStart() << "\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...
98void 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  DOUT << "Marking Edge Executable: " << Source->getNameStart()
103       << " -> " << Dest->getNameStart() << "\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.
120void 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 == Context->getFalse()] = 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  SwitchInst &SI = cast<SwitchInst>(TI);
169  LatticeVal SCValue;
170  if (AggressiveUndef)
171    SCValue = getOrInitValueState(SI.getCondition());
172  else
173    SCValue = getLatticeState(SI.getCondition());
174
175  if (SCValue == LatticeFunc->getOverdefinedVal() ||
176      SCValue == LatticeFunc->getUntrackedVal()) {
177    // All destinations are executable!
178    Succs.assign(TI.getNumSuccessors(), true);
179    return;
180  }
181
182  // If undefined, neither is feasible yet.
183  if (SCValue == LatticeFunc->getUndefVal())
184    return;
185
186  Constant *C = LatticeFunc->GetConstant(SCValue, SI.getCondition(), *this);
187  if (C == 0 || !isa<ConstantInt>(C)) {
188    // All destinations are executable!
189    Succs.assign(TI.getNumSuccessors(), true);
190    return;
191  }
192
193  Succs[SI.findCaseValue(cast<ConstantInt>(C))] = true;
194}
195
196
197/// isEdgeFeasible - Return true if the control flow edge from the 'From'
198/// basic block to the 'To' basic block is currently feasible...
199bool SparseSolver::isEdgeFeasible(BasicBlock *From, BasicBlock *To,
200                                  bool AggressiveUndef) {
201  SmallVector<bool, 16> SuccFeasible;
202  TerminatorInst *TI = From->getTerminator();
203  getFeasibleSuccessors(*TI, SuccFeasible, AggressiveUndef);
204
205  for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i)
206    if (TI->getSuccessor(i) == To && SuccFeasible[i])
207      return true;
208
209  return false;
210}
211
212void SparseSolver::visitTerminatorInst(TerminatorInst &TI) {
213  SmallVector<bool, 16> SuccFeasible;
214  getFeasibleSuccessors(TI, SuccFeasible, true);
215
216  BasicBlock *BB = TI.getParent();
217
218  // Mark all feasible successors executable...
219  for (unsigned i = 0, e = SuccFeasible.size(); i != e; ++i)
220    if (SuccFeasible[i])
221      markEdgeExecutable(BB, TI.getSuccessor(i));
222}
223
224void SparseSolver::visitPHINode(PHINode &PN) {
225  LatticeVal PNIV = getOrInitValueState(&PN);
226  LatticeVal Overdefined = LatticeFunc->getOverdefinedVal();
227
228  // If this value is already overdefined (common) just return.
229  if (PNIV == Overdefined || PNIV == LatticeFunc->getUntrackedVal())
230    return;  // Quick exit
231
232  // Super-extra-high-degree PHI nodes are unlikely to ever be interesting,
233  // and slow us down a lot.  Just mark them overdefined.
234  if (PN.getNumIncomingValues() > 64) {
235    UpdateState(PN, Overdefined);
236    return;
237  }
238
239  // Look at all of the executable operands of the PHI node.  If any of them
240  // are overdefined, the PHI becomes overdefined as well.  Otherwise, ask the
241  // transfer function to give us the merge of the incoming values.
242  for (unsigned i = 0, e = PN.getNumIncomingValues(); i != e; ++i) {
243    // If the edge is not yet known to be feasible, it doesn't impact the PHI.
244    if (!isEdgeFeasible(PN.getIncomingBlock(i), PN.getParent(), true))
245      continue;
246
247    // Merge in this value.
248    LatticeVal OpVal = getOrInitValueState(PN.getIncomingValue(i));
249    if (OpVal != PNIV)
250      PNIV = LatticeFunc->MergeValues(PNIV, OpVal);
251
252    if (PNIV == Overdefined)
253      break;  // Rest of input values don't matter.
254  }
255
256  // Update the PHI with the compute value, which is the merge of the inputs.
257  UpdateState(PN, PNIV);
258}
259
260
261void SparseSolver::visitInst(Instruction &I) {
262  // PHIs are handled by the propagation logic, they are never passed into the
263  // transfer functions.
264  if (PHINode *PN = dyn_cast<PHINode>(&I))
265    return visitPHINode(*PN);
266
267  // Otherwise, ask the transfer function what the result is.  If this is
268  // something that we care about, remember it.
269  LatticeVal IV = LatticeFunc->ComputeInstructionState(I, *this);
270  if (IV != LatticeFunc->getUntrackedVal())
271    UpdateState(I, IV);
272
273  if (TerminatorInst *TI = dyn_cast<TerminatorInst>(&I))
274    visitTerminatorInst(*TI);
275}
276
277void SparseSolver::Solve(Function &F) {
278  MarkBlockExecutable(&F.getEntryBlock());
279
280  // Process the work lists until they are empty!
281  while (!BBWorkList.empty() || !InstWorkList.empty()) {
282    // Process the instruction work list.
283    while (!InstWorkList.empty()) {
284      Instruction *I = InstWorkList.back();
285      InstWorkList.pop_back();
286
287      DOUT << "\nPopped off I-WL: " << *I;
288
289      // "I" got into the work list because it made a transition.  See if any
290      // users are both live and in need of updating.
291      for (Value::use_iterator UI = I->use_begin(), E = I->use_end();
292           UI != E; ++UI) {
293        Instruction *U = cast<Instruction>(*UI);
294        if (BBExecutable.count(U->getParent()))   // Inst is executable?
295          visitInst(*U);
296      }
297    }
298
299    // Process the basic block work list.
300    while (!BBWorkList.empty()) {
301      BasicBlock *BB = BBWorkList.back();
302      BBWorkList.pop_back();
303
304      DOUT << "\nPopped off BBWL: " << *BB;
305
306      // Notify all instructions in this basic block that they are newly
307      // executable.
308      for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I)
309        visitInst(*I);
310    }
311  }
312}
313
314void SparseSolver::Print(Function &F, std::ostream &OS) const {
315  OS << "\nFUNCTION: " << F.getNameStr() << "\n";
316  for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB) {
317    if (!BBExecutable.count(BB))
318      OS << "INFEASIBLE: ";
319    OS << "\t";
320    if (BB->hasName())
321      OS << BB->getNameStr() << ":\n";
322    else
323      OS << "; anon bb\n";
324    for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) {
325      LatticeFunc->PrintValue(getLatticeState(I), OS);
326      OS << *I;
327    }
328
329    OS << "\n";
330  }
331}
332
333