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 #include "llvm/Transforms/Scalar.h"
25 #include "llvm/Constants.h"
26 #include "llvm/Function.h"
27 #include "llvm/GlobalVariable.h"
28 #include "llvm/Instructions.h"
29 #include "llvm/Pass.h"
30 #include "llvm/Type.h"
31 #include "llvm/Support/InstVisitor.h"
32 #include "Support/Debug.h"
33 #include "Support/Statistic.h"
34 #include "Support/STLExtras.h"
39 // InstVal class - This class represents the different lattice values that an
40 // instruction may occupy. It is a simple class with value semantics.
43 Statistic<> NumInstRemoved("sccp", "Number of instructions removed");
47 undefined, // This instruction has no known value
48 constant, // This instruction has a constant value
49 overdefined // This instruction has an unknown value
50 } LatticeValue; // The current lattice position
51 Constant *ConstantVal; // If Constant value, the current value
53 inline InstVal() : LatticeValue(undefined), ConstantVal(0) {}
55 // markOverdefined - Return true if this is a new status to be in...
56 inline bool markOverdefined() {
57 if (LatticeValue != overdefined) {
58 LatticeValue = overdefined;
64 // markConstant - Return true if this is a new status for us...
65 inline bool markConstant(Constant *V) {
66 if (LatticeValue != constant) {
67 LatticeValue = constant;
71 assert(ConstantVal == V && "Marking constant with different value");
76 inline bool isUndefined() const { return LatticeValue == undefined; }
77 inline bool isConstant() const { return LatticeValue == constant; }
78 inline bool isOverdefined() const { return LatticeValue == overdefined; }
80 inline Constant *getConstant() const {
81 assert(isConstant() && "Cannot get the constant of a non-constant!");
86 } // end anonymous namespace
89 //===----------------------------------------------------------------------===//
92 // This class does all of the work of Sparse Conditional Constant Propagation.
95 class SCCP : public FunctionPass, public InstVisitor<SCCP> {
96 std::set<BasicBlock*> BBExecutable;// The basic blocks that are executable
97 std::map<Value*, InstVal> ValueState; // The state each value is in...
99 std::vector<Instruction*> InstWorkList;// The instruction work list
100 std::vector<BasicBlock*> BBWorkList; // The BasicBlock work list
102 /// UsersOfOverdefinedPHIs - Keep track of any users of PHI nodes that are not
103 /// overdefined, despite the fact that the PHI node is overdefined.
104 std::multimap<PHINode*, Instruction*> UsersOfOverdefinedPHIs;
106 /// KnownFeasibleEdges - Entries in this set are edges which have already had
107 /// PHI nodes retriggered.
108 typedef std::pair<BasicBlock*,BasicBlock*> Edge;
109 std::set<Edge> KnownFeasibleEdges;
112 // runOnFunction - Run the Sparse Conditional Constant Propagation algorithm,
113 // and return true if the function was modified.
115 bool runOnFunction(Function &F);
117 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
118 AU.setPreservesCFG();
122 //===--------------------------------------------------------------------===//
123 // The implementation of this class
126 friend class InstVisitor<SCCP>; // Allow callbacks from visitor
128 // markValueOverdefined - Make a value be marked as "constant". If the value
129 // is not already a constant, add it to the instruction work list so that
130 // the users of the instruction are updated later.
132 inline void markConstant(InstVal &IV, Instruction *I, Constant *C) {
133 if (IV.markConstant(C)) {
134 DEBUG(std::cerr << "markConstant: " << *C << ": " << *I);
135 InstWorkList.push_back(I);
138 inline void markConstant(Instruction *I, Constant *C) {
139 markConstant(ValueState[I], I, C);
142 // markValueOverdefined - Make a value be marked as "overdefined". If the
143 // value is not already overdefined, add it to the instruction work list so
144 // that the users of the instruction are updated later.
146 inline void markOverdefined(InstVal &IV, Instruction *I) {
147 if (IV.markOverdefined()) {
148 DEBUG(std::cerr << "markOverdefined: " << *I);
149 InstWorkList.push_back(I); // Only instructions go on the work list
152 inline void markOverdefined(Instruction *I) {
153 markOverdefined(ValueState[I], I);
156 // getValueState - Return the InstVal object that corresponds to the value.
157 // This function is necessary because not all values should start out in the
158 // underdefined state... Argument's should be overdefined, and
159 // constants should be marked as constants. If a value is not known to be an
160 // Instruction object, then use this accessor to get its value from the map.
162 inline InstVal &getValueState(Value *V) {
163 std::map<Value*, InstVal>::iterator I = ValueState.find(V);
164 if (I != ValueState.end()) return I->second; // Common case, in the map
166 if (Constant *CPV = dyn_cast<Constant>(V)) { // Constants are constant
167 ValueState[CPV].markConstant(CPV);
168 } else if (isa<Argument>(V)) { // Arguments are overdefined
169 ValueState[V].markOverdefined();
170 } else if (GlobalValue *GV = dyn_cast<GlobalValue>(V)) {
171 // The address of a global is a constant...
172 ValueState[V].markConstant(ConstantPointerRef::get(GV));
174 // All others are underdefined by default...
175 return ValueState[V];
178 // markEdgeExecutable - Mark a basic block as executable, adding it to the BB
179 // work list if it is not already executable...
181 void markEdgeExecutable(BasicBlock *Source, BasicBlock *Dest) {
182 if (!KnownFeasibleEdges.insert(Edge(Source, Dest)).second)
183 return; // This edge is already known to be executable!
185 if (BBExecutable.count(Dest)) {
186 DEBUG(std::cerr << "Marking Edge Executable: " << Source->getName()
187 << " -> " << Dest->getName() << "\n");
189 // The destination is already executable, but we just made an edge
190 // feasible that wasn't before. Revisit the PHI nodes in the block
191 // because they have potentially new operands.
192 for (BasicBlock::iterator I = Dest->begin();
193 PHINode *PN = dyn_cast<PHINode>(I); ++I)
197 DEBUG(std::cerr << "Marking Block Executable: " << Dest->getName()<<"\n");
198 BBExecutable.insert(Dest); // Basic block is executable!
199 BBWorkList.push_back(Dest); // Add the block to the work list!
204 // visit implementations - Something changed in this instruction... Either an
205 // operand made a transition, or the instruction is newly executable. Change
206 // the value type of I to reflect these changes if appropriate.
208 void visitPHINode(PHINode &I);
211 void visitReturnInst(ReturnInst &I) { /*does not have an effect*/ }
212 void visitTerminatorInst(TerminatorInst &TI);
214 void visitCastInst(CastInst &I);
215 void visitBinaryOperator(Instruction &I);
216 void visitShiftInst(ShiftInst &I) { visitBinaryOperator(I); }
218 // Instructions that cannot be folded away...
219 void visitStoreInst (Instruction &I) { /*returns void*/ }
220 void visitLoadInst (LoadInst &I);
221 void visitGetElementPtrInst(GetElementPtrInst &I);
222 void visitCallInst (Instruction &I) { markOverdefined(&I); }
223 void visitInvokeInst (TerminatorInst &I) {
224 if (I.getType() != Type::VoidTy) markOverdefined(&I);
225 visitTerminatorInst(I);
227 void visitUnwindInst (TerminatorInst &I) { /*returns void*/ }
228 void visitAllocationInst(Instruction &I) { markOverdefined(&I); }
229 void visitVANextInst (Instruction &I) { markOverdefined(&I); }
230 void visitVAArgInst (Instruction &I) { markOverdefined(&I); }
231 void visitFreeInst (Instruction &I) { /*returns void*/ }
233 void visitInstruction(Instruction &I) {
234 // If a new instruction is added to LLVM that we don't handle...
235 std::cerr << "SCCP: Don't know how to handle: " << I;
236 markOverdefined(&I); // Just in case
239 // getFeasibleSuccessors - Return a vector of booleans to indicate which
240 // successors are reachable from a given terminator instruction.
242 void getFeasibleSuccessors(TerminatorInst &TI, std::vector<bool> &Succs);
244 // isEdgeFeasible - Return true if the control flow edge from the 'From' basic
245 // block to the 'To' basic block is currently feasible...
247 bool isEdgeFeasible(BasicBlock *From, BasicBlock *To);
249 // OperandChangedState - This method is invoked on all of the users of an
250 // instruction that was just changed state somehow.... Based on this
251 // information, we need to update the specified user of this instruction.
253 void OperandChangedState(User *U) {
254 // Only instructions use other variable values!
255 Instruction &I = cast<Instruction>(*U);
256 if (BBExecutable.count(I.getParent())) // Inst is executable?
261 RegisterOpt<SCCP> X("sccp", "Sparse Conditional Constant Propagation");
262 } // end anonymous namespace
265 // createSCCPPass - This is the public interface to this file...
266 Pass *llvm::createSCCPPass() {
271 //===----------------------------------------------------------------------===//
272 // SCCP Class Implementation
275 // runOnFunction() - Run the Sparse Conditional Constant Propagation algorithm,
276 // and return true if the function was modified.
278 bool SCCP::runOnFunction(Function &F) {
279 // Mark the first block of the function as being executable...
280 BBExecutable.insert(F.begin()); // Basic block is executable!
281 BBWorkList.push_back(F.begin()); // Add the block to the work list!
283 // Process the work lists until their are empty!
284 while (!BBWorkList.empty() || !InstWorkList.empty()) {
285 // Process the instruction work list...
286 while (!InstWorkList.empty()) {
287 Instruction *I = InstWorkList.back();
288 InstWorkList.pop_back();
290 DEBUG(std::cerr << "\nPopped off I-WL: " << I);
292 // "I" got into the work list because it either made the transition from
293 // bottom to constant, or to Overdefined.
295 // Update all of the users of this instruction's value...
297 for_each(I->use_begin(), I->use_end(),
298 bind_obj(this, &SCCP::OperandChangedState));
301 // Process the basic block work list...
302 while (!BBWorkList.empty()) {
303 BasicBlock *BB = BBWorkList.back();
304 BBWorkList.pop_back();
306 DEBUG(std::cerr << "\nPopped off BBWL: " << BB);
308 // Notify all instructions in this basic block that they are newly
315 for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I)
316 if (!BBExecutable.count(I))
317 std::cerr << "BasicBlock Dead:" << *I;
320 // Iterate over all of the instructions in a function, replacing them with
321 // constants if we have found them to be of constant values.
323 bool MadeChanges = false;
324 for (Function::iterator BB = F.begin(), BBE = F.end(); BB != BBE; ++BB)
325 for (BasicBlock::iterator BI = BB->begin(); BI != BB->end();) {
326 Instruction &Inst = *BI;
327 InstVal &IV = ValueState[&Inst];
328 if (IV.isConstant()) {
329 Constant *Const = IV.getConstant();
330 DEBUG(std::cerr << "Constant: " << Const << " = " << Inst);
332 // Replaces all of the uses of a variable with uses of the constant.
333 Inst.replaceAllUsesWith(Const);
335 // Remove the operator from the list of definitions... and delete it.
336 BI = BB->getInstList().erase(BI);
338 // Hey, we just changed something!
346 // Reset state so that the next invocation will have empty data structures
347 BBExecutable.clear();
349 std::vector<Instruction*>().swap(InstWorkList);
350 std::vector<BasicBlock*>().swap(BBWorkList);
356 // getFeasibleSuccessors - Return a vector of booleans to indicate which
357 // successors are reachable from a given terminator instruction.
359 void SCCP::getFeasibleSuccessors(TerminatorInst &TI, std::vector<bool> &Succs) {
360 Succs.resize(TI.getNumSuccessors());
361 if (BranchInst *BI = dyn_cast<BranchInst>(&TI)) {
362 if (BI->isUnconditional()) {
365 InstVal &BCValue = getValueState(BI->getCondition());
366 if (BCValue.isOverdefined() ||
367 (BCValue.isConstant() && !isa<ConstantBool>(BCValue.getConstant()))) {
368 // Overdefined condition variables, and branches on unfoldable constant
369 // conditions, mean the branch could go either way.
370 Succs[0] = Succs[1] = true;
371 } else if (BCValue.isConstant()) {
372 // Constant condition variables mean the branch can only go a single way
373 Succs[BCValue.getConstant() == ConstantBool::False] = true;
376 } else if (InvokeInst *II = dyn_cast<InvokeInst>(&TI)) {
377 // Invoke instructions successors are always executable.
378 Succs[0] = Succs[1] = true;
379 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(&TI)) {
380 InstVal &SCValue = getValueState(SI->getCondition());
381 if (SCValue.isOverdefined() || // Overdefined condition?
382 (SCValue.isConstant() && !isa<ConstantInt>(SCValue.getConstant()))) {
383 // All destinations are executable!
384 Succs.assign(TI.getNumSuccessors(), true);
385 } else if (SCValue.isConstant()) {
386 Constant *CPV = SCValue.getConstant();
387 // Make sure to skip the "default value" which isn't a value
388 for (unsigned i = 1, E = SI->getNumSuccessors(); i != E; ++i) {
389 if (SI->getSuccessorValue(i) == CPV) {// Found the right branch...
395 // Constant value not equal to any of the branches... must execute
396 // default branch then...
400 std::cerr << "SCCP: Don't know how to handle: " << TI;
401 Succs.assign(TI.getNumSuccessors(), true);
406 // isEdgeFeasible - Return true if the control flow edge from the 'From' basic
407 // block to the 'To' basic block is currently feasible...
409 bool SCCP::isEdgeFeasible(BasicBlock *From, BasicBlock *To) {
410 assert(BBExecutable.count(To) && "Dest should always be alive!");
412 // Make sure the source basic block is executable!!
413 if (!BBExecutable.count(From)) return false;
415 // Check to make sure this edge itself is actually feasible now...
416 TerminatorInst *TI = From->getTerminator();
417 if (BranchInst *BI = dyn_cast<BranchInst>(TI)) {
418 if (BI->isUnconditional())
421 InstVal &BCValue = getValueState(BI->getCondition());
422 if (BCValue.isOverdefined()) {
423 // Overdefined condition variables mean the branch could go either way.
425 } else if (BCValue.isConstant()) {
426 // Not branching on an evaluatable constant?
427 if (!isa<ConstantBool>(BCValue.getConstant())) return true;
429 // Constant condition variables mean the branch can only go a single way
430 return BI->getSuccessor(BCValue.getConstant() ==
431 ConstantBool::False) == To;
435 } else if (InvokeInst *II = dyn_cast<InvokeInst>(TI)) {
436 // Invoke instructions successors are always executable.
438 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
439 InstVal &SCValue = getValueState(SI->getCondition());
440 if (SCValue.isOverdefined()) { // Overdefined condition?
441 // All destinations are executable!
443 } else if (SCValue.isConstant()) {
444 Constant *CPV = SCValue.getConstant();
445 if (!isa<ConstantInt>(CPV))
446 return true; // not a foldable constant?
448 // Make sure to skip the "default value" which isn't a value
449 for (unsigned i = 1, E = SI->getNumSuccessors(); i != E; ++i)
450 if (SI->getSuccessorValue(i) == CPV) // Found the taken branch...
451 return SI->getSuccessor(i) == To;
453 // Constant value not equal to any of the branches... must execute
454 // default branch then...
455 return SI->getDefaultDest() == To;
459 std::cerr << "Unknown terminator instruction: " << *TI;
464 // visit Implementations - Something changed in this instruction... Either an
465 // operand made a transition, or the instruction is newly executable. Change
466 // the value type of I to reflect these changes if appropriate. This method
467 // makes sure to do the following actions:
469 // 1. If a phi node merges two constants in, and has conflicting value coming
470 // from different branches, or if the PHI node merges in an overdefined
471 // value, then the PHI node becomes overdefined.
472 // 2. If a phi node merges only constants in, and they all agree on value, the
473 // PHI node becomes a constant value equal to that.
474 // 3. If V <- x (op) y && isConstant(x) && isConstant(y) V = Constant
475 // 4. If V <- x (op) y && (isOverdefined(x) || isOverdefined(y)) V = Overdefined
476 // 5. If V <- MEM or V <- CALL or V <- (unknown) then V = Overdefined
477 // 6. If a conditional branch has a value that is constant, make the selected
478 // destination executable
479 // 7. If a conditional branch has a value that is overdefined, make all
480 // successors executable.
482 void SCCP::visitPHINode(PHINode &PN) {
483 InstVal &PNIV = getValueState(&PN);
484 if (PNIV.isOverdefined()) {
485 // There may be instructions using this PHI node that are not overdefined
486 // themselves. If so, make sure that they know that the PHI node operand
488 std::multimap<PHINode*, Instruction*>::iterator I, E;
489 tie(I, E) = UsersOfOverdefinedPHIs.equal_range(&PN);
491 std::vector<Instruction*> Users;
492 Users.reserve(std::distance(I, E));
493 for (; I != E; ++I) Users.push_back(I->second);
494 while (!Users.empty()) {
499 return; // Quick exit
502 // Look at all of the executable operands of the PHI node. If any of them
503 // are overdefined, the PHI becomes overdefined as well. If they are all
504 // constant, and they agree with each other, the PHI becomes the identical
505 // constant. If they are constant and don't agree, the PHI is overdefined.
506 // If there are no executable operands, the PHI remains undefined.
508 Constant *OperandVal = 0;
509 for (unsigned i = 0, e = PN.getNumIncomingValues(); i != e; ++i) {
510 InstVal &IV = getValueState(PN.getIncomingValue(i));
511 if (IV.isUndefined()) continue; // Doesn't influence PHI node.
513 if (isEdgeFeasible(PN.getIncomingBlock(i), PN.getParent())) {
514 if (IV.isOverdefined()) { // PHI node becomes overdefined!
515 markOverdefined(PNIV, &PN);
519 if (OperandVal == 0) { // Grab the first value...
520 OperandVal = IV.getConstant();
521 } else { // Another value is being merged in!
522 // There is already a reachable operand. If we conflict with it,
523 // then the PHI node becomes overdefined. If we agree with it, we
526 // Check to see if there are two different constants merging...
527 if (IV.getConstant() != OperandVal) {
528 // Yes there is. This means the PHI node is not constant.
529 // You must be overdefined poor PHI.
531 markOverdefined(PNIV, &PN); // The PHI node now becomes overdefined
532 return; // I'm done analyzing you
538 // If we exited the loop, this means that the PHI node only has constant
539 // arguments that agree with each other(and OperandVal is the constant) or
540 // OperandVal is null because there are no defined incoming arguments. If
541 // this is the case, the PHI remains undefined.
544 markConstant(PNIV, &PN, OperandVal); // Acquire operand value
547 void SCCP::visitTerminatorInst(TerminatorInst &TI) {
548 std::vector<bool> SuccFeasible;
549 getFeasibleSuccessors(TI, SuccFeasible);
551 BasicBlock *BB = TI.getParent();
553 // Mark all feasible successors executable...
554 for (unsigned i = 0, e = SuccFeasible.size(); i != e; ++i)
556 markEdgeExecutable(BB, TI.getSuccessor(i));
559 void SCCP::visitCastInst(CastInst &I) {
560 Value *V = I.getOperand(0);
561 InstVal &VState = getValueState(V);
562 if (VState.isOverdefined()) // Inherit overdefinedness of operand
564 else if (VState.isConstant()) // Propagate constant value
565 markConstant(&I, ConstantExpr::getCast(VState.getConstant(), I.getType()));
568 // Handle BinaryOperators and Shift Instructions...
569 void SCCP::visitBinaryOperator(Instruction &I) {
570 InstVal &IV = ValueState[&I];
571 if (IV.isOverdefined()) return;
573 InstVal &V1State = getValueState(I.getOperand(0));
574 InstVal &V2State = getValueState(I.getOperand(1));
576 if (V1State.isOverdefined() || V2State.isOverdefined()) {
577 // If both operands are PHI nodes, it is possible that this instruction has
578 // a constant value, despite the fact that the PHI node doesn't. Check for
579 // this condition now.
580 if (PHINode *PN1 = dyn_cast<PHINode>(I.getOperand(0)))
581 if (PHINode *PN2 = dyn_cast<PHINode>(I.getOperand(1)))
582 if (PN1->getParent() == PN2->getParent()) {
583 // Since the two PHI nodes are in the same basic block, they must have
584 // entries for the same predecessors. Walk the predecessor list, and
585 // if all of the incoming values are constants, and the result of
586 // evaluating this expression with all incoming value pairs is the
587 // same, then this expression is a constant even though the PHI node
588 // is not a constant!
590 for (unsigned i = 0, e = PN1->getNumIncomingValues(); i != e; ++i) {
591 InstVal &In1 = getValueState(PN1->getIncomingValue(i));
592 BasicBlock *InBlock = PN1->getIncomingBlock(i);
593 InstVal &In2 =getValueState(PN2->getIncomingValueForBlock(InBlock));
595 if (In1.isOverdefined() || In2.isOverdefined()) {
596 Result.markOverdefined();
597 break; // Cannot fold this operation over the PHI nodes!
598 } else if (In1.isConstant() && In2.isConstant()) {
599 Constant *V = ConstantExpr::get(I.getOpcode(), In1.getConstant(),
601 if (Result.isUndefined())
602 Result.markConstant(V);
603 else if (Result.isConstant() && Result.getConstant() != V) {
604 Result.markOverdefined();
610 // If we found a constant value here, then we know the instruction is
611 // constant despite the fact that the PHI nodes are overdefined.
612 if (Result.isConstant()) {
613 markConstant(IV, &I, Result.getConstant());
614 // Remember that this instruction is virtually using the PHI node
616 UsersOfOverdefinedPHIs.insert(std::make_pair(PN1, &I));
617 UsersOfOverdefinedPHIs.insert(std::make_pair(PN2, &I));
619 } else if (Result.isUndefined()) {
623 // Okay, this really is overdefined now. Since we might have
624 // speculatively thought that this was not overdefined before, and
625 // added ourselves to the UsersOfOverdefinedPHIs list for the PHIs,
626 // make sure to clean out any entries that we put there, for
628 std::multimap<PHINode*, Instruction*>::iterator It, E;
629 tie(It, E) = UsersOfOverdefinedPHIs.equal_range(PN1);
631 if (It->second == &I) {
632 UsersOfOverdefinedPHIs.erase(It++);
636 tie(It, E) = UsersOfOverdefinedPHIs.equal_range(PN2);
638 if (It->second == &I) {
639 UsersOfOverdefinedPHIs.erase(It++);
645 markOverdefined(IV, &I);
646 } else if (V1State.isConstant() && V2State.isConstant()) {
647 markConstant(IV, &I, ConstantExpr::get(I.getOpcode(), V1State.getConstant(),
648 V2State.getConstant()));
652 // Handle getelementptr instructions... if all operands are constants then we
653 // can turn this into a getelementptr ConstantExpr.
655 void SCCP::visitGetElementPtrInst(GetElementPtrInst &I) {
656 InstVal &IV = ValueState[&I];
657 if (IV.isOverdefined()) return;
659 std::vector<Constant*> Operands;
660 Operands.reserve(I.getNumOperands());
662 for (unsigned i = 0, e = I.getNumOperands(); i != e; ++i) {
663 InstVal &State = getValueState(I.getOperand(i));
664 if (State.isUndefined())
665 return; // Operands are not resolved yet...
666 else if (State.isOverdefined()) {
667 markOverdefined(IV, &I);
670 assert(State.isConstant() && "Unknown state!");
671 Operands.push_back(State.getConstant());
674 Constant *Ptr = Operands[0];
675 Operands.erase(Operands.begin()); // Erase the pointer from idx list...
677 markConstant(IV, &I, ConstantExpr::getGetElementPtr(Ptr, Operands));
680 /// GetGEPGlobalInitializer - Given a constant and a getelementptr constantexpr,
681 /// return the constant value being addressed by the constant expression, or
682 /// null if something is funny.
684 static Constant *GetGEPGlobalInitializer(Constant *C, ConstantExpr *CE) {
685 if (CE->getOperand(1) != Constant::getNullValue(Type::LongTy))
686 return 0; // Do not allow stepping over the value!
688 // Loop over all of the operands, tracking down which value we are
690 for (unsigned i = 2, e = CE->getNumOperands(); i != e; ++i)
691 if (ConstantUInt *CU = dyn_cast<ConstantUInt>(CE->getOperand(i))) {
692 ConstantStruct *CS = dyn_cast<ConstantStruct>(C);
693 if (CS == 0) return 0;
694 if (CU->getValue() >= CS->getValues().size()) return 0;
695 C = cast<Constant>(CS->getValues()[CU->getValue()]);
696 } else if (ConstantSInt *CS = dyn_cast<ConstantSInt>(CE->getOperand(i))) {
697 ConstantArray *CA = dyn_cast<ConstantArray>(C);
698 if (CA == 0) return 0;
699 if ((uint64_t)CS->getValue() >= CA->getValues().size()) return 0;
700 C = cast<Constant>(CA->getValues()[CS->getValue()]);
706 // Handle load instructions. If the operand is a constant pointer to a constant
707 // global, we can replace the load with the loaded constant value!
708 void SCCP::visitLoadInst(LoadInst &I) {
709 InstVal &IV = ValueState[&I];
710 if (IV.isOverdefined()) return;
712 InstVal &PtrVal = getValueState(I.getOperand(0));
713 if (PtrVal.isUndefined()) return; // The pointer is not resolved yet!
714 if (PtrVal.isConstant() && !I.isVolatile()) {
715 Value *Ptr = PtrVal.getConstant();
716 if (ConstantPointerRef *CPR = dyn_cast<ConstantPointerRef>(Ptr))
717 Ptr = CPR->getValue();
719 // Transform load (constant global) into the value loaded.
720 if (GlobalVariable *GV = dyn_cast<GlobalVariable>(Ptr))
721 if (GV->isConstant() && !GV->isExternal()) {
722 markConstant(IV, &I, GV->getInitializer());
726 // Transform load (constantexpr_GEP global, 0, ...) into the value loaded.
727 if (ConstantExpr *CE = dyn_cast<ConstantExpr>(Ptr))
728 if (CE->getOpcode() == Instruction::GetElementPtr)
729 if (ConstantPointerRef *G
730 = dyn_cast<ConstantPointerRef>(CE->getOperand(0)))
731 if (GlobalVariable *GV = dyn_cast<GlobalVariable>(G->getValue()))
732 if (GV->isConstant() && !GV->isExternal())
734 GetGEPGlobalInitializer(GV->getInitializer(), CE)) {
735 markConstant(IV, &I, V);
740 // Otherwise we cannot say for certain what value this load will produce.
742 markOverdefined(IV, &I);