1 //===- SCCP.cpp - Sparse Conditional Constant Propagation -----------------===//
3 // The LLVM Compiler Infrastructure
5 // This file was developed by the LLVM research group and is distributed under
6 // the University of Illinois Open Source License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This file implements sparse conditional constant propagation and merging:
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
19 // * This pass has a habit of making definitions be dead. It is a good idea
20 // to to run a DCE pass sometime after running this pass.
22 //===----------------------------------------------------------------------===//
24 #define DEBUG_TYPE "sccp"
25 #include "llvm/Transforms/Scalar.h"
26 #include "llvm/Transforms/IPO.h"
27 #include "llvm/Constants.h"
28 #include "llvm/DerivedTypes.h"
29 #include "llvm/Instructions.h"
30 #include "llvm/Pass.h"
31 #include "llvm/Support/InstVisitor.h"
32 #include "llvm/Transforms/Utils/Local.h"
33 #include "llvm/Support/CallSite.h"
34 #include "llvm/Support/Debug.h"
35 #include "llvm/ADT/hash_map"
36 #include "llvm/ADT/Statistic.h"
37 #include "llvm/ADT/STLExtras.h"
42 // LatticeVal class - This class represents the different lattice values that an
43 // instruction may occupy. It is a simple class with value semantics.
49 undefined, // This instruction has no known value
50 constant, // This instruction has a constant value
51 overdefined // This instruction has an unknown value
52 } LatticeValue; // The current lattice position
53 Constant *ConstantVal; // If Constant value, the current value
55 inline LatticeVal() : LatticeValue(undefined), ConstantVal(0) {}
57 // markOverdefined - Return true if this is a new status to be in...
58 inline bool markOverdefined() {
59 if (LatticeValue != overdefined) {
60 LatticeValue = overdefined;
66 // markConstant - Return true if this is a new status for us...
67 inline bool markConstant(Constant *V) {
68 if (LatticeValue != constant) {
69 LatticeValue = constant;
73 assert(ConstantVal == V && "Marking constant with different value");
78 inline bool isUndefined() const { return LatticeValue == undefined; }
79 inline bool isConstant() const { return LatticeValue == constant; }
80 inline bool isOverdefined() const { return LatticeValue == overdefined; }
82 inline Constant *getConstant() const {
83 assert(isConstant() && "Cannot get the constant of a non-constant!");
88 } // end anonymous namespace
91 //===----------------------------------------------------------------------===//
93 /// SCCPSolver - This class is a general purpose solver for Sparse Conditional
94 /// Constant Propagation.
96 class SCCPSolver : public InstVisitor<SCCPSolver> {
97 std::set<BasicBlock*> BBExecutable;// The basic blocks that are executable
98 hash_map<Value*, LatticeVal> ValueState; // The state each value is in...
100 /// GlobalValue - If we are tracking any values for the contents of a global
101 /// variable, we keep a mapping from the constant accessor to the element of
102 /// the global, to the currently known value. If the value becomes
103 /// overdefined, it's entry is simply removed from this map.
104 hash_map<GlobalVariable*, LatticeVal> TrackedGlobals;
106 /// TrackedFunctionRetVals - If we are tracking arguments into and the return
107 /// value out of a function, it will have an entry in this map, indicating
108 /// what the known return value for the function is.
109 hash_map<Function*, LatticeVal> TrackedFunctionRetVals;
111 // The reason for two worklists is that overdefined is the lowest state
112 // on the lattice, and moving things to overdefined as fast as possible
113 // makes SCCP converge much faster.
114 // By having a separate worklist, we accomplish this because everything
115 // possibly overdefined will become overdefined at the soonest possible
117 std::vector<Value*> OverdefinedInstWorkList;
118 std::vector<Value*> InstWorkList;
121 std::vector<BasicBlock*> BBWorkList; // The BasicBlock work list
123 /// UsersOfOverdefinedPHIs - Keep track of any users of PHI nodes that are not
124 /// overdefined, despite the fact that the PHI node is overdefined.
125 std::multimap<PHINode*, Instruction*> UsersOfOverdefinedPHIs;
127 /// KnownFeasibleEdges - Entries in this set are edges which have already had
128 /// PHI nodes retriggered.
129 typedef std::pair<BasicBlock*,BasicBlock*> Edge;
130 std::set<Edge> KnownFeasibleEdges;
133 /// MarkBlockExecutable - This method can be used by clients to mark all of
134 /// the blocks that are known to be intrinsically live in the processed unit.
135 void MarkBlockExecutable(BasicBlock *BB) {
136 DEBUG(std::cerr << "Marking Block Executable: " << BB->getName() << "\n");
137 BBExecutable.insert(BB); // Basic block is executable!
138 BBWorkList.push_back(BB); // Add the block to the work list!
141 /// TrackValueOfGlobalVariable - Clients can use this method to
142 /// inform the SCCPSolver that it should track loads and stores to the
143 /// specified global variable if it can. This is only legal to call if
144 /// performing Interprocedural SCCP.
145 void TrackValueOfGlobalVariable(GlobalVariable *GV) {
146 const Type *ElTy = GV->getType()->getElementType();
147 if (ElTy->isFirstClassType()) {
148 LatticeVal &IV = TrackedGlobals[GV];
149 if (!isa<UndefValue>(GV->getInitializer()))
150 IV.markConstant(GV->getInitializer());
154 /// AddTrackedFunction - If the SCCP solver is supposed to track calls into
155 /// and out of the specified function (which cannot have its address taken),
156 /// this method must be called.
157 void AddTrackedFunction(Function *F) {
158 assert(F->hasInternalLinkage() && "Can only track internal functions!");
159 // Add an entry, F -> undef.
160 TrackedFunctionRetVals[F];
163 /// Solve - Solve for constants and executable blocks.
167 /// ResolveBranchesIn - While solving the dataflow for a function, we assume
168 /// that branches on undef values cannot reach any of their successors.
169 /// However, this is not a safe assumption. After we solve dataflow, this
170 /// method should be use to handle this. If this returns true, the solver
172 bool ResolveBranchesIn(Function &F);
174 /// getExecutableBlocks - Once we have solved for constants, return the set of
175 /// blocks that is known to be executable.
176 std::set<BasicBlock*> &getExecutableBlocks() {
180 /// getValueMapping - Once we have solved for constants, return the mapping of
181 /// LLVM values to LatticeVals.
182 hash_map<Value*, LatticeVal> &getValueMapping() {
186 /// getTrackedFunctionRetVals - Get the inferred return value map.
188 const hash_map<Function*, LatticeVal> &getTrackedFunctionRetVals() {
189 return TrackedFunctionRetVals;
192 /// getTrackedGlobals - Get and return the set of inferred initializers for
193 /// global variables.
194 const hash_map<GlobalVariable*, LatticeVal> &getTrackedGlobals() {
195 return TrackedGlobals;
200 // markConstant - Make a value be marked as "constant". If the value
201 // is not already a constant, add it to the instruction work list so that
202 // the users of the instruction are updated later.
204 inline void markConstant(LatticeVal &IV, Value *V, Constant *C) {
205 if (IV.markConstant(C)) {
206 DEBUG(std::cerr << "markConstant: " << *C << ": " << *V);
207 InstWorkList.push_back(V);
210 inline void markConstant(Value *V, Constant *C) {
211 markConstant(ValueState[V], V, C);
214 // markOverdefined - Make a value be marked as "overdefined". If the
215 // value is not already overdefined, add it to the overdefined instruction
216 // work list so that the users of the instruction are updated later.
218 inline void markOverdefined(LatticeVal &IV, Value *V) {
219 if (IV.markOverdefined()) {
220 DEBUG(std::cerr << "markOverdefined: ";
221 if (Function *F = dyn_cast<Function>(V))
222 std::cerr << "Function '" << F->getName() << "'\n";
225 // Only instructions go on the work list
226 OverdefinedInstWorkList.push_back(V);
229 inline void markOverdefined(Value *V) {
230 markOverdefined(ValueState[V], V);
233 inline void mergeInValue(LatticeVal &IV, Value *V, LatticeVal &MergeWithV) {
234 if (IV.isOverdefined() || MergeWithV.isUndefined())
236 if (MergeWithV.isOverdefined())
237 markOverdefined(IV, V);
238 else if (IV.isUndefined())
239 markConstant(IV, V, MergeWithV.getConstant());
240 else if (IV.getConstant() != MergeWithV.getConstant())
241 markOverdefined(IV, V);
244 // getValueState - Return the LatticeVal object that corresponds to the value.
245 // This function is necessary because not all values should start out in the
246 // underdefined state... Argument's should be overdefined, and
247 // constants should be marked as constants. If a value is not known to be an
248 // Instruction object, then use this accessor to get its value from the map.
250 inline LatticeVal &getValueState(Value *V) {
251 hash_map<Value*, LatticeVal>::iterator I = ValueState.find(V);
252 if (I != ValueState.end()) return I->second; // Common case, in the map
254 if (Constant *CPV = dyn_cast<Constant>(V)) {
255 if (isa<UndefValue>(V)) {
256 // Nothing to do, remain undefined.
258 ValueState[CPV].markConstant(CPV); // Constants are constant
261 // All others are underdefined by default...
262 return ValueState[V];
265 // markEdgeExecutable - Mark a basic block as executable, adding it to the BB
266 // work list if it is not already executable...
268 void markEdgeExecutable(BasicBlock *Source, BasicBlock *Dest) {
269 if (!KnownFeasibleEdges.insert(Edge(Source, Dest)).second)
270 return; // This edge is already known to be executable!
272 if (BBExecutable.count(Dest)) {
273 DEBUG(std::cerr << "Marking Edge Executable: " << Source->getName()
274 << " -> " << Dest->getName() << "\n");
276 // The destination is already executable, but we just made an edge
277 // feasible that wasn't before. Revisit the PHI nodes in the block
278 // because they have potentially new operands.
279 for (BasicBlock::iterator I = Dest->begin(); isa<PHINode>(I); ++I)
280 visitPHINode(*cast<PHINode>(I));
283 MarkBlockExecutable(Dest);
287 // getFeasibleSuccessors - Return a vector of booleans to indicate which
288 // successors are reachable from a given terminator instruction.
290 void getFeasibleSuccessors(TerminatorInst &TI, std::vector<bool> &Succs);
292 // isEdgeFeasible - Return true if the control flow edge from the 'From' basic
293 // block to the 'To' basic block is currently feasible...
295 bool isEdgeFeasible(BasicBlock *From, BasicBlock *To);
297 // OperandChangedState - This method is invoked on all of the users of an
298 // instruction that was just changed state somehow.... Based on this
299 // information, we need to update the specified user of this instruction.
301 void OperandChangedState(User *U) {
302 // Only instructions use other variable values!
303 Instruction &I = cast<Instruction>(*U);
304 if (BBExecutable.count(I.getParent())) // Inst is executable?
309 friend class InstVisitor<SCCPSolver>;
311 // visit implementations - Something changed in this instruction... Either an
312 // operand made a transition, or the instruction is newly executable. Change
313 // the value type of I to reflect these changes if appropriate.
315 void visitPHINode(PHINode &I);
318 void visitReturnInst(ReturnInst &I);
319 void visitTerminatorInst(TerminatorInst &TI);
321 void visitCastInst(CastInst &I);
322 void visitSelectInst(SelectInst &I);
323 void visitBinaryOperator(Instruction &I);
324 void visitShiftInst(ShiftInst &I) { visitBinaryOperator(I); }
326 // Instructions that cannot be folded away...
327 void visitStoreInst (Instruction &I);
328 void visitLoadInst (LoadInst &I);
329 void visitGetElementPtrInst(GetElementPtrInst &I);
330 void visitCallInst (CallInst &I) { visitCallSite(CallSite::get(&I)); }
331 void visitInvokeInst (InvokeInst &II) {
332 visitCallSite(CallSite::get(&II));
333 visitTerminatorInst(II);
335 void visitCallSite (CallSite CS);
336 void visitUnwindInst (TerminatorInst &I) { /*returns void*/ }
337 void visitUnreachableInst(TerminatorInst &I) { /*returns void*/ }
338 void visitAllocationInst(Instruction &I) { markOverdefined(&I); }
339 void visitVANextInst (Instruction &I) { markOverdefined(&I); }
340 void visitVAArgInst (Instruction &I) { markOverdefined(&I); }
341 void visitFreeInst (Instruction &I) { /*returns void*/ }
343 void visitInstruction(Instruction &I) {
344 // If a new instruction is added to LLVM that we don't handle...
345 std::cerr << "SCCP: Don't know how to handle: " << I;
346 markOverdefined(&I); // Just in case
350 // getFeasibleSuccessors - Return a vector of booleans to indicate which
351 // successors are reachable from a given terminator instruction.
353 void SCCPSolver::getFeasibleSuccessors(TerminatorInst &TI,
354 std::vector<bool> &Succs) {
355 Succs.resize(TI.getNumSuccessors());
356 if (BranchInst *BI = dyn_cast<BranchInst>(&TI)) {
357 if (BI->isUnconditional()) {
360 LatticeVal &BCValue = getValueState(BI->getCondition());
361 if (BCValue.isOverdefined() ||
362 (BCValue.isConstant() && !isa<ConstantBool>(BCValue.getConstant()))) {
363 // Overdefined condition variables, and branches on unfoldable constant
364 // conditions, mean the branch could go either way.
365 Succs[0] = Succs[1] = true;
366 } else if (BCValue.isConstant()) {
367 // Constant condition variables mean the branch can only go a single way
368 Succs[BCValue.getConstant() == ConstantBool::False] = true;
371 } else if (InvokeInst *II = dyn_cast<InvokeInst>(&TI)) {
372 // Invoke instructions successors are always executable.
373 Succs[0] = Succs[1] = true;
374 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(&TI)) {
375 LatticeVal &SCValue = getValueState(SI->getCondition());
376 if (SCValue.isOverdefined() || // Overdefined condition?
377 (SCValue.isConstant() && !isa<ConstantInt>(SCValue.getConstant()))) {
378 // All destinations are executable!
379 Succs.assign(TI.getNumSuccessors(), true);
380 } else if (SCValue.isConstant()) {
381 Constant *CPV = SCValue.getConstant();
382 // Make sure to skip the "default value" which isn't a value
383 for (unsigned i = 1, E = SI->getNumSuccessors(); i != E; ++i) {
384 if (SI->getSuccessorValue(i) == CPV) {// Found the right branch...
390 // Constant value not equal to any of the branches... must execute
391 // default branch then...
395 std::cerr << "SCCP: Don't know how to handle: " << TI;
396 Succs.assign(TI.getNumSuccessors(), true);
401 // isEdgeFeasible - Return true if the control flow edge from the 'From' basic
402 // block to the 'To' basic block is currently feasible...
404 bool SCCPSolver::isEdgeFeasible(BasicBlock *From, BasicBlock *To) {
405 assert(BBExecutable.count(To) && "Dest should always be alive!");
407 // Make sure the source basic block is executable!!
408 if (!BBExecutable.count(From)) return false;
410 // Check to make sure this edge itself is actually feasible now...
411 TerminatorInst *TI = From->getTerminator();
412 if (BranchInst *BI = dyn_cast<BranchInst>(TI)) {
413 if (BI->isUnconditional())
416 LatticeVal &BCValue = getValueState(BI->getCondition());
417 if (BCValue.isOverdefined()) {
418 // Overdefined condition variables mean the branch could go either way.
420 } else if (BCValue.isConstant()) {
421 // Not branching on an evaluatable constant?
422 if (!isa<ConstantBool>(BCValue.getConstant())) return true;
424 // Constant condition variables mean the branch can only go a single way
425 return BI->getSuccessor(BCValue.getConstant() ==
426 ConstantBool::False) == To;
430 } else if (InvokeInst *II = dyn_cast<InvokeInst>(TI)) {
431 // Invoke instructions successors are always executable.
433 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
434 LatticeVal &SCValue = getValueState(SI->getCondition());
435 if (SCValue.isOverdefined()) { // Overdefined condition?
436 // All destinations are executable!
438 } else if (SCValue.isConstant()) {
439 Constant *CPV = SCValue.getConstant();
440 if (!isa<ConstantInt>(CPV))
441 return true; // not a foldable constant?
443 // Make sure to skip the "default value" which isn't a value
444 for (unsigned i = 1, E = SI->getNumSuccessors(); i != E; ++i)
445 if (SI->getSuccessorValue(i) == CPV) // Found the taken branch...
446 return SI->getSuccessor(i) == To;
448 // Constant value not equal to any of the branches... must execute
449 // default branch then...
450 return SI->getDefaultDest() == To;
454 std::cerr << "Unknown terminator instruction: " << *TI;
459 // visit Implementations - Something changed in this instruction... Either an
460 // operand made a transition, or the instruction is newly executable. Change
461 // the value type of I to reflect these changes if appropriate. This method
462 // makes sure to do the following actions:
464 // 1. If a phi node merges two constants in, and has conflicting value coming
465 // from different branches, or if the PHI node merges in an overdefined
466 // value, then the PHI node becomes overdefined.
467 // 2. If a phi node merges only constants in, and they all agree on value, the
468 // PHI node becomes a constant value equal to that.
469 // 3. If V <- x (op) y && isConstant(x) && isConstant(y) V = Constant
470 // 4. If V <- x (op) y && (isOverdefined(x) || isOverdefined(y)) V = Overdefined
471 // 5. If V <- MEM or V <- CALL or V <- (unknown) then V = Overdefined
472 // 6. If a conditional branch has a value that is constant, make the selected
473 // destination executable
474 // 7. If a conditional branch has a value that is overdefined, make all
475 // successors executable.
477 void SCCPSolver::visitPHINode(PHINode &PN) {
478 LatticeVal &PNIV = getValueState(&PN);
479 if (PNIV.isOverdefined()) {
480 // There may be instructions using this PHI node that are not overdefined
481 // themselves. If so, make sure that they know that the PHI node operand
483 std::multimap<PHINode*, Instruction*>::iterator I, E;
484 tie(I, E) = UsersOfOverdefinedPHIs.equal_range(&PN);
486 std::vector<Instruction*> Users;
487 Users.reserve(std::distance(I, E));
488 for (; I != E; ++I) Users.push_back(I->second);
489 while (!Users.empty()) {
494 return; // Quick exit
497 // Super-extra-high-degree PHI nodes are unlikely to ever be marked constant,
498 // and slow us down a lot. Just mark them overdefined.
499 if (PN.getNumIncomingValues() > 64) {
500 markOverdefined(PNIV, &PN);
504 // Look at all of the executable operands of the PHI node. If any of them
505 // are overdefined, the PHI becomes overdefined as well. If they are all
506 // constant, and they agree with each other, the PHI becomes the identical
507 // constant. If they are constant and don't agree, the PHI is overdefined.
508 // If there are no executable operands, the PHI remains undefined.
510 Constant *OperandVal = 0;
511 for (unsigned i = 0, e = PN.getNumIncomingValues(); i != e; ++i) {
512 LatticeVal &IV = getValueState(PN.getIncomingValue(i));
513 if (IV.isUndefined()) continue; // Doesn't influence PHI node.
515 if (isEdgeFeasible(PN.getIncomingBlock(i), PN.getParent())) {
516 if (IV.isOverdefined()) { // PHI node becomes overdefined!
517 markOverdefined(PNIV, &PN);
521 if (OperandVal == 0) { // Grab the first value...
522 OperandVal = IV.getConstant();
523 } else { // Another value is being merged in!
524 // There is already a reachable operand. If we conflict with it,
525 // then the PHI node becomes overdefined. If we agree with it, we
528 // Check to see if there are two different constants merging...
529 if (IV.getConstant() != OperandVal) {
530 // Yes there is. This means the PHI node is not constant.
531 // You must be overdefined poor PHI.
533 markOverdefined(PNIV, &PN); // The PHI node now becomes overdefined
534 return; // I'm done analyzing you
540 // If we exited the loop, this means that the PHI node only has constant
541 // arguments that agree with each other(and OperandVal is the constant) or
542 // OperandVal is null because there are no defined incoming arguments. If
543 // this is the case, the PHI remains undefined.
546 markConstant(PNIV, &PN, OperandVal); // Acquire operand value
549 void SCCPSolver::visitReturnInst(ReturnInst &I) {
550 if (I.getNumOperands() == 0) return; // Ret void
552 // If we are tracking the return value of this function, merge it in.
553 Function *F = I.getParent()->getParent();
554 if (F->hasInternalLinkage() && !TrackedFunctionRetVals.empty()) {
555 hash_map<Function*, LatticeVal>::iterator TFRVI =
556 TrackedFunctionRetVals.find(F);
557 if (TFRVI != TrackedFunctionRetVals.end() &&
558 !TFRVI->second.isOverdefined()) {
559 LatticeVal &IV = getValueState(I.getOperand(0));
560 mergeInValue(TFRVI->second, F, IV);
566 void SCCPSolver::visitTerminatorInst(TerminatorInst &TI) {
567 std::vector<bool> SuccFeasible;
568 getFeasibleSuccessors(TI, SuccFeasible);
570 BasicBlock *BB = TI.getParent();
572 // Mark all feasible successors executable...
573 for (unsigned i = 0, e = SuccFeasible.size(); i != e; ++i)
575 markEdgeExecutable(BB, TI.getSuccessor(i));
578 void SCCPSolver::visitCastInst(CastInst &I) {
579 Value *V = I.getOperand(0);
580 LatticeVal &VState = getValueState(V);
581 if (VState.isOverdefined()) // Inherit overdefinedness of operand
583 else if (VState.isConstant()) // Propagate constant value
584 markConstant(&I, ConstantExpr::getCast(VState.getConstant(), I.getType()));
587 void SCCPSolver::visitSelectInst(SelectInst &I) {
588 LatticeVal &CondValue = getValueState(I.getCondition());
589 if (CondValue.isOverdefined())
591 else if (CondValue.isConstant()) {
592 if (CondValue.getConstant() == ConstantBool::True) {
593 LatticeVal &Val = getValueState(I.getTrueValue());
594 if (Val.isOverdefined())
596 else if (Val.isConstant())
597 markConstant(&I, Val.getConstant());
598 } else if (CondValue.getConstant() == ConstantBool::False) {
599 LatticeVal &Val = getValueState(I.getFalseValue());
600 if (Val.isOverdefined())
602 else if (Val.isConstant())
603 markConstant(&I, Val.getConstant());
609 // Handle BinaryOperators and Shift Instructions...
610 void SCCPSolver::visitBinaryOperator(Instruction &I) {
611 LatticeVal &IV = ValueState[&I];
612 if (IV.isOverdefined()) return;
614 LatticeVal &V1State = getValueState(I.getOperand(0));
615 LatticeVal &V2State = getValueState(I.getOperand(1));
617 if (V1State.isOverdefined() || V2State.isOverdefined()) {
618 // If this is an AND or OR with 0 or -1, it doesn't matter that the other
619 // operand is overdefined.
620 if (I.getOpcode() == Instruction::And || I.getOpcode() == Instruction::Or) {
621 LatticeVal *NonOverdefVal = 0;
622 if (!V1State.isOverdefined()) {
623 NonOverdefVal = &V1State;
624 } else if (!V2State.isOverdefined()) {
625 NonOverdefVal = &V2State;
629 if (NonOverdefVal->isUndefined()) {
630 // Could annihilate value.
631 if (I.getOpcode() == Instruction::And)
632 markConstant(IV, &I, Constant::getNullValue(I.getType()));
634 markConstant(IV, &I, ConstantInt::getAllOnesValue(I.getType()));
637 if (I.getOpcode() == Instruction::And) {
638 if (NonOverdefVal->getConstant()->isNullValue()) {
639 markConstant(IV, &I, NonOverdefVal->getConstant());
640 return; // X or 0 = -1
643 if (ConstantIntegral *CI =
644 dyn_cast<ConstantIntegral>(NonOverdefVal->getConstant()))
645 if (CI->isAllOnesValue()) {
646 markConstant(IV, &I, NonOverdefVal->getConstant());
647 return; // X or -1 = -1
655 // If both operands are PHI nodes, it is possible that this instruction has
656 // a constant value, despite the fact that the PHI node doesn't. Check for
657 // this condition now.
658 if (PHINode *PN1 = dyn_cast<PHINode>(I.getOperand(0)))
659 if (PHINode *PN2 = dyn_cast<PHINode>(I.getOperand(1)))
660 if (PN1->getParent() == PN2->getParent()) {
661 // Since the two PHI nodes are in the same basic block, they must have
662 // entries for the same predecessors. Walk the predecessor list, and
663 // if all of the incoming values are constants, and the result of
664 // evaluating this expression with all incoming value pairs is the
665 // same, then this expression is a constant even though the PHI node
666 // is not a constant!
668 for (unsigned i = 0, e = PN1->getNumIncomingValues(); i != e; ++i) {
669 LatticeVal &In1 = getValueState(PN1->getIncomingValue(i));
670 BasicBlock *InBlock = PN1->getIncomingBlock(i);
672 getValueState(PN2->getIncomingValueForBlock(InBlock));
674 if (In1.isOverdefined() || In2.isOverdefined()) {
675 Result.markOverdefined();
676 break; // Cannot fold this operation over the PHI nodes!
677 } else if (In1.isConstant() && In2.isConstant()) {
678 Constant *V = ConstantExpr::get(I.getOpcode(), In1.getConstant(),
680 if (Result.isUndefined())
681 Result.markConstant(V);
682 else if (Result.isConstant() && Result.getConstant() != V) {
683 Result.markOverdefined();
689 // If we found a constant value here, then we know the instruction is
690 // constant despite the fact that the PHI nodes are overdefined.
691 if (Result.isConstant()) {
692 markConstant(IV, &I, Result.getConstant());
693 // Remember that this instruction is virtually using the PHI node
695 UsersOfOverdefinedPHIs.insert(std::make_pair(PN1, &I));
696 UsersOfOverdefinedPHIs.insert(std::make_pair(PN2, &I));
698 } else if (Result.isUndefined()) {
702 // Okay, this really is overdefined now. Since we might have
703 // speculatively thought that this was not overdefined before, and
704 // added ourselves to the UsersOfOverdefinedPHIs list for the PHIs,
705 // make sure to clean out any entries that we put there, for
707 std::multimap<PHINode*, Instruction*>::iterator It, E;
708 tie(It, E) = UsersOfOverdefinedPHIs.equal_range(PN1);
710 if (It->second == &I) {
711 UsersOfOverdefinedPHIs.erase(It++);
715 tie(It, E) = UsersOfOverdefinedPHIs.equal_range(PN2);
717 if (It->second == &I) {
718 UsersOfOverdefinedPHIs.erase(It++);
724 markOverdefined(IV, &I);
725 } else if (V1State.isConstant() && V2State.isConstant()) {
726 markConstant(IV, &I, ConstantExpr::get(I.getOpcode(), V1State.getConstant(),
727 V2State.getConstant()));
731 // Handle getelementptr instructions... if all operands are constants then we
732 // can turn this into a getelementptr ConstantExpr.
734 void SCCPSolver::visitGetElementPtrInst(GetElementPtrInst &I) {
735 LatticeVal &IV = ValueState[&I];
736 if (IV.isOverdefined()) return;
738 std::vector<Constant*> Operands;
739 Operands.reserve(I.getNumOperands());
741 for (unsigned i = 0, e = I.getNumOperands(); i != e; ++i) {
742 LatticeVal &State = getValueState(I.getOperand(i));
743 if (State.isUndefined())
744 return; // Operands are not resolved yet...
745 else if (State.isOverdefined()) {
746 markOverdefined(IV, &I);
749 assert(State.isConstant() && "Unknown state!");
750 Operands.push_back(State.getConstant());
753 Constant *Ptr = Operands[0];
754 Operands.erase(Operands.begin()); // Erase the pointer from idx list...
756 markConstant(IV, &I, ConstantExpr::getGetElementPtr(Ptr, Operands));
759 /// GetGEPGlobalInitializer - Given a constant and a getelementptr constantexpr,
760 /// return the constant value being addressed by the constant expression, or
761 /// null if something is funny.
763 static Constant *GetGEPGlobalInitializer(Constant *C, ConstantExpr *CE) {
764 if (CE->getOperand(1) != Constant::getNullValue(CE->getOperand(1)->getType()))
765 return 0; // Do not allow stepping over the value!
767 // Loop over all of the operands, tracking down which value we are
769 for (unsigned i = 2, e = CE->getNumOperands(); i != e; ++i)
770 if (ConstantUInt *CU = dyn_cast<ConstantUInt>(CE->getOperand(i))) {
771 ConstantStruct *CS = dyn_cast<ConstantStruct>(C);
772 if (CS == 0) return 0;
773 if (CU->getValue() >= CS->getNumOperands()) return 0;
774 C = CS->getOperand((unsigned)CU->getValue());
775 } else if (ConstantSInt *CS = dyn_cast<ConstantSInt>(CE->getOperand(i))) {
776 ConstantArray *CA = dyn_cast<ConstantArray>(C);
777 if (CA == 0) return 0;
778 if ((uint64_t)CS->getValue() >= CA->getNumOperands()) return 0;
779 C = CA->getOperand((unsigned)CS->getValue());
785 void SCCPSolver::visitStoreInst(Instruction &SI) {
786 if (TrackedGlobals.empty() || !isa<GlobalVariable>(SI.getOperand(1)))
788 GlobalVariable *GV = cast<GlobalVariable>(SI.getOperand(1));
789 hash_map<GlobalVariable*, LatticeVal>::iterator I = TrackedGlobals.find(GV);
790 if (I == TrackedGlobals.end() || I->second.isOverdefined()) return;
792 // Get the value we are storing into the global.
793 LatticeVal &PtrVal = getValueState(SI.getOperand(0));
795 mergeInValue(I->second, GV, PtrVal);
796 if (I->second.isOverdefined())
797 TrackedGlobals.erase(I); // No need to keep tracking this!
801 // Handle load instructions. If the operand is a constant pointer to a constant
802 // global, we can replace the load with the loaded constant value!
803 void SCCPSolver::visitLoadInst(LoadInst &I) {
804 LatticeVal &IV = ValueState[&I];
805 if (IV.isOverdefined()) return;
807 LatticeVal &PtrVal = getValueState(I.getOperand(0));
808 if (PtrVal.isUndefined()) return; // The pointer is not resolved yet!
809 if (PtrVal.isConstant() && !I.isVolatile()) {
810 Value *Ptr = PtrVal.getConstant();
811 if (isa<ConstantPointerNull>(Ptr)) {
813 markConstant(IV, &I, Constant::getNullValue(I.getType()));
817 // Transform load (constant global) into the value loaded.
818 if (GlobalVariable *GV = dyn_cast<GlobalVariable>(Ptr)) {
819 if (GV->isConstant()) {
820 if (!GV->isExternal()) {
821 markConstant(IV, &I, GV->getInitializer());
824 } else if (!TrackedGlobals.empty()) {
825 // If we are tracking this global, merge in the known value for it.
826 hash_map<GlobalVariable*, LatticeVal>::iterator It =
827 TrackedGlobals.find(GV);
828 if (It != TrackedGlobals.end()) {
829 mergeInValue(IV, &I, It->second);
835 // Transform load (constantexpr_GEP global, 0, ...) into the value loaded.
836 if (ConstantExpr *CE = dyn_cast<ConstantExpr>(Ptr))
837 if (CE->getOpcode() == Instruction::GetElementPtr)
838 if (GlobalVariable *GV = dyn_cast<GlobalVariable>(CE->getOperand(0)))
839 if (GV->isConstant() && !GV->isExternal())
841 GetGEPGlobalInitializer(GV->getInitializer(), CE)) {
842 markConstant(IV, &I, V);
847 // Otherwise we cannot say for certain what value this load will produce.
849 markOverdefined(IV, &I);
852 void SCCPSolver::visitCallSite(CallSite CS) {
853 Function *F = CS.getCalledFunction();
855 // If we are tracking this function, we must make sure to bind arguments as
857 hash_map<Function*, LatticeVal>::iterator TFRVI =TrackedFunctionRetVals.end();
858 if (F && F->hasInternalLinkage())
859 TFRVI = TrackedFunctionRetVals.find(F);
861 if (TFRVI != TrackedFunctionRetVals.end()) {
862 // If this is the first call to the function hit, mark its entry block
864 if (!BBExecutable.count(F->begin()))
865 MarkBlockExecutable(F->begin());
867 CallSite::arg_iterator CAI = CS.arg_begin();
868 for (Function::arg_iterator AI = F->arg_begin(), E = F->arg_end();
869 AI != E; ++AI, ++CAI) {
870 LatticeVal &IV = ValueState[AI];
871 if (!IV.isOverdefined())
872 mergeInValue(IV, AI, getValueState(*CAI));
875 Instruction *I = CS.getInstruction();
876 if (I->getType() == Type::VoidTy) return;
878 LatticeVal &IV = ValueState[I];
879 if (IV.isOverdefined()) return;
881 // Propagate the return value of the function to the value of the instruction.
882 if (TFRVI != TrackedFunctionRetVals.end()) {
883 mergeInValue(IV, I, TFRVI->second);
887 if (F == 0 || !F->isExternal() || !canConstantFoldCallTo(F)) {
888 markOverdefined(IV, I);
892 std::vector<Constant*> Operands;
893 Operands.reserve(I->getNumOperands()-1);
895 for (CallSite::arg_iterator AI = CS.arg_begin(), E = CS.arg_end();
897 LatticeVal &State = getValueState(*AI);
898 if (State.isUndefined())
899 return; // Operands are not resolved yet...
900 else if (State.isOverdefined()) {
901 markOverdefined(IV, I);
904 assert(State.isConstant() && "Unknown state!");
905 Operands.push_back(State.getConstant());
908 if (Constant *C = ConstantFoldCall(F, Operands))
909 markConstant(IV, I, C);
911 markOverdefined(IV, I);
915 void SCCPSolver::Solve() {
916 // Process the work lists until they are empty!
917 while (!BBWorkList.empty() || !InstWorkList.empty() ||
918 !OverdefinedInstWorkList.empty()) {
919 // Process the instruction work list...
920 while (!OverdefinedInstWorkList.empty()) {
921 Value *I = OverdefinedInstWorkList.back();
922 OverdefinedInstWorkList.pop_back();
924 DEBUG(std::cerr << "\nPopped off OI-WL: " << *I);
926 // "I" got into the work list because it either made the transition from
927 // bottom to constant
929 // Anything on this worklist that is overdefined need not be visited
930 // since all of its users will have already been marked as overdefined
931 // Update all of the users of this instruction's value...
933 for (Value::use_iterator UI = I->use_begin(), E = I->use_end();
935 OperandChangedState(*UI);
937 // Process the instruction work list...
938 while (!InstWorkList.empty()) {
939 Value *I = InstWorkList.back();
940 InstWorkList.pop_back();
942 DEBUG(std::cerr << "\nPopped off I-WL: " << *I);
944 // "I" got into the work list because it either made the transition from
945 // bottom to constant
947 // Anything on this worklist that is overdefined need not be visited
948 // since all of its users will have already been marked as overdefined.
949 // Update all of the users of this instruction's value...
951 if (!getValueState(I).isOverdefined())
952 for (Value::use_iterator UI = I->use_begin(), E = I->use_end();
954 OperandChangedState(*UI);
957 // Process the basic block work list...
958 while (!BBWorkList.empty()) {
959 BasicBlock *BB = BBWorkList.back();
960 BBWorkList.pop_back();
962 DEBUG(std::cerr << "\nPopped off BBWL: " << *BB);
964 // Notify all instructions in this basic block that they are newly
971 /// ResolveBranchesIn - While solving the dataflow for a function, we assume
972 /// that branches on undef values cannot reach any of their successors.
973 /// However, this is not a safe assumption. After we solve dataflow, this
974 /// method should be use to handle this. If this returns true, the solver
976 bool SCCPSolver::ResolveBranchesIn(Function &F) {
977 bool BranchesResolved = false;
978 for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB)
979 if (BBExecutable.count(BB)) {
980 TerminatorInst *TI = BB->getTerminator();
981 if (BranchInst *BI = dyn_cast<BranchInst>(TI)) {
982 if (BI->isConditional()) {
983 LatticeVal &BCValue = getValueState(BI->getCondition());
984 if (BCValue.isUndefined()) {
985 BI->setCondition(ConstantBool::True);
986 BranchesResolved = true;
990 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
991 LatticeVal &SCValue = getValueState(SI->getCondition());
992 if (SCValue.isUndefined()) {
993 const Type *CondTy = SI->getCondition()->getType();
994 SI->setCondition(Constant::getNullValue(CondTy));
995 BranchesResolved = true;
1001 return BranchesResolved;
1006 Statistic<> NumInstRemoved("sccp", "Number of instructions removed");
1007 Statistic<> NumDeadBlocks ("sccp", "Number of basic blocks unreachable");
1009 //===--------------------------------------------------------------------===//
1011 /// SCCP Class - This class uses the SCCPSolver to implement a per-function
1012 /// Sparse Conditional COnstant Propagator.
1014 struct SCCP : public FunctionPass {
1015 // runOnFunction - Run the Sparse Conditional Constant Propagation
1016 // algorithm, and return true if the function was modified.
1018 bool runOnFunction(Function &F);
1020 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
1021 AU.setPreservesCFG();
1025 RegisterOpt<SCCP> X("sccp", "Sparse Conditional Constant Propagation");
1026 } // end anonymous namespace
1029 // createSCCPPass - This is the public interface to this file...
1030 FunctionPass *llvm::createSCCPPass() {
1035 // runOnFunction() - Run the Sparse Conditional Constant Propagation algorithm,
1036 // and return true if the function was modified.
1038 bool SCCP::runOnFunction(Function &F) {
1039 DEBUG(std::cerr << "SCCP on function '" << F.getName() << "'\n");
1042 // Mark the first block of the function as being executable.
1043 Solver.MarkBlockExecutable(F.begin());
1045 // Mark all arguments to the function as being overdefined.
1046 hash_map<Value*, LatticeVal> &Values = Solver.getValueMapping();
1047 for (Function::arg_iterator AI = F.arg_begin(), E = F.arg_end(); AI != E; ++AI)
1048 Values[AI].markOverdefined();
1050 // Solve for constants.
1051 bool ResolvedBranches = true;
1052 while (ResolvedBranches) {
1054 DEBUG(std::cerr << "RESOLVING UNDEF BRANCHES\n");
1055 ResolvedBranches = Solver.ResolveBranchesIn(F);
1058 bool MadeChanges = false;
1060 // If we decided that there are basic blocks that are dead in this function,
1061 // delete their contents now. Note that we cannot actually delete the blocks,
1062 // as we cannot modify the CFG of the function.
1064 std::set<BasicBlock*> &ExecutableBBs = Solver.getExecutableBlocks();
1065 for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB)
1066 if (!ExecutableBBs.count(BB)) {
1067 DEBUG(std::cerr << " BasicBlock Dead:" << *BB);
1070 // Delete the instructions backwards, as it has a reduced likelihood of
1071 // having to update as many def-use and use-def chains.
1072 std::vector<Instruction*> Insts;
1073 for (BasicBlock::iterator I = BB->begin(), E = BB->getTerminator();
1076 while (!Insts.empty()) {
1077 Instruction *I = Insts.back();
1079 if (!I->use_empty())
1080 I->replaceAllUsesWith(UndefValue::get(I->getType()));
1081 BB->getInstList().erase(I);
1086 // Iterate over all of the instructions in a function, replacing them with
1087 // constants if we have found them to be of constant values.
1089 for (BasicBlock::iterator BI = BB->begin(), E = BB->end(); BI != E; ) {
1090 Instruction *Inst = BI++;
1091 if (Inst->getType() != Type::VoidTy) {
1092 LatticeVal &IV = Values[Inst];
1093 if (IV.isConstant() || IV.isUndefined() &&
1094 !isa<TerminatorInst>(Inst)) {
1095 Constant *Const = IV.isConstant()
1096 ? IV.getConstant() : UndefValue::get(Inst->getType());
1097 DEBUG(std::cerr << " Constant: " << *Const << " = " << *Inst);
1099 // Replaces all of the uses of a variable with uses of the constant.
1100 Inst->replaceAllUsesWith(Const);
1102 // Delete the instruction.
1103 BB->getInstList().erase(Inst);
1105 // Hey, we just changed something!
1117 Statistic<> IPNumInstRemoved("ipsccp", "Number of instructions removed");
1118 Statistic<> IPNumDeadBlocks ("ipsccp", "Number of basic blocks unreachable");
1119 Statistic<> IPNumArgsElimed ("ipsccp",
1120 "Number of arguments constant propagated");
1121 Statistic<> IPNumGlobalConst("ipsccp",
1122 "Number of globals found to be constant");
1124 //===--------------------------------------------------------------------===//
1126 /// IPSCCP Class - This class implements interprocedural Sparse Conditional
1127 /// Constant Propagation.
1129 struct IPSCCP : public ModulePass {
1130 bool runOnModule(Module &M);
1134 Y("ipsccp", "Interprocedural Sparse Conditional Constant Propagation");
1135 } // end anonymous namespace
1137 // createIPSCCPPass - This is the public interface to this file...
1138 ModulePass *llvm::createIPSCCPPass() {
1139 return new IPSCCP();
1143 static bool AddressIsTaken(GlobalValue *GV) {
1144 // Delete any dead constantexpr klingons.
1145 GV->removeDeadConstantUsers();
1147 for (Value::use_iterator UI = GV->use_begin(), E = GV->use_end();
1149 if (StoreInst *SI = dyn_cast<StoreInst>(*UI)) {
1150 if (SI->getOperand(0) == GV || SI->isVolatile())
1151 return true; // Storing addr of GV.
1152 } else if (isa<InvokeInst>(*UI) || isa<CallInst>(*UI)) {
1153 // Make sure we are calling the function, not passing the address.
1154 CallSite CS = CallSite::get(cast<Instruction>(*UI));
1155 for (CallSite::arg_iterator AI = CS.arg_begin(),
1156 E = CS.arg_end(); AI != E; ++AI)
1159 } else if (LoadInst *LI = dyn_cast<LoadInst>(*UI)) {
1160 if (LI->isVolatile())
1168 bool IPSCCP::runOnModule(Module &M) {
1171 // Loop over all functions, marking arguments to those with their addresses
1172 // taken or that are external as overdefined.
1174 hash_map<Value*, LatticeVal> &Values = Solver.getValueMapping();
1175 for (Module::iterator F = M.begin(), E = M.end(); F != E; ++F)
1176 if (!F->hasInternalLinkage() || AddressIsTaken(F)) {
1177 if (!F->isExternal())
1178 Solver.MarkBlockExecutable(F->begin());
1179 for (Function::arg_iterator AI = F->arg_begin(), E = F->arg_end();
1181 Values[AI].markOverdefined();
1183 Solver.AddTrackedFunction(F);
1186 // Loop over global variables. We inform the solver about any internal global
1187 // variables that do not have their 'addresses taken'. If they don't have
1188 // their addresses taken, we can propagate constants through them.
1189 for (Module::global_iterator G = M.global_begin(), E = M.global_end();
1191 if (!G->isConstant() && G->hasInternalLinkage() && !AddressIsTaken(G))
1192 Solver.TrackValueOfGlobalVariable(G);
1194 // Solve for constants.
1195 bool ResolvedBranches = true;
1196 while (ResolvedBranches) {
1199 DEBUG(std::cerr << "RESOLVING UNDEF BRANCHES\n");
1200 ResolvedBranches = false;
1201 for (Module::iterator F = M.begin(), E = M.end(); F != E; ++F)
1202 ResolvedBranches |= Solver.ResolveBranchesIn(*F);
1205 bool MadeChanges = false;
1207 // Iterate over all of the instructions in the module, replacing them with
1208 // constants if we have found them to be of constant values.
1210 std::set<BasicBlock*> &ExecutableBBs = Solver.getExecutableBlocks();
1211 for (Module::iterator F = M.begin(), E = M.end(); F != E; ++F) {
1212 for (Function::arg_iterator AI = F->arg_begin(), E = F->arg_end();
1214 if (!AI->use_empty()) {
1215 LatticeVal &IV = Values[AI];
1216 if (IV.isConstant() || IV.isUndefined()) {
1217 Constant *CST = IV.isConstant() ?
1218 IV.getConstant() : UndefValue::get(AI->getType());
1219 DEBUG(std::cerr << "*** Arg " << *AI << " = " << *CST <<"\n");
1221 // Replaces all of the uses of a variable with uses of the
1223 AI->replaceAllUsesWith(CST);
1228 std::vector<BasicBlock*> BlocksToErase;
1229 for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB)
1230 if (!ExecutableBBs.count(BB)) {
1231 DEBUG(std::cerr << " BasicBlock Dead:" << *BB);
1234 // Delete the instructions backwards, as it has a reduced likelihood of
1235 // having to update as many def-use and use-def chains.
1236 std::vector<Instruction*> Insts;
1237 TerminatorInst *TI = BB->getTerminator();
1238 for (BasicBlock::iterator I = BB->begin(), E = TI; I != E; ++I)
1241 while (!Insts.empty()) {
1242 Instruction *I = Insts.back();
1244 if (!I->use_empty())
1245 I->replaceAllUsesWith(UndefValue::get(I->getType()));
1246 BB->getInstList().erase(I);
1251 for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i) {
1252 BasicBlock *Succ = TI->getSuccessor(i);
1253 if (Succ->begin() != Succ->end() && isa<PHINode>(Succ->begin()))
1254 TI->getSuccessor(i)->removePredecessor(BB);
1256 if (!TI->use_empty())
1257 TI->replaceAllUsesWith(UndefValue::get(TI->getType()));
1258 BB->getInstList().erase(TI);
1260 if (&*BB != &F->front())
1261 BlocksToErase.push_back(BB);
1263 new UnreachableInst(BB);
1266 for (BasicBlock::iterator BI = BB->begin(), E = BB->end(); BI != E; ) {
1267 Instruction *Inst = BI++;
1268 if (Inst->getType() != Type::VoidTy) {
1269 LatticeVal &IV = Values[Inst];
1270 if (IV.isConstant() || IV.isUndefined() &&
1271 !isa<TerminatorInst>(Inst)) {
1272 Constant *Const = IV.isConstant()
1273 ? IV.getConstant() : UndefValue::get(Inst->getType());
1274 DEBUG(std::cerr << " Constant: " << *Const << " = " << *Inst);
1276 // Replaces all of the uses of a variable with uses of the
1278 Inst->replaceAllUsesWith(Const);
1280 // Delete the instruction.
1281 if (!isa<TerminatorInst>(Inst) && !isa<CallInst>(Inst))
1282 BB->getInstList().erase(Inst);
1284 // Hey, we just changed something!
1292 // Now that all instructions in the function are constant folded, erase dead
1293 // blocks, because we can now use ConstantFoldTerminator to get rid of
1295 for (unsigned i = 0, e = BlocksToErase.size(); i != e; ++i) {
1296 // If there are any PHI nodes in this successor, drop entries for BB now.
1297 BasicBlock *DeadBB = BlocksToErase[i];
1298 while (!DeadBB->use_empty()) {
1299 Instruction *I = cast<Instruction>(DeadBB->use_back());
1300 bool Folded = ConstantFoldTerminator(I->getParent());
1301 assert(Folded && "Didn't fold away reference to block!");
1304 // Finally, delete the basic block.
1305 F->getBasicBlockList().erase(DeadBB);
1309 // If we inferred constant or undef return values for a function, we replaced
1310 // all call uses with the inferred value. This means we don't need to bother
1311 // actually returning anything from the function. Replace all return
1312 // instructions with return undef.
1313 const hash_map<Function*, LatticeVal> &RV =Solver.getTrackedFunctionRetVals();
1314 for (hash_map<Function*, LatticeVal>::const_iterator I = RV.begin(),
1315 E = RV.end(); I != E; ++I)
1316 if (!I->second.isOverdefined() &&
1317 I->first->getReturnType() != Type::VoidTy) {
1318 Function *F = I->first;
1319 for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB)
1320 if (ReturnInst *RI = dyn_cast<ReturnInst>(BB->getTerminator()))
1321 if (!isa<UndefValue>(RI->getOperand(0)))
1322 RI->setOperand(0, UndefValue::get(F->getReturnType()));
1325 // If we infered constant or undef values for globals variables, we can delete
1326 // the global and any stores that remain to it.
1327 const hash_map<GlobalVariable*, LatticeVal> &TG = Solver.getTrackedGlobals();
1328 for (hash_map<GlobalVariable*, LatticeVal>::const_iterator I = TG.begin(),
1329 E = TG.end(); I != E; ++I) {
1330 GlobalVariable *GV = I->first;
1331 assert(!I->second.isOverdefined() &&
1332 "Overdefined values should have been taken out of the map!");
1333 DEBUG(std::cerr << "Found that GV '" << GV->getName()<< "' is constant!\n");
1334 while (!GV->use_empty()) {
1335 StoreInst *SI = cast<StoreInst>(GV->use_back());
1336 SI->eraseFromParent();
1338 M.getGlobalList().erase(GV);