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1 //===- SCCP.cpp - Sparse Conditional Constant 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 sparse conditional constant propagation and merging:
11 //
12 // Specifically, this:
13 //   * Assumes values are constant unless proven otherwise
14 //   * Assumes BasicBlocks are dead unless proven otherwise
15 //   * Proves values to be constant, and replaces them with constants
16 //   * Proves conditional branches to be unconditional
17 //
18 //===----------------------------------------------------------------------===//
19 
20 #define DEBUG_TYPE "sccp"
21 #include "llvm/Transforms/Scalar.h"
22 #include "llvm/ADT/DenseMap.h"
23 #include "llvm/ADT/DenseSet.h"
24 #include "llvm/ADT/PointerIntPair.h"
25 #include "llvm/ADT/SmallPtrSet.h"
26 #include "llvm/ADT/SmallVector.h"
27 #include "llvm/ADT/Statistic.h"
28 #include "llvm/Analysis/ConstantFolding.h"
29 #include "llvm/IR/Constants.h"
30 #include "llvm/IR/DataLayout.h"
31 #include "llvm/IR/DerivedTypes.h"
32 #include "llvm/IR/Instructions.h"
33 #include "llvm/InstVisitor.h"
34 #include "llvm/Pass.h"
35 #include "llvm/Support/CallSite.h"
36 #include "llvm/Support/Debug.h"
37 #include "llvm/Support/ErrorHandling.h"
38 #include "llvm/Support/raw_ostream.h"
39 #include "llvm/Target/TargetLibraryInfo.h"
40 #include "llvm/Transforms/IPO.h"
41 #include "llvm/Transforms/Utils/Local.h"
42 #include <algorithm>
43 using namespace llvm;
44 
45 STATISTIC(NumInstRemoved, "Number of instructions removed");
46 STATISTIC(NumDeadBlocks , "Number of basic blocks unreachable");
47 
48 STATISTIC(IPNumInstRemoved, "Number of instructions removed by IPSCCP");
49 STATISTIC(IPNumArgsElimed ,"Number of arguments constant propagated by IPSCCP");
50 STATISTIC(IPNumGlobalConst, "Number of globals found to be constant by IPSCCP");
51 
52 namespace {
53 /// LatticeVal class - This class represents the different lattice values that
54 /// an LLVM value may occupy.  It is a simple class with value semantics.
55 ///
56 class LatticeVal {
57   enum LatticeValueTy {
58     /// undefined - This LLVM Value has no known value yet.
59     undefined,
60 
61     /// constant - This LLVM Value has a specific constant value.
62     constant,
63 
64     /// forcedconstant - This LLVM Value was thought to be undef until
65     /// ResolvedUndefsIn.  This is treated just like 'constant', but if merged
66     /// with another (different) constant, it goes to overdefined, instead of
67     /// asserting.
68     forcedconstant,
69 
70     /// overdefined - This instruction is not known to be constant, and we know
71     /// it has a value.
72     overdefined
73   };
74 
75   /// Val: This stores the current lattice value along with the Constant* for
76   /// the constant if this is a 'constant' or 'forcedconstant' value.
77   PointerIntPair<Constant *, 2, LatticeValueTy> Val;
78 
getLatticeValue() const79   LatticeValueTy getLatticeValue() const {
80     return Val.getInt();
81   }
82 
83 public:
LatticeVal()84   LatticeVal() : Val(0, undefined) {}
85 
isUndefined() const86   bool isUndefined() const { return getLatticeValue() == undefined; }
isConstant() const87   bool isConstant() const {
88     return getLatticeValue() == constant || getLatticeValue() == forcedconstant;
89   }
isOverdefined() const90   bool isOverdefined() const { return getLatticeValue() == overdefined; }
91 
getConstant() const92   Constant *getConstant() const {
93     assert(isConstant() && "Cannot get the constant of a non-constant!");
94     return Val.getPointer();
95   }
96 
97   /// markOverdefined - Return true if this is a change in status.
markOverdefined()98   bool markOverdefined() {
99     if (isOverdefined())
100       return false;
101 
102     Val.setInt(overdefined);
103     return true;
104   }
105 
106   /// markConstant - Return true if this is a change in status.
markConstant(Constant * V)107   bool markConstant(Constant *V) {
108     if (getLatticeValue() == constant) { // Constant but not forcedconstant.
109       assert(getConstant() == V && "Marking constant with different value");
110       return false;
111     }
112 
113     if (isUndefined()) {
114       Val.setInt(constant);
115       assert(V && "Marking constant with NULL");
116       Val.setPointer(V);
117     } else {
118       assert(getLatticeValue() == forcedconstant &&
119              "Cannot move from overdefined to constant!");
120       // Stay at forcedconstant if the constant is the same.
121       if (V == getConstant()) return false;
122 
123       // Otherwise, we go to overdefined.  Assumptions made based on the
124       // forced value are possibly wrong.  Assuming this is another constant
125       // could expose a contradiction.
126       Val.setInt(overdefined);
127     }
128     return true;
129   }
130 
131   /// getConstantInt - If this is a constant with a ConstantInt value, return it
132   /// otherwise return null.
getConstantInt() const133   ConstantInt *getConstantInt() const {
134     if (isConstant())
135       return dyn_cast<ConstantInt>(getConstant());
136     return 0;
137   }
138 
markForcedConstant(Constant * V)139   void markForcedConstant(Constant *V) {
140     assert(isUndefined() && "Can't force a defined value!");
141     Val.setInt(forcedconstant);
142     Val.setPointer(V);
143   }
144 };
145 } // end anonymous namespace.
146 
147 
148 namespace {
149 
150 //===----------------------------------------------------------------------===//
151 //
152 /// SCCPSolver - This class is a general purpose solver for Sparse Conditional
153 /// Constant Propagation.
154 ///
155 class SCCPSolver : public InstVisitor<SCCPSolver> {
156   const DataLayout *TD;
157   const TargetLibraryInfo *TLI;
158   SmallPtrSet<BasicBlock*, 8> BBExecutable; // The BBs that are executable.
159   DenseMap<Value*, LatticeVal> ValueState;  // The state each value is in.
160 
161   /// StructValueState - This maintains ValueState for values that have
162   /// StructType, for example for formal arguments, calls, insertelement, etc.
163   ///
164   DenseMap<std::pair<Value*, unsigned>, LatticeVal> StructValueState;
165 
166   /// GlobalValue - If we are tracking any values for the contents of a global
167   /// variable, we keep a mapping from the constant accessor to the element of
168   /// the global, to the currently known value.  If the value becomes
169   /// overdefined, it's entry is simply removed from this map.
170   DenseMap<GlobalVariable*, LatticeVal> TrackedGlobals;
171 
172   /// TrackedRetVals - If we are tracking arguments into and the return
173   /// value out of a function, it will have an entry in this map, indicating
174   /// what the known return value for the function is.
175   DenseMap<Function*, LatticeVal> TrackedRetVals;
176 
177   /// TrackedMultipleRetVals - Same as TrackedRetVals, but used for functions
178   /// that return multiple values.
179   DenseMap<std::pair<Function*, unsigned>, LatticeVal> TrackedMultipleRetVals;
180 
181   /// MRVFunctionsTracked - Each function in TrackedMultipleRetVals is
182   /// represented here for efficient lookup.
183   SmallPtrSet<Function*, 16> MRVFunctionsTracked;
184 
185   /// TrackingIncomingArguments - This is the set of functions for whose
186   /// arguments we make optimistic assumptions about and try to prove as
187   /// constants.
188   SmallPtrSet<Function*, 16> TrackingIncomingArguments;
189 
190   /// The reason for two worklists is that overdefined is the lowest state
191   /// on the lattice, and moving things to overdefined as fast as possible
192   /// makes SCCP converge much faster.
193   ///
194   /// By having a separate worklist, we accomplish this because everything
195   /// possibly overdefined will become overdefined at the soonest possible
196   /// point.
197   SmallVector<Value*, 64> OverdefinedInstWorkList;
198   SmallVector<Value*, 64> InstWorkList;
199 
200 
201   SmallVector<BasicBlock*, 64>  BBWorkList;  // The BasicBlock work list
202 
203   /// KnownFeasibleEdges - Entries in this set are edges which have already had
204   /// PHI nodes retriggered.
205   typedef std::pair<BasicBlock*, BasicBlock*> Edge;
206   DenseSet<Edge> KnownFeasibleEdges;
207 public:
SCCPSolver(const DataLayout * td,const TargetLibraryInfo * tli)208   SCCPSolver(const DataLayout *td, const TargetLibraryInfo *tli)
209     : TD(td), TLI(tli) {}
210 
211   /// MarkBlockExecutable - This method can be used by clients to mark all of
212   /// the blocks that are known to be intrinsically live in the processed unit.
213   ///
214   /// This returns true if the block was not considered live before.
MarkBlockExecutable(BasicBlock * BB)215   bool MarkBlockExecutable(BasicBlock *BB) {
216     if (!BBExecutable.insert(BB)) return false;
217     DEBUG(dbgs() << "Marking Block Executable: " << BB->getName() << '\n');
218     BBWorkList.push_back(BB);  // Add the block to the work list!
219     return true;
220   }
221 
222   /// TrackValueOfGlobalVariable - Clients can use this method to
223   /// inform the SCCPSolver that it should track loads and stores to the
224   /// specified global variable if it can.  This is only legal to call if
225   /// performing Interprocedural SCCP.
TrackValueOfGlobalVariable(GlobalVariable * GV)226   void TrackValueOfGlobalVariable(GlobalVariable *GV) {
227     // We only track the contents of scalar globals.
228     if (GV->getType()->getElementType()->isSingleValueType()) {
229       LatticeVal &IV = TrackedGlobals[GV];
230       if (!isa<UndefValue>(GV->getInitializer()))
231         IV.markConstant(GV->getInitializer());
232     }
233   }
234 
235   /// AddTrackedFunction - If the SCCP solver is supposed to track calls into
236   /// and out of the specified function (which cannot have its address taken),
237   /// this method must be called.
AddTrackedFunction(Function * F)238   void AddTrackedFunction(Function *F) {
239     // Add an entry, F -> undef.
240     if (StructType *STy = dyn_cast<StructType>(F->getReturnType())) {
241       MRVFunctionsTracked.insert(F);
242       for (unsigned i = 0, e = STy->getNumElements(); i != e; ++i)
243         TrackedMultipleRetVals.insert(std::make_pair(std::make_pair(F, i),
244                                                      LatticeVal()));
245     } else
246       TrackedRetVals.insert(std::make_pair(F, LatticeVal()));
247   }
248 
AddArgumentTrackedFunction(Function * F)249   void AddArgumentTrackedFunction(Function *F) {
250     TrackingIncomingArguments.insert(F);
251   }
252 
253   /// Solve - Solve for constants and executable blocks.
254   ///
255   void Solve();
256 
257   /// ResolvedUndefsIn - While solving the dataflow for a function, we assume
258   /// that branches on undef values cannot reach any of their successors.
259   /// However, this is not a safe assumption.  After we solve dataflow, this
260   /// method should be use to handle this.  If this returns true, the solver
261   /// should be rerun.
262   bool ResolvedUndefsIn(Function &F);
263 
isBlockExecutable(BasicBlock * BB) const264   bool isBlockExecutable(BasicBlock *BB) const {
265     return BBExecutable.count(BB);
266   }
267 
getLatticeValueFor(Value * V) const268   LatticeVal getLatticeValueFor(Value *V) const {
269     DenseMap<Value*, LatticeVal>::const_iterator I = ValueState.find(V);
270     assert(I != ValueState.end() && "V is not in valuemap!");
271     return I->second;
272   }
273 
274   /// getTrackedRetVals - Get the inferred return value map.
275   ///
getTrackedRetVals()276   const DenseMap<Function*, LatticeVal> &getTrackedRetVals() {
277     return TrackedRetVals;
278   }
279 
280   /// getTrackedGlobals - Get and return the set of inferred initializers for
281   /// global variables.
getTrackedGlobals()282   const DenseMap<GlobalVariable*, LatticeVal> &getTrackedGlobals() {
283     return TrackedGlobals;
284   }
285 
markOverdefined(Value * V)286   void markOverdefined(Value *V) {
287     assert(!V->getType()->isStructTy() && "Should use other method");
288     markOverdefined(ValueState[V], V);
289   }
290 
291   /// markAnythingOverdefined - Mark the specified value overdefined.  This
292   /// works with both scalars and structs.
markAnythingOverdefined(Value * V)293   void markAnythingOverdefined(Value *V) {
294     if (StructType *STy = dyn_cast<StructType>(V->getType()))
295       for (unsigned i = 0, e = STy->getNumElements(); i != e; ++i)
296         markOverdefined(getStructValueState(V, i), V);
297     else
298       markOverdefined(V);
299   }
300 
301 private:
302   // markConstant - Make a value be marked as "constant".  If the value
303   // is not already a constant, add it to the instruction work list so that
304   // the users of the instruction are updated later.
305   //
markConstant(LatticeVal & IV,Value * V,Constant * C)306   void markConstant(LatticeVal &IV, Value *V, Constant *C) {
307     if (!IV.markConstant(C)) return;
308     DEBUG(dbgs() << "markConstant: " << *C << ": " << *V << '\n');
309     if (IV.isOverdefined())
310       OverdefinedInstWorkList.push_back(V);
311     else
312       InstWorkList.push_back(V);
313   }
314 
markConstant(Value * V,Constant * C)315   void markConstant(Value *V, Constant *C) {
316     assert(!V->getType()->isStructTy() && "Should use other method");
317     markConstant(ValueState[V], V, C);
318   }
319 
markForcedConstant(Value * V,Constant * C)320   void markForcedConstant(Value *V, Constant *C) {
321     assert(!V->getType()->isStructTy() && "Should use other method");
322     LatticeVal &IV = ValueState[V];
323     IV.markForcedConstant(C);
324     DEBUG(dbgs() << "markForcedConstant: " << *C << ": " << *V << '\n');
325     if (IV.isOverdefined())
326       OverdefinedInstWorkList.push_back(V);
327     else
328       InstWorkList.push_back(V);
329   }
330 
331 
332   // markOverdefined - Make a value be marked as "overdefined". If the
333   // value is not already overdefined, add it to the overdefined instruction
334   // work list so that the users of the instruction are updated later.
markOverdefined(LatticeVal & IV,Value * V)335   void markOverdefined(LatticeVal &IV, Value *V) {
336     if (!IV.markOverdefined()) return;
337 
338     DEBUG(dbgs() << "markOverdefined: ";
339           if (Function *F = dyn_cast<Function>(V))
340             dbgs() << "Function '" << F->getName() << "'\n";
341           else
342             dbgs() << *V << '\n');
343     // Only instructions go on the work list
344     OverdefinedInstWorkList.push_back(V);
345   }
346 
mergeInValue(LatticeVal & IV,Value * V,LatticeVal MergeWithV)347   void mergeInValue(LatticeVal &IV, Value *V, LatticeVal MergeWithV) {
348     if (IV.isOverdefined() || MergeWithV.isUndefined())
349       return;  // Noop.
350     if (MergeWithV.isOverdefined())
351       markOverdefined(IV, V);
352     else if (IV.isUndefined())
353       markConstant(IV, V, MergeWithV.getConstant());
354     else if (IV.getConstant() != MergeWithV.getConstant())
355       markOverdefined(IV, V);
356   }
357 
mergeInValue(Value * V,LatticeVal MergeWithV)358   void mergeInValue(Value *V, LatticeVal MergeWithV) {
359     assert(!V->getType()->isStructTy() && "Should use other method");
360     mergeInValue(ValueState[V], V, MergeWithV);
361   }
362 
363 
364   /// getValueState - Return the LatticeVal object that corresponds to the
365   /// value.  This function handles the case when the value hasn't been seen yet
366   /// by properly seeding constants etc.
getValueState(Value * V)367   LatticeVal &getValueState(Value *V) {
368     assert(!V->getType()->isStructTy() && "Should use getStructValueState");
369 
370     std::pair<DenseMap<Value*, LatticeVal>::iterator, bool> I =
371       ValueState.insert(std::make_pair(V, LatticeVal()));
372     LatticeVal &LV = I.first->second;
373 
374     if (!I.second)
375       return LV;  // Common case, already in the map.
376 
377     if (Constant *C = dyn_cast<Constant>(V)) {
378       // Undef values remain undefined.
379       if (!isa<UndefValue>(V))
380         LV.markConstant(C);          // Constants are constant
381     }
382 
383     // All others are underdefined by default.
384     return LV;
385   }
386 
387   /// getStructValueState - Return the LatticeVal object that corresponds to the
388   /// value/field pair.  This function handles the case when the value hasn't
389   /// been seen yet by properly seeding constants etc.
getStructValueState(Value * V,unsigned i)390   LatticeVal &getStructValueState(Value *V, unsigned i) {
391     assert(V->getType()->isStructTy() && "Should use getValueState");
392     assert(i < cast<StructType>(V->getType())->getNumElements() &&
393            "Invalid element #");
394 
395     std::pair<DenseMap<std::pair<Value*, unsigned>, LatticeVal>::iterator,
396               bool> I = StructValueState.insert(
397                         std::make_pair(std::make_pair(V, i), LatticeVal()));
398     LatticeVal &LV = I.first->second;
399 
400     if (!I.second)
401       return LV;  // Common case, already in the map.
402 
403     if (Constant *C = dyn_cast<Constant>(V)) {
404       Constant *Elt = C->getAggregateElement(i);
405 
406       if (Elt == 0)
407         LV.markOverdefined();      // Unknown sort of constant.
408       else if (isa<UndefValue>(Elt))
409         ; // Undef values remain undefined.
410       else
411         LV.markConstant(Elt);      // Constants are constant.
412     }
413 
414     // All others are underdefined by default.
415     return LV;
416   }
417 
418 
419   /// markEdgeExecutable - Mark a basic block as executable, adding it to the BB
420   /// work list if it is not already executable.
markEdgeExecutable(BasicBlock * Source,BasicBlock * Dest)421   void markEdgeExecutable(BasicBlock *Source, BasicBlock *Dest) {
422     if (!KnownFeasibleEdges.insert(Edge(Source, Dest)).second)
423       return;  // This edge is already known to be executable!
424 
425     if (!MarkBlockExecutable(Dest)) {
426       // If the destination is already executable, we just made an *edge*
427       // feasible that wasn't before.  Revisit the PHI nodes in the block
428       // because they have potentially new operands.
429       DEBUG(dbgs() << "Marking Edge Executable: " << Source->getName()
430             << " -> " << Dest->getName() << '\n');
431 
432       PHINode *PN;
433       for (BasicBlock::iterator I = Dest->begin();
434            (PN = dyn_cast<PHINode>(I)); ++I)
435         visitPHINode(*PN);
436     }
437   }
438 
439   // getFeasibleSuccessors - Return a vector of booleans to indicate which
440   // successors are reachable from a given terminator instruction.
441   //
442   void getFeasibleSuccessors(TerminatorInst &TI, SmallVectorImpl<bool> &Succs);
443 
444   // isEdgeFeasible - Return true if the control flow edge from the 'From' basic
445   // block to the 'To' basic block is currently feasible.
446   //
447   bool isEdgeFeasible(BasicBlock *From, BasicBlock *To);
448 
449   // OperandChangedState - This method is invoked on all of the users of an
450   // instruction that was just changed state somehow.  Based on this
451   // information, we need to update the specified user of this instruction.
452   //
OperandChangedState(Instruction * I)453   void OperandChangedState(Instruction *I) {
454     if (BBExecutable.count(I->getParent()))   // Inst is executable?
455       visit(*I);
456   }
457 
458 private:
459   friend class InstVisitor<SCCPSolver>;
460 
461   // visit implementations - Something changed in this instruction.  Either an
462   // operand made a transition, or the instruction is newly executable.  Change
463   // the value type of I to reflect these changes if appropriate.
464   void visitPHINode(PHINode &I);
465 
466   // Terminators
467   void visitReturnInst(ReturnInst &I);
468   void visitTerminatorInst(TerminatorInst &TI);
469 
470   void visitCastInst(CastInst &I);
471   void visitSelectInst(SelectInst &I);
472   void visitBinaryOperator(Instruction &I);
473   void visitCmpInst(CmpInst &I);
474   void visitExtractElementInst(ExtractElementInst &I);
475   void visitInsertElementInst(InsertElementInst &I);
476   void visitShuffleVectorInst(ShuffleVectorInst &I);
477   void visitExtractValueInst(ExtractValueInst &EVI);
478   void visitInsertValueInst(InsertValueInst &IVI);
visitLandingPadInst(LandingPadInst & I)479   void visitLandingPadInst(LandingPadInst &I) { markAnythingOverdefined(&I); }
480 
481   // Instructions that cannot be folded away.
482   void visitStoreInst     (StoreInst &I);
483   void visitLoadInst      (LoadInst &I);
484   void visitGetElementPtrInst(GetElementPtrInst &I);
visitCallInst(CallInst & I)485   void visitCallInst      (CallInst &I) {
486     visitCallSite(&I);
487   }
visitInvokeInst(InvokeInst & II)488   void visitInvokeInst    (InvokeInst &II) {
489     visitCallSite(&II);
490     visitTerminatorInst(II);
491   }
492   void visitCallSite      (CallSite CS);
visitResumeInst(TerminatorInst & I)493   void visitResumeInst    (TerminatorInst &I) { /*returns void*/ }
visitUnwindInst(TerminatorInst & I)494   void visitUnwindInst    (TerminatorInst &I) { /*returns void*/ }
visitUnreachableInst(TerminatorInst & I)495   void visitUnreachableInst(TerminatorInst &I) { /*returns void*/ }
visitFenceInst(FenceInst & I)496   void visitFenceInst     (FenceInst &I) { /*returns void*/ }
visitAtomicCmpXchgInst(AtomicCmpXchgInst & I)497   void visitAtomicCmpXchgInst (AtomicCmpXchgInst &I) { markOverdefined(&I); }
visitAtomicRMWInst(AtomicRMWInst & I)498   void visitAtomicRMWInst (AtomicRMWInst &I) { markOverdefined(&I); }
visitAllocaInst(Instruction & I)499   void visitAllocaInst    (Instruction &I) { markOverdefined(&I); }
visitVAArgInst(Instruction & I)500   void visitVAArgInst     (Instruction &I) { markAnythingOverdefined(&I); }
501 
visitInstruction(Instruction & I)502   void visitInstruction(Instruction &I) {
503     // If a new instruction is added to LLVM that we don't handle.
504     dbgs() << "SCCP: Don't know how to handle: " << I << '\n';
505     markAnythingOverdefined(&I);   // Just in case
506   }
507 };
508 
509 } // end anonymous namespace
510 
511 
512 // getFeasibleSuccessors - Return a vector of booleans to indicate which
513 // successors are reachable from a given terminator instruction.
514 //
getFeasibleSuccessors(TerminatorInst & TI,SmallVectorImpl<bool> & Succs)515 void SCCPSolver::getFeasibleSuccessors(TerminatorInst &TI,
516                                        SmallVectorImpl<bool> &Succs) {
517   Succs.resize(TI.getNumSuccessors());
518   if (BranchInst *BI = dyn_cast<BranchInst>(&TI)) {
519     if (BI->isUnconditional()) {
520       Succs[0] = true;
521       return;
522     }
523 
524     LatticeVal BCValue = getValueState(BI->getCondition());
525     ConstantInt *CI = BCValue.getConstantInt();
526     if (CI == 0) {
527       // Overdefined condition variables, and branches on unfoldable constant
528       // conditions, mean the branch could go either way.
529       if (!BCValue.isUndefined())
530         Succs[0] = Succs[1] = true;
531       return;
532     }
533 
534     // Constant condition variables mean the branch can only go a single way.
535     Succs[CI->isZero()] = true;
536     return;
537   }
538 
539   if (isa<InvokeInst>(TI)) {
540     // Invoke instructions successors are always executable.
541     Succs[0] = Succs[1] = true;
542     return;
543   }
544 
545   if (SwitchInst *SI = dyn_cast<SwitchInst>(&TI)) {
546     if (!SI->getNumCases()) {
547       Succs[0] = true;
548       return;
549     }
550     LatticeVal SCValue = getValueState(SI->getCondition());
551     ConstantInt *CI = SCValue.getConstantInt();
552 
553     if (CI == 0) {   // Overdefined or undefined condition?
554       // All destinations are executable!
555       if (!SCValue.isUndefined())
556         Succs.assign(TI.getNumSuccessors(), true);
557       return;
558     }
559 
560     Succs[SI->findCaseValue(CI).getSuccessorIndex()] = true;
561     return;
562   }
563 
564   // TODO: This could be improved if the operand is a [cast of a] BlockAddress.
565   if (isa<IndirectBrInst>(&TI)) {
566     // Just mark all destinations executable!
567     Succs.assign(TI.getNumSuccessors(), true);
568     return;
569   }
570 
571 #ifndef NDEBUG
572   dbgs() << "Unknown terminator instruction: " << TI << '\n';
573 #endif
574   llvm_unreachable("SCCP: Don't know how to handle this terminator!");
575 }
576 
577 
578 // isEdgeFeasible - Return true if the control flow edge from the 'From' basic
579 // block to the 'To' basic block is currently feasible.
580 //
isEdgeFeasible(BasicBlock * From,BasicBlock * To)581 bool SCCPSolver::isEdgeFeasible(BasicBlock *From, BasicBlock *To) {
582   assert(BBExecutable.count(To) && "Dest should always be alive!");
583 
584   // Make sure the source basic block is executable!!
585   if (!BBExecutable.count(From)) return false;
586 
587   // Check to make sure this edge itself is actually feasible now.
588   TerminatorInst *TI = From->getTerminator();
589   if (BranchInst *BI = dyn_cast<BranchInst>(TI)) {
590     if (BI->isUnconditional())
591       return true;
592 
593     LatticeVal BCValue = getValueState(BI->getCondition());
594 
595     // Overdefined condition variables mean the branch could go either way,
596     // undef conditions mean that neither edge is feasible yet.
597     ConstantInt *CI = BCValue.getConstantInt();
598     if (CI == 0)
599       return !BCValue.isUndefined();
600 
601     // Constant condition variables mean the branch can only go a single way.
602     return BI->getSuccessor(CI->isZero()) == To;
603   }
604 
605   // Invoke instructions successors are always executable.
606   if (isa<InvokeInst>(TI))
607     return true;
608 
609   if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
610     if (SI->getNumCases() < 1)
611       return true;
612 
613     LatticeVal SCValue = getValueState(SI->getCondition());
614     ConstantInt *CI = SCValue.getConstantInt();
615 
616     if (CI == 0)
617       return !SCValue.isUndefined();
618 
619     return SI->findCaseValue(CI).getCaseSuccessor() == To;
620   }
621 
622   // Just mark all destinations executable!
623   // TODO: This could be improved if the operand is a [cast of a] BlockAddress.
624   if (isa<IndirectBrInst>(TI))
625     return true;
626 
627 #ifndef NDEBUG
628   dbgs() << "Unknown terminator instruction: " << *TI << '\n';
629 #endif
630   llvm_unreachable(0);
631 }
632 
633 // visit Implementations - Something changed in this instruction, either an
634 // operand made a transition, or the instruction is newly executable.  Change
635 // the value type of I to reflect these changes if appropriate.  This method
636 // makes sure to do the following actions:
637 //
638 // 1. If a phi node merges two constants in, and has conflicting value coming
639 //    from different branches, or if the PHI node merges in an overdefined
640 //    value, then the PHI node becomes overdefined.
641 // 2. If a phi node merges only constants in, and they all agree on value, the
642 //    PHI node becomes a constant value equal to that.
643 // 3. If V <- x (op) y && isConstant(x) && isConstant(y) V = Constant
644 // 4. If V <- x (op) y && (isOverdefined(x) || isOverdefined(y)) V = Overdefined
645 // 5. If V <- MEM or V <- CALL or V <- (unknown) then V = Overdefined
646 // 6. If a conditional branch has a value that is constant, make the selected
647 //    destination executable
648 // 7. If a conditional branch has a value that is overdefined, make all
649 //    successors executable.
650 //
visitPHINode(PHINode & PN)651 void SCCPSolver::visitPHINode(PHINode &PN) {
652   // If this PN returns a struct, just mark the result overdefined.
653   // TODO: We could do a lot better than this if code actually uses this.
654   if (PN.getType()->isStructTy())
655     return markAnythingOverdefined(&PN);
656 
657   if (getValueState(&PN).isOverdefined())
658     return;  // Quick exit
659 
660   // Super-extra-high-degree PHI nodes are unlikely to ever be marked constant,
661   // and slow us down a lot.  Just mark them overdefined.
662   if (PN.getNumIncomingValues() > 64)
663     return markOverdefined(&PN);
664 
665   // Look at all of the executable operands of the PHI node.  If any of them
666   // are overdefined, the PHI becomes overdefined as well.  If they are all
667   // constant, and they agree with each other, the PHI becomes the identical
668   // constant.  If they are constant and don't agree, the PHI is overdefined.
669   // If there are no executable operands, the PHI remains undefined.
670   //
671   Constant *OperandVal = 0;
672   for (unsigned i = 0, e = PN.getNumIncomingValues(); i != e; ++i) {
673     LatticeVal IV = getValueState(PN.getIncomingValue(i));
674     if (IV.isUndefined()) continue;  // Doesn't influence PHI node.
675 
676     if (!isEdgeFeasible(PN.getIncomingBlock(i), PN.getParent()))
677       continue;
678 
679     if (IV.isOverdefined())    // PHI node becomes overdefined!
680       return markOverdefined(&PN);
681 
682     if (OperandVal == 0) {   // Grab the first value.
683       OperandVal = IV.getConstant();
684       continue;
685     }
686 
687     // There is already a reachable operand.  If we conflict with it,
688     // then the PHI node becomes overdefined.  If we agree with it, we
689     // can continue on.
690 
691     // Check to see if there are two different constants merging, if so, the PHI
692     // node is overdefined.
693     if (IV.getConstant() != OperandVal)
694       return markOverdefined(&PN);
695   }
696 
697   // If we exited the loop, this means that the PHI node only has constant
698   // arguments that agree with each other(and OperandVal is the constant) or
699   // OperandVal is null because there are no defined incoming arguments.  If
700   // this is the case, the PHI remains undefined.
701   //
702   if (OperandVal)
703     markConstant(&PN, OperandVal);      // Acquire operand value
704 }
705 
visitReturnInst(ReturnInst & I)706 void SCCPSolver::visitReturnInst(ReturnInst &I) {
707   if (I.getNumOperands() == 0) return;  // ret void
708 
709   Function *F = I.getParent()->getParent();
710   Value *ResultOp = I.getOperand(0);
711 
712   // If we are tracking the return value of this function, merge it in.
713   if (!TrackedRetVals.empty() && !ResultOp->getType()->isStructTy()) {
714     DenseMap<Function*, LatticeVal>::iterator TFRVI =
715       TrackedRetVals.find(F);
716     if (TFRVI != TrackedRetVals.end()) {
717       mergeInValue(TFRVI->second, F, getValueState(ResultOp));
718       return;
719     }
720   }
721 
722   // Handle functions that return multiple values.
723   if (!TrackedMultipleRetVals.empty()) {
724     if (StructType *STy = dyn_cast<StructType>(ResultOp->getType()))
725       if (MRVFunctionsTracked.count(F))
726         for (unsigned i = 0, e = STy->getNumElements(); i != e; ++i)
727           mergeInValue(TrackedMultipleRetVals[std::make_pair(F, i)], F,
728                        getStructValueState(ResultOp, i));
729 
730   }
731 }
732 
visitTerminatorInst(TerminatorInst & TI)733 void SCCPSolver::visitTerminatorInst(TerminatorInst &TI) {
734   SmallVector<bool, 16> SuccFeasible;
735   getFeasibleSuccessors(TI, SuccFeasible);
736 
737   BasicBlock *BB = TI.getParent();
738 
739   // Mark all feasible successors executable.
740   for (unsigned i = 0, e = SuccFeasible.size(); i != e; ++i)
741     if (SuccFeasible[i])
742       markEdgeExecutable(BB, TI.getSuccessor(i));
743 }
744 
visitCastInst(CastInst & I)745 void SCCPSolver::visitCastInst(CastInst &I) {
746   LatticeVal OpSt = getValueState(I.getOperand(0));
747   if (OpSt.isOverdefined())          // Inherit overdefinedness of operand
748     markOverdefined(&I);
749   else if (OpSt.isConstant())        // Propagate constant value
750     markConstant(&I, ConstantExpr::getCast(I.getOpcode(),
751                                            OpSt.getConstant(), I.getType()));
752 }
753 
754 
visitExtractValueInst(ExtractValueInst & EVI)755 void SCCPSolver::visitExtractValueInst(ExtractValueInst &EVI) {
756   // If this returns a struct, mark all elements over defined, we don't track
757   // structs in structs.
758   if (EVI.getType()->isStructTy())
759     return markAnythingOverdefined(&EVI);
760 
761   // If this is extracting from more than one level of struct, we don't know.
762   if (EVI.getNumIndices() != 1)
763     return markOverdefined(&EVI);
764 
765   Value *AggVal = EVI.getAggregateOperand();
766   if (AggVal->getType()->isStructTy()) {
767     unsigned i = *EVI.idx_begin();
768     LatticeVal EltVal = getStructValueState(AggVal, i);
769     mergeInValue(getValueState(&EVI), &EVI, EltVal);
770   } else {
771     // Otherwise, must be extracting from an array.
772     return markOverdefined(&EVI);
773   }
774 }
775 
visitInsertValueInst(InsertValueInst & IVI)776 void SCCPSolver::visitInsertValueInst(InsertValueInst &IVI) {
777   StructType *STy = dyn_cast<StructType>(IVI.getType());
778   if (STy == 0)
779     return markOverdefined(&IVI);
780 
781   // If this has more than one index, we can't handle it, drive all results to
782   // undef.
783   if (IVI.getNumIndices() != 1)
784     return markAnythingOverdefined(&IVI);
785 
786   Value *Aggr = IVI.getAggregateOperand();
787   unsigned Idx = *IVI.idx_begin();
788 
789   // Compute the result based on what we're inserting.
790   for (unsigned i = 0, e = STy->getNumElements(); i != e; ++i) {
791     // This passes through all values that aren't the inserted element.
792     if (i != Idx) {
793       LatticeVal EltVal = getStructValueState(Aggr, i);
794       mergeInValue(getStructValueState(&IVI, i), &IVI, EltVal);
795       continue;
796     }
797 
798     Value *Val = IVI.getInsertedValueOperand();
799     if (Val->getType()->isStructTy())
800       // We don't track structs in structs.
801       markOverdefined(getStructValueState(&IVI, i), &IVI);
802     else {
803       LatticeVal InVal = getValueState(Val);
804       mergeInValue(getStructValueState(&IVI, i), &IVI, InVal);
805     }
806   }
807 }
808 
visitSelectInst(SelectInst & I)809 void SCCPSolver::visitSelectInst(SelectInst &I) {
810   // If this select returns a struct, just mark the result overdefined.
811   // TODO: We could do a lot better than this if code actually uses this.
812   if (I.getType()->isStructTy())
813     return markAnythingOverdefined(&I);
814 
815   LatticeVal CondValue = getValueState(I.getCondition());
816   if (CondValue.isUndefined())
817     return;
818 
819   if (ConstantInt *CondCB = CondValue.getConstantInt()) {
820     Value *OpVal = CondCB->isZero() ? I.getFalseValue() : I.getTrueValue();
821     mergeInValue(&I, getValueState(OpVal));
822     return;
823   }
824 
825   // Otherwise, the condition is overdefined or a constant we can't evaluate.
826   // See if we can produce something better than overdefined based on the T/F
827   // value.
828   LatticeVal TVal = getValueState(I.getTrueValue());
829   LatticeVal FVal = getValueState(I.getFalseValue());
830 
831   // select ?, C, C -> C.
832   if (TVal.isConstant() && FVal.isConstant() &&
833       TVal.getConstant() == FVal.getConstant())
834     return markConstant(&I, FVal.getConstant());
835 
836   if (TVal.isUndefined())   // select ?, undef, X -> X.
837     return mergeInValue(&I, FVal);
838   if (FVal.isUndefined())   // select ?, X, undef -> X.
839     return mergeInValue(&I, TVal);
840   markOverdefined(&I);
841 }
842 
843 // Handle Binary Operators.
visitBinaryOperator(Instruction & I)844 void SCCPSolver::visitBinaryOperator(Instruction &I) {
845   LatticeVal V1State = getValueState(I.getOperand(0));
846   LatticeVal V2State = getValueState(I.getOperand(1));
847 
848   LatticeVal &IV = ValueState[&I];
849   if (IV.isOverdefined()) return;
850 
851   if (V1State.isConstant() && V2State.isConstant())
852     return markConstant(IV, &I,
853                         ConstantExpr::get(I.getOpcode(), V1State.getConstant(),
854                                           V2State.getConstant()));
855 
856   // If something is undef, wait for it to resolve.
857   if (!V1State.isOverdefined() && !V2State.isOverdefined())
858     return;
859 
860   // Otherwise, one of our operands is overdefined.  Try to produce something
861   // better than overdefined with some tricks.
862 
863   // If this is an AND or OR with 0 or -1, it doesn't matter that the other
864   // operand is overdefined.
865   if (I.getOpcode() == Instruction::And || I.getOpcode() == Instruction::Or) {
866     LatticeVal *NonOverdefVal = 0;
867     if (!V1State.isOverdefined())
868       NonOverdefVal = &V1State;
869     else if (!V2State.isOverdefined())
870       NonOverdefVal = &V2State;
871 
872     if (NonOverdefVal) {
873       if (NonOverdefVal->isUndefined()) {
874         // Could annihilate value.
875         if (I.getOpcode() == Instruction::And)
876           markConstant(IV, &I, Constant::getNullValue(I.getType()));
877         else if (VectorType *PT = dyn_cast<VectorType>(I.getType()))
878           markConstant(IV, &I, Constant::getAllOnesValue(PT));
879         else
880           markConstant(IV, &I,
881                        Constant::getAllOnesValue(I.getType()));
882         return;
883       }
884 
885       if (I.getOpcode() == Instruction::And) {
886         // X and 0 = 0
887         if (NonOverdefVal->getConstant()->isNullValue())
888           return markConstant(IV, &I, NonOverdefVal->getConstant());
889       } else {
890         if (ConstantInt *CI = NonOverdefVal->getConstantInt())
891           if (CI->isAllOnesValue())     // X or -1 = -1
892             return markConstant(IV, &I, NonOverdefVal->getConstant());
893       }
894     }
895   }
896 
897 
898   markOverdefined(&I);
899 }
900 
901 // Handle ICmpInst instruction.
visitCmpInst(CmpInst & I)902 void SCCPSolver::visitCmpInst(CmpInst &I) {
903   LatticeVal V1State = getValueState(I.getOperand(0));
904   LatticeVal V2State = getValueState(I.getOperand(1));
905 
906   LatticeVal &IV = ValueState[&I];
907   if (IV.isOverdefined()) return;
908 
909   if (V1State.isConstant() && V2State.isConstant())
910     return markConstant(IV, &I, ConstantExpr::getCompare(I.getPredicate(),
911                                                          V1State.getConstant(),
912                                                         V2State.getConstant()));
913 
914   // If operands are still undefined, wait for it to resolve.
915   if (!V1State.isOverdefined() && !V2State.isOverdefined())
916     return;
917 
918   markOverdefined(&I);
919 }
920 
visitExtractElementInst(ExtractElementInst & I)921 void SCCPSolver::visitExtractElementInst(ExtractElementInst &I) {
922   // TODO : SCCP does not handle vectors properly.
923   return markOverdefined(&I);
924 
925 #if 0
926   LatticeVal &ValState = getValueState(I.getOperand(0));
927   LatticeVal &IdxState = getValueState(I.getOperand(1));
928 
929   if (ValState.isOverdefined() || IdxState.isOverdefined())
930     markOverdefined(&I);
931   else if(ValState.isConstant() && IdxState.isConstant())
932     markConstant(&I, ConstantExpr::getExtractElement(ValState.getConstant(),
933                                                      IdxState.getConstant()));
934 #endif
935 }
936 
visitInsertElementInst(InsertElementInst & I)937 void SCCPSolver::visitInsertElementInst(InsertElementInst &I) {
938   // TODO : SCCP does not handle vectors properly.
939   return markOverdefined(&I);
940 #if 0
941   LatticeVal &ValState = getValueState(I.getOperand(0));
942   LatticeVal &EltState = getValueState(I.getOperand(1));
943   LatticeVal &IdxState = getValueState(I.getOperand(2));
944 
945   if (ValState.isOverdefined() || EltState.isOverdefined() ||
946       IdxState.isOverdefined())
947     markOverdefined(&I);
948   else if(ValState.isConstant() && EltState.isConstant() &&
949           IdxState.isConstant())
950     markConstant(&I, ConstantExpr::getInsertElement(ValState.getConstant(),
951                                                     EltState.getConstant(),
952                                                     IdxState.getConstant()));
953   else if (ValState.isUndefined() && EltState.isConstant() &&
954            IdxState.isConstant())
955     markConstant(&I,ConstantExpr::getInsertElement(UndefValue::get(I.getType()),
956                                                    EltState.getConstant(),
957                                                    IdxState.getConstant()));
958 #endif
959 }
960 
visitShuffleVectorInst(ShuffleVectorInst & I)961 void SCCPSolver::visitShuffleVectorInst(ShuffleVectorInst &I) {
962   // TODO : SCCP does not handle vectors properly.
963   return markOverdefined(&I);
964 #if 0
965   LatticeVal &V1State   = getValueState(I.getOperand(0));
966   LatticeVal &V2State   = getValueState(I.getOperand(1));
967   LatticeVal &MaskState = getValueState(I.getOperand(2));
968 
969   if (MaskState.isUndefined() ||
970       (V1State.isUndefined() && V2State.isUndefined()))
971     return;  // Undefined output if mask or both inputs undefined.
972 
973   if (V1State.isOverdefined() || V2State.isOverdefined() ||
974       MaskState.isOverdefined()) {
975     markOverdefined(&I);
976   } else {
977     // A mix of constant/undef inputs.
978     Constant *V1 = V1State.isConstant() ?
979         V1State.getConstant() : UndefValue::get(I.getType());
980     Constant *V2 = V2State.isConstant() ?
981         V2State.getConstant() : UndefValue::get(I.getType());
982     Constant *Mask = MaskState.isConstant() ?
983       MaskState.getConstant() : UndefValue::get(I.getOperand(2)->getType());
984     markConstant(&I, ConstantExpr::getShuffleVector(V1, V2, Mask));
985   }
986 #endif
987 }
988 
989 // Handle getelementptr instructions.  If all operands are constants then we
990 // can turn this into a getelementptr ConstantExpr.
991 //
visitGetElementPtrInst(GetElementPtrInst & I)992 void SCCPSolver::visitGetElementPtrInst(GetElementPtrInst &I) {
993   if (ValueState[&I].isOverdefined()) return;
994 
995   SmallVector<Constant*, 8> Operands;
996   Operands.reserve(I.getNumOperands());
997 
998   for (unsigned i = 0, e = I.getNumOperands(); i != e; ++i) {
999     LatticeVal State = getValueState(I.getOperand(i));
1000     if (State.isUndefined())
1001       return;  // Operands are not resolved yet.
1002 
1003     if (State.isOverdefined())
1004       return markOverdefined(&I);
1005 
1006     assert(State.isConstant() && "Unknown state!");
1007     Operands.push_back(State.getConstant());
1008   }
1009 
1010   Constant *Ptr = Operands[0];
1011   ArrayRef<Constant *> Indices(Operands.begin() + 1, Operands.end());
1012   markConstant(&I, ConstantExpr::getGetElementPtr(Ptr, Indices));
1013 }
1014 
visitStoreInst(StoreInst & SI)1015 void SCCPSolver::visitStoreInst(StoreInst &SI) {
1016   // If this store is of a struct, ignore it.
1017   if (SI.getOperand(0)->getType()->isStructTy())
1018     return;
1019 
1020   if (TrackedGlobals.empty() || !isa<GlobalVariable>(SI.getOperand(1)))
1021     return;
1022 
1023   GlobalVariable *GV = cast<GlobalVariable>(SI.getOperand(1));
1024   DenseMap<GlobalVariable*, LatticeVal>::iterator I = TrackedGlobals.find(GV);
1025   if (I == TrackedGlobals.end() || I->second.isOverdefined()) return;
1026 
1027   // Get the value we are storing into the global, then merge it.
1028   mergeInValue(I->second, GV, getValueState(SI.getOperand(0)));
1029   if (I->second.isOverdefined())
1030     TrackedGlobals.erase(I);      // No need to keep tracking this!
1031 }
1032 
1033 
1034 // Handle load instructions.  If the operand is a constant pointer to a constant
1035 // global, we can replace the load with the loaded constant value!
visitLoadInst(LoadInst & I)1036 void SCCPSolver::visitLoadInst(LoadInst &I) {
1037   // If this load is of a struct, just mark the result overdefined.
1038   if (I.getType()->isStructTy())
1039     return markAnythingOverdefined(&I);
1040 
1041   LatticeVal PtrVal = getValueState(I.getOperand(0));
1042   if (PtrVal.isUndefined()) return;   // The pointer is not resolved yet!
1043 
1044   LatticeVal &IV = ValueState[&I];
1045   if (IV.isOverdefined()) return;
1046 
1047   if (!PtrVal.isConstant() || I.isVolatile())
1048     return markOverdefined(IV, &I);
1049 
1050   Constant *Ptr = PtrVal.getConstant();
1051 
1052   // load null -> null
1053   if (isa<ConstantPointerNull>(Ptr) && I.getPointerAddressSpace() == 0)
1054     return markConstant(IV, &I, Constant::getNullValue(I.getType()));
1055 
1056   // Transform load (constant global) into the value loaded.
1057   if (GlobalVariable *GV = dyn_cast<GlobalVariable>(Ptr)) {
1058     if (!TrackedGlobals.empty()) {
1059       // If we are tracking this global, merge in the known value for it.
1060       DenseMap<GlobalVariable*, LatticeVal>::iterator It =
1061         TrackedGlobals.find(GV);
1062       if (It != TrackedGlobals.end()) {
1063         mergeInValue(IV, &I, It->second);
1064         return;
1065       }
1066     }
1067   }
1068 
1069   // Transform load from a constant into a constant if possible.
1070   if (Constant *C = ConstantFoldLoadFromConstPtr(Ptr, TD))
1071     return markConstant(IV, &I, C);
1072 
1073   // Otherwise we cannot say for certain what value this load will produce.
1074   // Bail out.
1075   markOverdefined(IV, &I);
1076 }
1077 
visitCallSite(CallSite CS)1078 void SCCPSolver::visitCallSite(CallSite CS) {
1079   Function *F = CS.getCalledFunction();
1080   Instruction *I = CS.getInstruction();
1081 
1082   // The common case is that we aren't tracking the callee, either because we
1083   // are not doing interprocedural analysis or the callee is indirect, or is
1084   // external.  Handle these cases first.
1085   if (F == 0 || F->isDeclaration()) {
1086 CallOverdefined:
1087     // Void return and not tracking callee, just bail.
1088     if (I->getType()->isVoidTy()) return;
1089 
1090     // Otherwise, if we have a single return value case, and if the function is
1091     // a declaration, maybe we can constant fold it.
1092     if (F && F->isDeclaration() && !I->getType()->isStructTy() &&
1093         canConstantFoldCallTo(F)) {
1094 
1095       SmallVector<Constant*, 8> Operands;
1096       for (CallSite::arg_iterator AI = CS.arg_begin(), E = CS.arg_end();
1097            AI != E; ++AI) {
1098         LatticeVal State = getValueState(*AI);
1099 
1100         if (State.isUndefined())
1101           return;  // Operands are not resolved yet.
1102         if (State.isOverdefined())
1103           return markOverdefined(I);
1104         assert(State.isConstant() && "Unknown state!");
1105         Operands.push_back(State.getConstant());
1106       }
1107 
1108       // If we can constant fold this, mark the result of the call as a
1109       // constant.
1110       if (Constant *C = ConstantFoldCall(F, Operands, TLI))
1111         return markConstant(I, C);
1112     }
1113 
1114     // Otherwise, we don't know anything about this call, mark it overdefined.
1115     return markAnythingOverdefined(I);
1116   }
1117 
1118   // If this is a local function that doesn't have its address taken, mark its
1119   // entry block executable and merge in the actual arguments to the call into
1120   // the formal arguments of the function.
1121   if (!TrackingIncomingArguments.empty() && TrackingIncomingArguments.count(F)){
1122     MarkBlockExecutable(F->begin());
1123 
1124     // Propagate information from this call site into the callee.
1125     CallSite::arg_iterator CAI = CS.arg_begin();
1126     for (Function::arg_iterator AI = F->arg_begin(), E = F->arg_end();
1127          AI != E; ++AI, ++CAI) {
1128       // If this argument is byval, and if the function is not readonly, there
1129       // will be an implicit copy formed of the input aggregate.
1130       if (AI->hasByValAttr() && !F->onlyReadsMemory()) {
1131         markOverdefined(AI);
1132         continue;
1133       }
1134 
1135       if (StructType *STy = dyn_cast<StructType>(AI->getType())) {
1136         for (unsigned i = 0, e = STy->getNumElements(); i != e; ++i) {
1137           LatticeVal CallArg = getStructValueState(*CAI, i);
1138           mergeInValue(getStructValueState(AI, i), AI, CallArg);
1139         }
1140       } else {
1141         mergeInValue(AI, getValueState(*CAI));
1142       }
1143     }
1144   }
1145 
1146   // If this is a single/zero retval case, see if we're tracking the function.
1147   if (StructType *STy = dyn_cast<StructType>(F->getReturnType())) {
1148     if (!MRVFunctionsTracked.count(F))
1149       goto CallOverdefined;  // Not tracking this callee.
1150 
1151     // If we are tracking this callee, propagate the result of the function
1152     // into this call site.
1153     for (unsigned i = 0, e = STy->getNumElements(); i != e; ++i)
1154       mergeInValue(getStructValueState(I, i), I,
1155                    TrackedMultipleRetVals[std::make_pair(F, i)]);
1156   } else {
1157     DenseMap<Function*, LatticeVal>::iterator TFRVI = TrackedRetVals.find(F);
1158     if (TFRVI == TrackedRetVals.end())
1159       goto CallOverdefined;  // Not tracking this callee.
1160 
1161     // If so, propagate the return value of the callee into this call result.
1162     mergeInValue(I, TFRVI->second);
1163   }
1164 }
1165 
Solve()1166 void SCCPSolver::Solve() {
1167   // Process the work lists until they are empty!
1168   while (!BBWorkList.empty() || !InstWorkList.empty() ||
1169          !OverdefinedInstWorkList.empty()) {
1170     // Process the overdefined instruction's work list first, which drives other
1171     // things to overdefined more quickly.
1172     while (!OverdefinedInstWorkList.empty()) {
1173       Value *I = OverdefinedInstWorkList.pop_back_val();
1174 
1175       DEBUG(dbgs() << "\nPopped off OI-WL: " << *I << '\n');
1176 
1177       // "I" got into the work list because it either made the transition from
1178       // bottom to constant, or to overdefined.
1179       //
1180       // Anything on this worklist that is overdefined need not be visited
1181       // since all of its users will have already been marked as overdefined
1182       // Update all of the users of this instruction's value.
1183       //
1184       for (Value::use_iterator UI = I->use_begin(), E = I->use_end();
1185            UI != E; ++UI)
1186         if (Instruction *I = dyn_cast<Instruction>(*UI))
1187           OperandChangedState(I);
1188     }
1189 
1190     // Process the instruction work list.
1191     while (!InstWorkList.empty()) {
1192       Value *I = InstWorkList.pop_back_val();
1193 
1194       DEBUG(dbgs() << "\nPopped off I-WL: " << *I << '\n');
1195 
1196       // "I" got into the work list because it made the transition from undef to
1197       // constant.
1198       //
1199       // Anything on this worklist that is overdefined need not be visited
1200       // since all of its users will have already been marked as overdefined.
1201       // Update all of the users of this instruction's value.
1202       //
1203       if (I->getType()->isStructTy() || !getValueState(I).isOverdefined())
1204         for (Value::use_iterator UI = I->use_begin(), E = I->use_end();
1205              UI != E; ++UI)
1206           if (Instruction *I = dyn_cast<Instruction>(*UI))
1207             OperandChangedState(I);
1208     }
1209 
1210     // Process the basic block work list.
1211     while (!BBWorkList.empty()) {
1212       BasicBlock *BB = BBWorkList.back();
1213       BBWorkList.pop_back();
1214 
1215       DEBUG(dbgs() << "\nPopped off BBWL: " << *BB << '\n');
1216 
1217       // Notify all instructions in this basic block that they are newly
1218       // executable.
1219       visit(BB);
1220     }
1221   }
1222 }
1223 
1224 /// ResolvedUndefsIn - While solving the dataflow for a function, we assume
1225 /// that branches on undef values cannot reach any of their successors.
1226 /// However, this is not a safe assumption.  After we solve dataflow, this
1227 /// method should be use to handle this.  If this returns true, the solver
1228 /// should be rerun.
1229 ///
1230 /// This method handles this by finding an unresolved branch and marking it one
1231 /// of the edges from the block as being feasible, even though the condition
1232 /// doesn't say it would otherwise be.  This allows SCCP to find the rest of the
1233 /// CFG and only slightly pessimizes the analysis results (by marking one,
1234 /// potentially infeasible, edge feasible).  This cannot usefully modify the
1235 /// constraints on the condition of the branch, as that would impact other users
1236 /// of the value.
1237 ///
1238 /// This scan also checks for values that use undefs, whose results are actually
1239 /// defined.  For example, 'zext i8 undef to i32' should produce all zeros
1240 /// conservatively, as "(zext i8 X -> i32) & 0xFF00" must always return zero,
1241 /// even if X isn't defined.
ResolvedUndefsIn(Function & F)1242 bool SCCPSolver::ResolvedUndefsIn(Function &F) {
1243   for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB) {
1244     if (!BBExecutable.count(BB))
1245       continue;
1246 
1247     for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) {
1248       // Look for instructions which produce undef values.
1249       if (I->getType()->isVoidTy()) continue;
1250 
1251       if (StructType *STy = dyn_cast<StructType>(I->getType())) {
1252         // Only a few things that can be structs matter for undef.
1253 
1254         // Tracked calls must never be marked overdefined in ResolvedUndefsIn.
1255         if (CallSite CS = CallSite(I))
1256           if (Function *F = CS.getCalledFunction())
1257             if (MRVFunctionsTracked.count(F))
1258               continue;
1259 
1260         // extractvalue and insertvalue don't need to be marked; they are
1261         // tracked as precisely as their operands.
1262         if (isa<ExtractValueInst>(I) || isa<InsertValueInst>(I))
1263           continue;
1264 
1265         // Send the results of everything else to overdefined.  We could be
1266         // more precise than this but it isn't worth bothering.
1267         for (unsigned i = 0, e = STy->getNumElements(); i != e; ++i) {
1268           LatticeVal &LV = getStructValueState(I, i);
1269           if (LV.isUndefined())
1270             markOverdefined(LV, I);
1271         }
1272         continue;
1273       }
1274 
1275       LatticeVal &LV = getValueState(I);
1276       if (!LV.isUndefined()) continue;
1277 
1278       // extractvalue is safe; check here because the argument is a struct.
1279       if (isa<ExtractValueInst>(I))
1280         continue;
1281 
1282       // Compute the operand LatticeVals, for convenience below.
1283       // Anything taking a struct is conservatively assumed to require
1284       // overdefined markings.
1285       if (I->getOperand(0)->getType()->isStructTy()) {
1286         markOverdefined(I);
1287         return true;
1288       }
1289       LatticeVal Op0LV = getValueState(I->getOperand(0));
1290       LatticeVal Op1LV;
1291       if (I->getNumOperands() == 2) {
1292         if (I->getOperand(1)->getType()->isStructTy()) {
1293           markOverdefined(I);
1294           return true;
1295         }
1296 
1297         Op1LV = getValueState(I->getOperand(1));
1298       }
1299       // If this is an instructions whose result is defined even if the input is
1300       // not fully defined, propagate the information.
1301       Type *ITy = I->getType();
1302       switch (I->getOpcode()) {
1303       case Instruction::Add:
1304       case Instruction::Sub:
1305       case Instruction::Trunc:
1306       case Instruction::FPTrunc:
1307       case Instruction::BitCast:
1308         break; // Any undef -> undef
1309       case Instruction::FSub:
1310       case Instruction::FAdd:
1311       case Instruction::FMul:
1312       case Instruction::FDiv:
1313       case Instruction::FRem:
1314         // Floating-point binary operation: be conservative.
1315         if (Op0LV.isUndefined() && Op1LV.isUndefined())
1316           markForcedConstant(I, Constant::getNullValue(ITy));
1317         else
1318           markOverdefined(I);
1319         return true;
1320       case Instruction::ZExt:
1321       case Instruction::SExt:
1322       case Instruction::FPToUI:
1323       case Instruction::FPToSI:
1324       case Instruction::FPExt:
1325       case Instruction::PtrToInt:
1326       case Instruction::IntToPtr:
1327       case Instruction::SIToFP:
1328       case Instruction::UIToFP:
1329         // undef -> 0; some outputs are impossible
1330         markForcedConstant(I, Constant::getNullValue(ITy));
1331         return true;
1332       case Instruction::Mul:
1333       case Instruction::And:
1334         // Both operands undef -> undef
1335         if (Op0LV.isUndefined() && Op1LV.isUndefined())
1336           break;
1337         // undef * X -> 0.   X could be zero.
1338         // undef & X -> 0.   X could be zero.
1339         markForcedConstant(I, Constant::getNullValue(ITy));
1340         return true;
1341 
1342       case Instruction::Or:
1343         // Both operands undef -> undef
1344         if (Op0LV.isUndefined() && Op1LV.isUndefined())
1345           break;
1346         // undef | X -> -1.   X could be -1.
1347         markForcedConstant(I, Constant::getAllOnesValue(ITy));
1348         return true;
1349 
1350       case Instruction::Xor:
1351         // undef ^ undef -> 0; strictly speaking, this is not strictly
1352         // necessary, but we try to be nice to people who expect this
1353         // behavior in simple cases
1354         if (Op0LV.isUndefined() && Op1LV.isUndefined()) {
1355           markForcedConstant(I, Constant::getNullValue(ITy));
1356           return true;
1357         }
1358         // undef ^ X -> undef
1359         break;
1360 
1361       case Instruction::SDiv:
1362       case Instruction::UDiv:
1363       case Instruction::SRem:
1364       case Instruction::URem:
1365         // X / undef -> undef.  No change.
1366         // X % undef -> undef.  No change.
1367         if (Op1LV.isUndefined()) break;
1368 
1369         // undef / X -> 0.   X could be maxint.
1370         // undef % X -> 0.   X could be 1.
1371         markForcedConstant(I, Constant::getNullValue(ITy));
1372         return true;
1373 
1374       case Instruction::AShr:
1375         // X >>a undef -> undef.
1376         if (Op1LV.isUndefined()) break;
1377 
1378         // undef >>a X -> all ones
1379         markForcedConstant(I, Constant::getAllOnesValue(ITy));
1380         return true;
1381       case Instruction::LShr:
1382       case Instruction::Shl:
1383         // X << undef -> undef.
1384         // X >> undef -> undef.
1385         if (Op1LV.isUndefined()) break;
1386 
1387         // undef << X -> 0
1388         // undef >> X -> 0
1389         markForcedConstant(I, Constant::getNullValue(ITy));
1390         return true;
1391       case Instruction::Select:
1392         Op1LV = getValueState(I->getOperand(1));
1393         // undef ? X : Y  -> X or Y.  There could be commonality between X/Y.
1394         if (Op0LV.isUndefined()) {
1395           if (!Op1LV.isConstant())  // Pick the constant one if there is any.
1396             Op1LV = getValueState(I->getOperand(2));
1397         } else if (Op1LV.isUndefined()) {
1398           // c ? undef : undef -> undef.  No change.
1399           Op1LV = getValueState(I->getOperand(2));
1400           if (Op1LV.isUndefined())
1401             break;
1402           // Otherwise, c ? undef : x -> x.
1403         } else {
1404           // Leave Op1LV as Operand(1)'s LatticeValue.
1405         }
1406 
1407         if (Op1LV.isConstant())
1408           markForcedConstant(I, Op1LV.getConstant());
1409         else
1410           markOverdefined(I);
1411         return true;
1412       case Instruction::Load:
1413         // A load here means one of two things: a load of undef from a global,
1414         // a load from an unknown pointer.  Either way, having it return undef
1415         // is okay.
1416         break;
1417       case Instruction::ICmp:
1418         // X == undef -> undef.  Other comparisons get more complicated.
1419         if (cast<ICmpInst>(I)->isEquality())
1420           break;
1421         markOverdefined(I);
1422         return true;
1423       case Instruction::Call:
1424       case Instruction::Invoke: {
1425         // There are two reasons a call can have an undef result
1426         // 1. It could be tracked.
1427         // 2. It could be constant-foldable.
1428         // Because of the way we solve return values, tracked calls must
1429         // never be marked overdefined in ResolvedUndefsIn.
1430         if (Function *F = CallSite(I).getCalledFunction())
1431           if (TrackedRetVals.count(F))
1432             break;
1433 
1434         // If the call is constant-foldable, we mark it overdefined because
1435         // we do not know what return values are valid.
1436         markOverdefined(I);
1437         return true;
1438       }
1439       default:
1440         // If we don't know what should happen here, conservatively mark it
1441         // overdefined.
1442         markOverdefined(I);
1443         return true;
1444       }
1445     }
1446 
1447     // Check to see if we have a branch or switch on an undefined value.  If so
1448     // we force the branch to go one way or the other to make the successor
1449     // values live.  It doesn't really matter which way we force it.
1450     TerminatorInst *TI = BB->getTerminator();
1451     if (BranchInst *BI = dyn_cast<BranchInst>(TI)) {
1452       if (!BI->isConditional()) continue;
1453       if (!getValueState(BI->getCondition()).isUndefined())
1454         continue;
1455 
1456       // If the input to SCCP is actually branch on undef, fix the undef to
1457       // false.
1458       if (isa<UndefValue>(BI->getCondition())) {
1459         BI->setCondition(ConstantInt::getFalse(BI->getContext()));
1460         markEdgeExecutable(BB, TI->getSuccessor(1));
1461         return true;
1462       }
1463 
1464       // Otherwise, it is a branch on a symbolic value which is currently
1465       // considered to be undef.  Handle this by forcing the input value to the
1466       // branch to false.
1467       markForcedConstant(BI->getCondition(),
1468                          ConstantInt::getFalse(TI->getContext()));
1469       return true;
1470     }
1471 
1472     if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
1473       if (!SI->getNumCases())
1474         continue;
1475       if (!getValueState(SI->getCondition()).isUndefined())
1476         continue;
1477 
1478       // If the input to SCCP is actually switch on undef, fix the undef to
1479       // the first constant.
1480       if (isa<UndefValue>(SI->getCondition())) {
1481         SI->setCondition(SI->case_begin().getCaseValue());
1482         markEdgeExecutable(BB, SI->case_begin().getCaseSuccessor());
1483         return true;
1484       }
1485 
1486       markForcedConstant(SI->getCondition(), SI->case_begin().getCaseValue());
1487       return true;
1488     }
1489   }
1490 
1491   return false;
1492 }
1493 
1494 
1495 namespace {
1496   //===--------------------------------------------------------------------===//
1497   //
1498   /// SCCP Class - This class uses the SCCPSolver to implement a per-function
1499   /// Sparse Conditional Constant Propagator.
1500   ///
1501   struct SCCP : public FunctionPass {
getAnalysisUsage__anona95eefe80311::SCCP1502     virtual void getAnalysisUsage(AnalysisUsage &AU) const {
1503       AU.addRequired<TargetLibraryInfo>();
1504     }
1505     static char ID; // Pass identification, replacement for typeid
SCCP__anona95eefe80311::SCCP1506     SCCP() : FunctionPass(ID) {
1507       initializeSCCPPass(*PassRegistry::getPassRegistry());
1508     }
1509 
1510     // runOnFunction - Run the Sparse Conditional Constant Propagation
1511     // algorithm, and return true if the function was modified.
1512     //
1513     bool runOnFunction(Function &F);
1514   };
1515 } // end anonymous namespace
1516 
1517 char SCCP::ID = 0;
1518 INITIALIZE_PASS(SCCP, "sccp",
1519                 "Sparse Conditional Constant Propagation", false, false)
1520 
1521 // createSCCPPass - This is the public interface to this file.
createSCCPPass()1522 FunctionPass *llvm::createSCCPPass() {
1523   return new SCCP();
1524 }
1525 
DeleteInstructionInBlock(BasicBlock * BB)1526 static void DeleteInstructionInBlock(BasicBlock *BB) {
1527   DEBUG(dbgs() << "  BasicBlock Dead:" << *BB);
1528   ++NumDeadBlocks;
1529 
1530   // Check to see if there are non-terminating instructions to delete.
1531   if (isa<TerminatorInst>(BB->begin()))
1532     return;
1533 
1534   // Delete the instructions backwards, as it has a reduced likelihood of having
1535   // to update as many def-use and use-def chains.
1536   Instruction *EndInst = BB->getTerminator(); // Last not to be deleted.
1537   while (EndInst != BB->begin()) {
1538     // Delete the next to last instruction.
1539     BasicBlock::iterator I = EndInst;
1540     Instruction *Inst = --I;
1541     if (!Inst->use_empty())
1542       Inst->replaceAllUsesWith(UndefValue::get(Inst->getType()));
1543     if (isa<LandingPadInst>(Inst)) {
1544       EndInst = Inst;
1545       continue;
1546     }
1547     BB->getInstList().erase(Inst);
1548     ++NumInstRemoved;
1549   }
1550 }
1551 
1552 // runOnFunction() - Run the Sparse Conditional Constant Propagation algorithm,
1553 // and return true if the function was modified.
1554 //
runOnFunction(Function & F)1555 bool SCCP::runOnFunction(Function &F) {
1556   DEBUG(dbgs() << "SCCP on function '" << F.getName() << "'\n");
1557   const DataLayout *TD = getAnalysisIfAvailable<DataLayout>();
1558   const TargetLibraryInfo *TLI = &getAnalysis<TargetLibraryInfo>();
1559   SCCPSolver Solver(TD, TLI);
1560 
1561   // Mark the first block of the function as being executable.
1562   Solver.MarkBlockExecutable(F.begin());
1563 
1564   // Mark all arguments to the function as being overdefined.
1565   for (Function::arg_iterator AI = F.arg_begin(), E = F.arg_end(); AI != E;++AI)
1566     Solver.markAnythingOverdefined(AI);
1567 
1568   // Solve for constants.
1569   bool ResolvedUndefs = true;
1570   while (ResolvedUndefs) {
1571     Solver.Solve();
1572     DEBUG(dbgs() << "RESOLVING UNDEFs\n");
1573     ResolvedUndefs = Solver.ResolvedUndefsIn(F);
1574   }
1575 
1576   bool MadeChanges = false;
1577 
1578   // If we decided that there are basic blocks that are dead in this function,
1579   // delete their contents now.  Note that we cannot actually delete the blocks,
1580   // as we cannot modify the CFG of the function.
1581 
1582   for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB) {
1583     if (!Solver.isBlockExecutable(BB)) {
1584       DeleteInstructionInBlock(BB);
1585       MadeChanges = true;
1586       continue;
1587     }
1588 
1589     // Iterate over all of the instructions in a function, replacing them with
1590     // constants if we have found them to be of constant values.
1591     //
1592     for (BasicBlock::iterator BI = BB->begin(), E = BB->end(); BI != E; ) {
1593       Instruction *Inst = BI++;
1594       if (Inst->getType()->isVoidTy() || isa<TerminatorInst>(Inst))
1595         continue;
1596 
1597       // TODO: Reconstruct structs from their elements.
1598       if (Inst->getType()->isStructTy())
1599         continue;
1600 
1601       LatticeVal IV = Solver.getLatticeValueFor(Inst);
1602       if (IV.isOverdefined())
1603         continue;
1604 
1605       Constant *Const = IV.isConstant()
1606         ? IV.getConstant() : UndefValue::get(Inst->getType());
1607       DEBUG(dbgs() << "  Constant: " << *Const << " = " << *Inst << '\n');
1608 
1609       // Replaces all of the uses of a variable with uses of the constant.
1610       Inst->replaceAllUsesWith(Const);
1611 
1612       // Delete the instruction.
1613       Inst->eraseFromParent();
1614 
1615       // Hey, we just changed something!
1616       MadeChanges = true;
1617       ++NumInstRemoved;
1618     }
1619   }
1620 
1621   return MadeChanges;
1622 }
1623 
1624 namespace {
1625   //===--------------------------------------------------------------------===//
1626   //
1627   /// IPSCCP Class - This class implements interprocedural Sparse Conditional
1628   /// Constant Propagation.
1629   ///
1630   struct IPSCCP : public ModulePass {
getAnalysisUsage__anona95eefe80411::IPSCCP1631     virtual void getAnalysisUsage(AnalysisUsage &AU) const {
1632       AU.addRequired<TargetLibraryInfo>();
1633     }
1634     static char ID;
IPSCCP__anona95eefe80411::IPSCCP1635     IPSCCP() : ModulePass(ID) {
1636       initializeIPSCCPPass(*PassRegistry::getPassRegistry());
1637     }
1638     bool runOnModule(Module &M);
1639   };
1640 } // end anonymous namespace
1641 
1642 char IPSCCP::ID = 0;
1643 INITIALIZE_PASS_BEGIN(IPSCCP, "ipsccp",
1644                 "Interprocedural Sparse Conditional Constant Propagation",
1645                 false, false)
INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfo)1646 INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfo)
1647 INITIALIZE_PASS_END(IPSCCP, "ipsccp",
1648                 "Interprocedural Sparse Conditional Constant Propagation",
1649                 false, false)
1650 
1651 // createIPSCCPPass - This is the public interface to this file.
1652 ModulePass *llvm::createIPSCCPPass() {
1653   return new IPSCCP();
1654 }
1655 
1656 
AddressIsTaken(const GlobalValue * GV)1657 static bool AddressIsTaken(const GlobalValue *GV) {
1658   // Delete any dead constantexpr klingons.
1659   GV->removeDeadConstantUsers();
1660 
1661   for (Value::const_use_iterator UI = GV->use_begin(), E = GV->use_end();
1662        UI != E; ++UI) {
1663     const User *U = *UI;
1664     if (const StoreInst *SI = dyn_cast<StoreInst>(U)) {
1665       if (SI->getOperand(0) == GV || SI->isVolatile())
1666         return true;  // Storing addr of GV.
1667     } else if (isa<InvokeInst>(U) || isa<CallInst>(U)) {
1668       // Make sure we are calling the function, not passing the address.
1669       ImmutableCallSite CS(cast<Instruction>(U));
1670       if (!CS.isCallee(UI))
1671         return true;
1672     } else if (const LoadInst *LI = dyn_cast<LoadInst>(U)) {
1673       if (LI->isVolatile())
1674         return true;
1675     } else if (isa<BlockAddress>(U)) {
1676       // blockaddress doesn't take the address of the function, it takes addr
1677       // of label.
1678     } else {
1679       return true;
1680     }
1681   }
1682   return false;
1683 }
1684 
runOnModule(Module & M)1685 bool IPSCCP::runOnModule(Module &M) {
1686   const DataLayout *TD = getAnalysisIfAvailable<DataLayout>();
1687   const TargetLibraryInfo *TLI = &getAnalysis<TargetLibraryInfo>();
1688   SCCPSolver Solver(TD, TLI);
1689 
1690   // AddressTakenFunctions - This set keeps track of the address-taken functions
1691   // that are in the input.  As IPSCCP runs through and simplifies code,
1692   // functions that were address taken can end up losing their
1693   // address-taken-ness.  Because of this, we keep track of their addresses from
1694   // the first pass so we can use them for the later simplification pass.
1695   SmallPtrSet<Function*, 32> AddressTakenFunctions;
1696 
1697   // Loop over all functions, marking arguments to those with their addresses
1698   // taken or that are external as overdefined.
1699   //
1700   for (Module::iterator F = M.begin(), E = M.end(); F != E; ++F) {
1701     if (F->isDeclaration())
1702       continue;
1703 
1704     // If this is a strong or ODR definition of this function, then we can
1705     // propagate information about its result into callsites of it.
1706     if (!F->mayBeOverridden())
1707       Solver.AddTrackedFunction(F);
1708 
1709     // If this function only has direct calls that we can see, we can track its
1710     // arguments and return value aggressively, and can assume it is not called
1711     // unless we see evidence to the contrary.
1712     if (F->hasLocalLinkage()) {
1713       if (AddressIsTaken(F))
1714         AddressTakenFunctions.insert(F);
1715       else {
1716         Solver.AddArgumentTrackedFunction(F);
1717         continue;
1718       }
1719     }
1720 
1721     // Assume the function is called.
1722     Solver.MarkBlockExecutable(F->begin());
1723 
1724     // Assume nothing about the incoming arguments.
1725     for (Function::arg_iterator AI = F->arg_begin(), E = F->arg_end();
1726          AI != E; ++AI)
1727       Solver.markAnythingOverdefined(AI);
1728   }
1729 
1730   // Loop over global variables.  We inform the solver about any internal global
1731   // variables that do not have their 'addresses taken'.  If they don't have
1732   // their addresses taken, we can propagate constants through them.
1733   for (Module::global_iterator G = M.global_begin(), E = M.global_end();
1734        G != E; ++G)
1735     if (!G->isConstant() && G->hasLocalLinkage() && !AddressIsTaken(G))
1736       Solver.TrackValueOfGlobalVariable(G);
1737 
1738   // Solve for constants.
1739   bool ResolvedUndefs = true;
1740   while (ResolvedUndefs) {
1741     Solver.Solve();
1742 
1743     DEBUG(dbgs() << "RESOLVING UNDEFS\n");
1744     ResolvedUndefs = false;
1745     for (Module::iterator F = M.begin(), E = M.end(); F != E; ++F)
1746       ResolvedUndefs |= Solver.ResolvedUndefsIn(*F);
1747   }
1748 
1749   bool MadeChanges = false;
1750 
1751   // Iterate over all of the instructions in the module, replacing them with
1752   // constants if we have found them to be of constant values.
1753   //
1754   SmallVector<BasicBlock*, 512> BlocksToErase;
1755 
1756   for (Module::iterator F = M.begin(), E = M.end(); F != E; ++F) {
1757     if (Solver.isBlockExecutable(F->begin())) {
1758       for (Function::arg_iterator AI = F->arg_begin(), E = F->arg_end();
1759            AI != E; ++AI) {
1760         if (AI->use_empty() || AI->getType()->isStructTy()) continue;
1761 
1762         // TODO: Could use getStructLatticeValueFor to find out if the entire
1763         // result is a constant and replace it entirely if so.
1764 
1765         LatticeVal IV = Solver.getLatticeValueFor(AI);
1766         if (IV.isOverdefined()) continue;
1767 
1768         Constant *CST = IV.isConstant() ?
1769         IV.getConstant() : UndefValue::get(AI->getType());
1770         DEBUG(dbgs() << "***  Arg " << *AI << " = " << *CST <<"\n");
1771 
1772         // Replaces all of the uses of a variable with uses of the
1773         // constant.
1774         AI->replaceAllUsesWith(CST);
1775         ++IPNumArgsElimed;
1776       }
1777     }
1778 
1779     for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB) {
1780       if (!Solver.isBlockExecutable(BB)) {
1781         DeleteInstructionInBlock(BB);
1782         MadeChanges = true;
1783 
1784         TerminatorInst *TI = BB->getTerminator();
1785         for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i) {
1786           BasicBlock *Succ = TI->getSuccessor(i);
1787           if (!Succ->empty() && isa<PHINode>(Succ->begin()))
1788             TI->getSuccessor(i)->removePredecessor(BB);
1789         }
1790         if (!TI->use_empty())
1791           TI->replaceAllUsesWith(UndefValue::get(TI->getType()));
1792         TI->eraseFromParent();
1793 
1794         if (&*BB != &F->front())
1795           BlocksToErase.push_back(BB);
1796         else
1797           new UnreachableInst(M.getContext(), BB);
1798         continue;
1799       }
1800 
1801       for (BasicBlock::iterator BI = BB->begin(), E = BB->end(); BI != E; ) {
1802         Instruction *Inst = BI++;
1803         if (Inst->getType()->isVoidTy() || Inst->getType()->isStructTy())
1804           continue;
1805 
1806         // TODO: Could use getStructLatticeValueFor to find out if the entire
1807         // result is a constant and replace it entirely if so.
1808 
1809         LatticeVal IV = Solver.getLatticeValueFor(Inst);
1810         if (IV.isOverdefined())
1811           continue;
1812 
1813         Constant *Const = IV.isConstant()
1814           ? IV.getConstant() : UndefValue::get(Inst->getType());
1815         DEBUG(dbgs() << "  Constant: " << *Const << " = " << *Inst << '\n');
1816 
1817         // Replaces all of the uses of a variable with uses of the
1818         // constant.
1819         Inst->replaceAllUsesWith(Const);
1820 
1821         // Delete the instruction.
1822         if (!isa<CallInst>(Inst) && !isa<TerminatorInst>(Inst))
1823           Inst->eraseFromParent();
1824 
1825         // Hey, we just changed something!
1826         MadeChanges = true;
1827         ++IPNumInstRemoved;
1828       }
1829     }
1830 
1831     // Now that all instructions in the function are constant folded, erase dead
1832     // blocks, because we can now use ConstantFoldTerminator to get rid of
1833     // in-edges.
1834     for (unsigned i = 0, e = BlocksToErase.size(); i != e; ++i) {
1835       // If there are any PHI nodes in this successor, drop entries for BB now.
1836       BasicBlock *DeadBB = BlocksToErase[i];
1837       for (Value::use_iterator UI = DeadBB->use_begin(), UE = DeadBB->use_end();
1838            UI != UE; ) {
1839         // Grab the user and then increment the iterator early, as the user
1840         // will be deleted. Step past all adjacent uses from the same user.
1841         Instruction *I = dyn_cast<Instruction>(*UI);
1842         do { ++UI; } while (UI != UE && *UI == I);
1843 
1844         // Ignore blockaddress users; BasicBlock's dtor will handle them.
1845         if (!I) continue;
1846 
1847         bool Folded = ConstantFoldTerminator(I->getParent());
1848         if (!Folded) {
1849           // The constant folder may not have been able to fold the terminator
1850           // if this is a branch or switch on undef.  Fold it manually as a
1851           // branch to the first successor.
1852 #ifndef NDEBUG
1853           if (BranchInst *BI = dyn_cast<BranchInst>(I)) {
1854             assert(BI->isConditional() && isa<UndefValue>(BI->getCondition()) &&
1855                    "Branch should be foldable!");
1856           } else if (SwitchInst *SI = dyn_cast<SwitchInst>(I)) {
1857             assert(isa<UndefValue>(SI->getCondition()) && "Switch should fold");
1858           } else {
1859             llvm_unreachable("Didn't fold away reference to block!");
1860           }
1861 #endif
1862 
1863           // Make this an uncond branch to the first successor.
1864           TerminatorInst *TI = I->getParent()->getTerminator();
1865           BranchInst::Create(TI->getSuccessor(0), TI);
1866 
1867           // Remove entries in successor phi nodes to remove edges.
1868           for (unsigned i = 1, e = TI->getNumSuccessors(); i != e; ++i)
1869             TI->getSuccessor(i)->removePredecessor(TI->getParent());
1870 
1871           // Remove the old terminator.
1872           TI->eraseFromParent();
1873         }
1874       }
1875 
1876       // Finally, delete the basic block.
1877       F->getBasicBlockList().erase(DeadBB);
1878     }
1879     BlocksToErase.clear();
1880   }
1881 
1882   // If we inferred constant or undef return values for a function, we replaced
1883   // all call uses with the inferred value.  This means we don't need to bother
1884   // actually returning anything from the function.  Replace all return
1885   // instructions with return undef.
1886   //
1887   // Do this in two stages: first identify the functions we should process, then
1888   // actually zap their returns.  This is important because we can only do this
1889   // if the address of the function isn't taken.  In cases where a return is the
1890   // last use of a function, the order of processing functions would affect
1891   // whether other functions are optimizable.
1892   SmallVector<ReturnInst*, 8> ReturnsToZap;
1893 
1894   // TODO: Process multiple value ret instructions also.
1895   const DenseMap<Function*, LatticeVal> &RV = Solver.getTrackedRetVals();
1896   for (DenseMap<Function*, LatticeVal>::const_iterator I = RV.begin(),
1897        E = RV.end(); I != E; ++I) {
1898     Function *F = I->first;
1899     if (I->second.isOverdefined() || F->getReturnType()->isVoidTy())
1900       continue;
1901 
1902     // We can only do this if we know that nothing else can call the function.
1903     if (!F->hasLocalLinkage() || AddressTakenFunctions.count(F))
1904       continue;
1905 
1906     for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB)
1907       if (ReturnInst *RI = dyn_cast<ReturnInst>(BB->getTerminator()))
1908         if (!isa<UndefValue>(RI->getOperand(0)))
1909           ReturnsToZap.push_back(RI);
1910   }
1911 
1912   // Zap all returns which we've identified as zap to change.
1913   for (unsigned i = 0, e = ReturnsToZap.size(); i != e; ++i) {
1914     Function *F = ReturnsToZap[i]->getParent()->getParent();
1915     ReturnsToZap[i]->setOperand(0, UndefValue::get(F->getReturnType()));
1916   }
1917 
1918   // If we inferred constant or undef values for globals variables, we can
1919   // delete the global and any stores that remain to it.
1920   const DenseMap<GlobalVariable*, LatticeVal> &TG = Solver.getTrackedGlobals();
1921   for (DenseMap<GlobalVariable*, LatticeVal>::const_iterator I = TG.begin(),
1922          E = TG.end(); I != E; ++I) {
1923     GlobalVariable *GV = I->first;
1924     assert(!I->second.isOverdefined() &&
1925            "Overdefined values should have been taken out of the map!");
1926     DEBUG(dbgs() << "Found that GV '" << GV->getName() << "' is constant!\n");
1927     while (!GV->use_empty()) {
1928       StoreInst *SI = cast<StoreInst>(GV->use_back());
1929       SI->eraseFromParent();
1930     }
1931     M.getGlobalList().erase(GV);
1932     ++IPNumGlobalConst;
1933   }
1934 
1935   return MadeChanges;
1936 }
1937