SparsePropagation.cpp revision f3ef5332fa3f4d5ec72c178a2b19dac363a19383 
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"
21using namespace llvm;
22
23#define DEBUG_TYPE "sparseprop"
24
25//===----------------------------------------------------------------------===//
26//                  AbstractLatticeFunction Implementation
27//===----------------------------------------------------------------------===//
28
29AbstractLatticeFunction::~AbstractLatticeFunction() {}
30
31/// PrintValue - Render the specified lattice value to the specified stream.
32void 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///
53SparseSolver::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.
79void 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.
91void 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...
99void 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.
121void 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...
205bool 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
218void 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
230void 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
277void 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
293void 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
329void SparseSolver::Print(Function &F, raw_ostream &OS) const {
330  OS << "\nFUNCTION: " << F.getName() << "\n";
331  for (auto &BB : F) {
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 (auto &I : BB) {
340      LatticeFunc->PrintValue(getLatticeState(&I), OS);
341      OS << I << "\n";
342    }
343
344    OS << "\n";
345  }
346}
347
348