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 
~AbstractLatticeFunction()29 AbstractLatticeFunction::~AbstractLatticeFunction() {}
30 
31 /// PrintValue - Render the specified lattice value to the specified stream.
PrintValue(LatticeVal V,raw_ostream & OS)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 ///
getOrInitValueState(Value * V)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.
UpdateState(Instruction & Inst,LatticeVal V)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.
MarkBlockExecutable(BasicBlock * BB)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...
markEdgeExecutable(BasicBlock * Source,BasicBlock * Dest)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.
getFeasibleSuccessors(TerminatorInst & TI,SmallVectorImpl<bool> & Succs,bool AggressiveUndef)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...
isEdgeFeasible(BasicBlock * From,BasicBlock * To,bool AggressiveUndef)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 
visitTerminatorInst(TerminatorInst & TI)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 
visitPHINode(PHINode & PN)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 
visitInst(Instruction & I)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 
Solve(Function & F)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 
Print(Function & F,raw_ostream & OS) const329 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