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/ConstantHandling.h"
26 #include "llvm/Function.h"
27 #include "llvm/GlobalVariable.h"
28 #include "llvm/Instructions.h"
29 #include "llvm/Pass.h"
30 #include "llvm/Support/InstVisitor.h"
31 #include "Support/Debug.h"
32 #include "Support/Statistic.h"
33 #include "Support/STLExtras.h"
38 // InstVal class - This class represents the different lattice values that an
39 // instruction may occupy. It is a simple class with value semantics.
42 Statistic<> NumInstRemoved("sccp", "Number of instructions removed");
46 undefined, // This instruction has no known value
47 constant, // This instruction has a constant value
48 overdefined // This instruction has an unknown value
49 } LatticeValue; // The current lattice position
50 Constant *ConstantVal; // If Constant value, the current value
52 inline InstVal() : LatticeValue(undefined), ConstantVal(0) {}
54 // markOverdefined - Return true if this is a new status to be in...
55 inline bool markOverdefined() {
56 if (LatticeValue != overdefined) {
57 LatticeValue = overdefined;
63 // markConstant - Return true if this is a new status for us...
64 inline bool markConstant(Constant *V) {
65 if (LatticeValue != constant) {
66 LatticeValue = constant;
70 assert(ConstantVal == V && "Marking constant with different value");
75 inline bool isUndefined() const { return LatticeValue == undefined; }
76 inline bool isConstant() const { return LatticeValue == constant; }
77 inline bool isOverdefined() const { return LatticeValue == overdefined; }
79 inline Constant *getConstant() const {
80 assert(isConstant() && "Cannot get the constant of a non-constant!");
85 } // end anonymous namespace
88 //===----------------------------------------------------------------------===//
91 // This class does all of the work of Sparse Conditional Constant Propagation.
94 class SCCP : public FunctionPass, public InstVisitor<SCCP> {
95 std::set<BasicBlock*> BBExecutable;// The basic blocks that are executable
96 std::map<Value*, InstVal> ValueState; // The state each value is in...
98 std::vector<Instruction*> InstWorkList;// The instruction work list
99 std::vector<BasicBlock*> BBWorkList; // The BasicBlock work list
101 /// UsersOfOverdefinedPHIs - Keep track of any users of PHI nodes that are not
102 /// overdefined, despite the fact that the PHI node is overdefined.
103 std::multimap<PHINode*, Instruction*> UsersOfOverdefinedPHIs;
105 /// KnownFeasibleEdges - Entries in this set are edges which have already had
106 /// PHI nodes retriggered.
107 typedef std::pair<BasicBlock*,BasicBlock*> Edge;
108 std::set<Edge> KnownFeasibleEdges;
111 // runOnFunction - Run the Sparse Conditional Constant Propagation algorithm,
112 // and return true if the function was modified.
114 bool runOnFunction(Function &F);
116 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
117 AU.setPreservesCFG();
121 //===--------------------------------------------------------------------===//
122 // The implementation of this class
125 friend class InstVisitor<SCCP>; // Allow callbacks from visitor
127 // markValueOverdefined - Make a value be marked as "constant". If the value
128 // is not already a constant, add it to the instruction work list so that
129 // the users of the instruction are updated later.
131 inline void markConstant(InstVal &IV, Instruction *I, Constant *C) {
132 if (IV.markConstant(C)) {
133 DEBUG(std::cerr << "markConstant: " << *C << ": " << *I);
134 InstWorkList.push_back(I);
137 inline void markConstant(Instruction *I, Constant *C) {
138 markConstant(ValueState[I], I, C);
141 // markValueOverdefined - Make a value be marked as "overdefined". If the
142 // value is not already overdefined, add it to the instruction work list so
143 // that the users of the instruction are updated later.
145 inline void markOverdefined(InstVal &IV, Instruction *I) {
146 if (IV.markOverdefined()) {
147 DEBUG(std::cerr << "markOverdefined: " << *I);
148 InstWorkList.push_back(I); // Only instructions go on the work list
151 inline void markOverdefined(Instruction *I) {
152 markOverdefined(ValueState[I], I);
155 // getValueState - Return the InstVal object that corresponds to the value.
156 // This function is necessary because not all values should start out in the
157 // underdefined state... Argument's should be overdefined, and
158 // constants should be marked as constants. If a value is not known to be an
159 // Instruction object, then use this accessor to get its value from the map.
161 inline InstVal &getValueState(Value *V) {
162 std::map<Value*, InstVal>::iterator I = ValueState.find(V);
163 if (I != ValueState.end()) return I->second; // Common case, in the map
165 if (Constant *CPV = dyn_cast<Constant>(V)) { // Constants are constant
166 ValueState[CPV].markConstant(CPV);
167 } else if (isa<Argument>(V)) { // Arguments are overdefined
168 ValueState[V].markOverdefined();
169 } else if (GlobalValue *GV = dyn_cast<GlobalValue>(V)) {
170 // The address of a global is a constant...
171 ValueState[V].markConstant(ConstantPointerRef::get(GV));
173 // All others are underdefined by default...
174 return ValueState[V];
177 // markEdgeExecutable - Mark a basic block as executable, adding it to the BB
178 // work list if it is not already executable...
180 void markEdgeExecutable(BasicBlock *Source, BasicBlock *Dest) {
181 if (!KnownFeasibleEdges.insert(Edge(Source, Dest)).second)
182 return; // This edge is already known to be executable!
184 if (BBExecutable.count(Dest)) {
185 DEBUG(std::cerr << "Marking Edge Executable: " << Source->getName()
186 << " -> " << Dest->getName() << "\n");
188 // The destination is already executable, but we just made an edge
189 // feasible that wasn't before. Revisit the PHI nodes in the block
190 // because they have potentially new operands.
191 for (BasicBlock::iterator I = Dest->begin();
192 PHINode *PN = dyn_cast<PHINode>(I); ++I)
196 DEBUG(std::cerr << "Marking Block Executable: " << Dest->getName()<<"\n");
197 BBExecutable.insert(Dest); // Basic block is executable!
198 BBWorkList.push_back(Dest); // Add the block to the work list!
203 // visit implementations - Something changed in this instruction... Either an
204 // operand made a transition, or the instruction is newly executable. Change
205 // the value type of I to reflect these changes if appropriate.
207 void visitPHINode(PHINode &I);
210 void visitReturnInst(ReturnInst &I) { /*does not have an effect*/ }
211 void visitTerminatorInst(TerminatorInst &TI);
213 void visitCastInst(CastInst &I);
214 void visitBinaryOperator(Instruction &I);
215 void visitShiftInst(ShiftInst &I) { visitBinaryOperator(I); }
217 // Instructions that cannot be folded away...
218 void visitStoreInst (Instruction &I) { /*returns void*/ }
219 void visitLoadInst (LoadInst &I);
220 void visitGetElementPtrInst(GetElementPtrInst &I);
221 void visitCallInst (Instruction &I) { markOverdefined(&I); }
222 void visitInvokeInst (TerminatorInst &I) {
223 if (I.getType() != Type::VoidTy) markOverdefined(&I);
224 visitTerminatorInst(I);
226 void visitUnwindInst (TerminatorInst &I) { /*returns void*/ }
227 void visitAllocationInst(Instruction &I) { markOverdefined(&I); }
228 void visitVANextInst (Instruction &I) { markOverdefined(&I); }
229 void visitVAArgInst (Instruction &I) { markOverdefined(&I); }
230 void visitFreeInst (Instruction &I) { /*returns void*/ }
232 void visitInstruction(Instruction &I) {
233 // If a new instruction is added to LLVM that we don't handle...
234 std::cerr << "SCCP: Don't know how to handle: " << I;
235 markOverdefined(&I); // Just in case
238 // getFeasibleSuccessors - Return a vector of booleans to indicate which
239 // successors are reachable from a given terminator instruction.
241 void getFeasibleSuccessors(TerminatorInst &TI, std::vector<bool> &Succs);
243 // isEdgeFeasible - Return true if the control flow edge from the 'From' basic
244 // block to the 'To' basic block is currently feasible...
246 bool isEdgeFeasible(BasicBlock *From, BasicBlock *To);
248 // OperandChangedState - This method is invoked on all of the users of an
249 // instruction that was just changed state somehow.... Based on this
250 // information, we need to update the specified user of this instruction.
252 void OperandChangedState(User *U) {
253 // Only instructions use other variable values!
254 Instruction &I = cast<Instruction>(*U);
255 if (BBExecutable.count(I.getParent())) // Inst is executable?
260 RegisterOpt<SCCP> X("sccp", "Sparse Conditional Constant Propagation");
261 } // end anonymous namespace
264 // createSCCPPass - This is the public interface to this file...
265 Pass *llvm::createSCCPPass() {
270 //===----------------------------------------------------------------------===//
271 // SCCP Class Implementation
274 // runOnFunction() - Run the Sparse Conditional Constant Propagation algorithm,
275 // and return true if the function was modified.
277 bool SCCP::runOnFunction(Function &F) {
278 // Mark the first block of the function as being executable...
279 BBExecutable.insert(F.begin()); // Basic block is executable!
280 BBWorkList.push_back(F.begin()); // Add the block to the work list!
282 // Process the work lists until their are empty!
283 while (!BBWorkList.empty() || !InstWorkList.empty()) {
284 // Process the instruction work list...
285 while (!InstWorkList.empty()) {
286 Instruction *I = InstWorkList.back();
287 InstWorkList.pop_back();
289 DEBUG(std::cerr << "\nPopped off I-WL: " << I);
291 // "I" got into the work list because it either made the transition from
292 // bottom to constant, or to Overdefined.
294 // Update all of the users of this instruction's value...
296 for_each(I->use_begin(), I->use_end(),
297 bind_obj(this, &SCCP::OperandChangedState));
300 // Process the basic block work list...
301 while (!BBWorkList.empty()) {
302 BasicBlock *BB = BBWorkList.back();
303 BBWorkList.pop_back();
305 DEBUG(std::cerr << "\nPopped off BBWL: " << BB);
307 // Notify all instructions in this basic block that they are newly
314 for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I)
315 if (!BBExecutable.count(I))
316 std::cerr << "BasicBlock Dead:" << *I;
319 // Iterate over all of the instructions in a function, replacing them with
320 // constants if we have found them to be of constant values.
322 bool MadeChanges = false;
323 for (Function::iterator BB = F.begin(), BBE = F.end(); BB != BBE; ++BB)
324 for (BasicBlock::iterator BI = BB->begin(); BI != BB->end();) {
325 Instruction &Inst = *BI;
326 InstVal &IV = ValueState[&Inst];
327 if (IV.isConstant()) {
328 Constant *Const = IV.getConstant();
329 DEBUG(std::cerr << "Constant: " << Const << " = " << Inst);
331 // Replaces all of the uses of a variable with uses of the constant.
332 Inst.replaceAllUsesWith(Const);
334 // Remove the operator from the list of definitions... and delete it.
335 BI = BB->getInstList().erase(BI);
337 // Hey, we just changed something!
345 // Reset state so that the next invocation will have empty data structures
346 BBExecutable.clear();
348 std::vector<Instruction*>().swap(InstWorkList);
349 std::vector<BasicBlock*>().swap(BBWorkList);
355 // getFeasibleSuccessors - Return a vector of booleans to indicate which
356 // successors are reachable from a given terminator instruction.
358 void SCCP::getFeasibleSuccessors(TerminatorInst &TI, std::vector<bool> &Succs) {
359 Succs.resize(TI.getNumSuccessors());
360 if (BranchInst *BI = dyn_cast<BranchInst>(&TI)) {
361 if (BI->isUnconditional()) {
364 InstVal &BCValue = getValueState(BI->getCondition());
365 if (BCValue.isOverdefined()) {
366 // Overdefined condition variables mean the branch could go either way.
367 Succs[0] = Succs[1] = true;
368 } else if (BCValue.isConstant()) {
369 // Constant condition variables mean the branch can only go a single way
370 Succs[BCValue.getConstant() == ConstantBool::False] = true;
373 } else if (InvokeInst *II = dyn_cast<InvokeInst>(&TI)) {
374 // Invoke instructions successors are always executable.
375 Succs[0] = Succs[1] = true;
376 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(&TI)) {
377 InstVal &SCValue = getValueState(SI->getCondition());
378 if (SCValue.isOverdefined()) { // Overdefined condition?
379 // All destinations are executable!
380 Succs.assign(TI.getNumSuccessors(), true);
381 } else if (SCValue.isConstant()) {
382 Constant *CPV = SCValue.getConstant();
383 // Make sure to skip the "default value" which isn't a value
384 for (unsigned i = 1, E = SI->getNumSuccessors(); i != E; ++i) {
385 if (SI->getSuccessorValue(i) == CPV) {// Found the right branch...
391 // Constant value not equal to any of the branches... must execute
392 // default branch then...
396 std::cerr << "SCCP: Don't know how to handle: " << TI;
397 Succs.assign(TI.getNumSuccessors(), true);
402 // isEdgeFeasible - Return true if the control flow edge from the 'From' basic
403 // block to the 'To' basic block is currently feasible...
405 bool SCCP::isEdgeFeasible(BasicBlock *From, BasicBlock *To) {
406 assert(BBExecutable.count(To) && "Dest should always be alive!");
408 // Make sure the source basic block is executable!!
409 if (!BBExecutable.count(From)) return false;
411 // Check to make sure this edge itself is actually feasible now...
412 TerminatorInst *TI = From->getTerminator();
413 if (BranchInst *BI = dyn_cast<BranchInst>(TI)) {
414 if (BI->isUnconditional())
417 InstVal &BCValue = getValueState(BI->getCondition());
418 if (BCValue.isOverdefined()) {
419 // Overdefined condition variables mean the branch could go either way.
421 } else if (BCValue.isConstant()) {
422 // Constant condition variables mean the branch can only go a single way
423 return BI->getSuccessor(BCValue.getConstant() ==
424 ConstantBool::False) == To;
428 } else if (InvokeInst *II = dyn_cast<InvokeInst>(TI)) {
429 // Invoke instructions successors are always executable.
431 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
432 InstVal &SCValue = getValueState(SI->getCondition());
433 if (SCValue.isOverdefined()) { // Overdefined condition?
434 // All destinations are executable!
436 } else if (SCValue.isConstant()) {
437 Constant *CPV = SCValue.getConstant();
438 // Make sure to skip the "default value" which isn't a value
439 for (unsigned i = 1, E = SI->getNumSuccessors(); i != E; ++i)
440 if (SI->getSuccessorValue(i) == CPV) // Found the taken branch...
441 return SI->getSuccessor(i) == To;
443 // Constant value not equal to any of the branches... must execute
444 // default branch then...
445 return SI->getDefaultDest() == To;
449 std::cerr << "Unknown terminator instruction: " << *TI;
454 // visit Implementations - Something changed in this instruction... Either an
455 // operand made a transition, or the instruction is newly executable. Change
456 // the value type of I to reflect these changes if appropriate. This method
457 // makes sure to do the following actions:
459 // 1. If a phi node merges two constants in, and has conflicting value coming
460 // from different branches, or if the PHI node merges in an overdefined
461 // value, then the PHI node becomes overdefined.
462 // 2. If a phi node merges only constants in, and they all agree on value, the
463 // PHI node becomes a constant value equal to that.
464 // 3. If V <- x (op) y && isConstant(x) && isConstant(y) V = Constant
465 // 4. If V <- x (op) y && (isOverdefined(x) || isOverdefined(y)) V = Overdefined
466 // 5. If V <- MEM or V <- CALL or V <- (unknown) then V = Overdefined
467 // 6. If a conditional branch has a value that is constant, make the selected
468 // destination executable
469 // 7. If a conditional branch has a value that is overdefined, make all
470 // successors executable.
472 void SCCP::visitPHINode(PHINode &PN) {
473 InstVal &PNIV = getValueState(&PN);
474 if (PNIV.isOverdefined()) {
475 // There may be instructions using this PHI node that are not overdefined
476 // themselves. If so, make sure that they know that the PHI node operand
478 std::multimap<PHINode*, Instruction*>::iterator I, E;
479 tie(I, E) = UsersOfOverdefinedPHIs.equal_range(&PN);
481 std::vector<Instruction*> Users;
482 Users.reserve(std::distance(I, E));
483 for (; I != E; ++I) Users.push_back(I->second);
484 while (!Users.empty()) {
489 return; // Quick exit
492 // Look at all of the executable operands of the PHI node. If any of them
493 // are overdefined, the PHI becomes overdefined as well. If they are all
494 // constant, and they agree with each other, the PHI becomes the identical
495 // constant. If they are constant and don't agree, the PHI is overdefined.
496 // If there are no executable operands, the PHI remains undefined.
498 Constant *OperandVal = 0;
499 for (unsigned i = 0, e = PN.getNumIncomingValues(); i != e; ++i) {
500 InstVal &IV = getValueState(PN.getIncomingValue(i));
501 if (IV.isUndefined()) continue; // Doesn't influence PHI node.
503 if (isEdgeFeasible(PN.getIncomingBlock(i), PN.getParent())) {
504 if (IV.isOverdefined()) { // PHI node becomes overdefined!
505 markOverdefined(PNIV, &PN);
509 if (OperandVal == 0) { // Grab the first value...
510 OperandVal = IV.getConstant();
511 } else { // Another value is being merged in!
512 // There is already a reachable operand. If we conflict with it,
513 // then the PHI node becomes overdefined. If we agree with it, we
516 // Check to see if there are two different constants merging...
517 if (IV.getConstant() != OperandVal) {
518 // Yes there is. This means the PHI node is not constant.
519 // You must be overdefined poor PHI.
521 markOverdefined(PNIV, &PN); // The PHI node now becomes overdefined
522 return; // I'm done analyzing you
528 // If we exited the loop, this means that the PHI node only has constant
529 // arguments that agree with each other(and OperandVal is the constant) or
530 // OperandVal is null because there are no defined incoming arguments. If
531 // this is the case, the PHI remains undefined.
534 markConstant(PNIV, &PN, OperandVal); // Acquire operand value
537 void SCCP::visitTerminatorInst(TerminatorInst &TI) {
538 std::vector<bool> SuccFeasible;
539 getFeasibleSuccessors(TI, SuccFeasible);
541 BasicBlock *BB = TI.getParent();
543 // Mark all feasible successors executable...
544 for (unsigned i = 0, e = SuccFeasible.size(); i != e; ++i)
546 markEdgeExecutable(BB, TI.getSuccessor(i));
549 void SCCP::visitCastInst(CastInst &I) {
550 Value *V = I.getOperand(0);
551 InstVal &VState = getValueState(V);
552 if (VState.isOverdefined()) { // Inherit overdefinedness of operand
554 } else if (VState.isConstant()) { // Propagate constant value
556 ConstantFoldCastInstruction(VState.getConstant(), I.getType());
558 if (Result) // If this instruction constant folds!
559 markConstant(&I, Result);
561 markOverdefined(&I); // Don't know how to fold this instruction. :(
565 // Handle BinaryOperators and Shift Instructions...
566 void SCCP::visitBinaryOperator(Instruction &I) {
567 InstVal &IV = ValueState[&I];
568 if (IV.isOverdefined()) return;
570 InstVal &V1State = getValueState(I.getOperand(0));
571 InstVal &V2State = getValueState(I.getOperand(1));
573 if (V1State.isOverdefined() || V2State.isOverdefined()) {
574 // If both operands are PHI nodes, it is possible that this instruction has
575 // a constant value, despite the fact that the PHI node doesn't. Check for
576 // this condition now.
577 if (PHINode *PN1 = dyn_cast<PHINode>(I.getOperand(0)))
578 if (PHINode *PN2 = dyn_cast<PHINode>(I.getOperand(1)))
579 if (PN1->getParent() == PN2->getParent()) {
580 // Since the two PHI nodes are in the same basic block, they must have
581 // entries for the same predecessors. Walk the predecessor list, and
582 // if all of the incoming values are constants, and the result of
583 // evaluating this expression with all incoming value pairs is the
584 // same, then this expression is a constant even though the PHI node
585 // is not a constant!
587 for (unsigned i = 0, e = PN1->getNumIncomingValues(); i != e; ++i) {
588 InstVal &In1 = getValueState(PN1->getIncomingValue(i));
589 BasicBlock *InBlock = PN1->getIncomingBlock(i);
590 InstVal &In2 =getValueState(PN2->getIncomingValueForBlock(InBlock));
592 if (In1.isOverdefined() || In2.isOverdefined()) {
593 Result.markOverdefined();
594 break; // Cannot fold this operation over the PHI nodes!
595 } else if (In1.isConstant() && In2.isConstant()) {
597 if (isa<BinaryOperator>(I))
598 Val = ConstantExpr::get(I.getOpcode(), In1.getConstant(),
601 assert(isa<ShiftInst>(I) &&
602 "Can only handle binops and shifts here!");
603 Val = ConstantExpr::getShift(I.getOpcode(), In1.getConstant(),
606 if (Result.isUndefined())
607 Result.markConstant(Val);
608 else if (Result.isConstant() && Result.getConstant() != Val) {
609 Result.markOverdefined();
615 // If we found a constant value here, then we know the instruction is
616 // constant despite the fact that the PHI nodes are overdefined.
617 if (Result.isConstant()) {
618 markConstant(IV, &I, Result.getConstant());
619 // Remember that this instruction is virtually using the PHI node
621 UsersOfOverdefinedPHIs.insert(std::make_pair(PN1, &I));
622 UsersOfOverdefinedPHIs.insert(std::make_pair(PN2, &I));
624 } else if (Result.isUndefined()) {
628 // Okay, this really is overdefined now. Since we might have
629 // speculatively thought that this was not overdefined before, and
630 // added ourselves to the UsersOfOverdefinedPHIs list for the PHIs,
631 // make sure to clean out any entries that we put there, for
633 std::multimap<PHINode*, Instruction*>::iterator It, E;
634 tie(It, E) = UsersOfOverdefinedPHIs.equal_range(PN1);
636 if (It->second == &I) {
637 UsersOfOverdefinedPHIs.erase(It++);
641 tie(It, E) = UsersOfOverdefinedPHIs.equal_range(PN2);
643 if (It->second == &I) {
644 UsersOfOverdefinedPHIs.erase(It++);
650 markOverdefined(IV, &I);
651 } else if (V1State.isConstant() && V2State.isConstant()) {
652 Constant *Result = 0;
653 if (isa<BinaryOperator>(I))
654 Result = ConstantExpr::get(I.getOpcode(), V1State.getConstant(),
655 V2State.getConstant());
657 assert (isa<ShiftInst>(I) && "Can only handle binops and shifts here!");
658 Result = ConstantExpr::getShift(I.getOpcode(), V1State.getConstant(),
659 V2State.getConstant());
662 markConstant(IV, &I, Result); // This instruction constant folds!
666 // Handle getelementptr instructions... if all operands are constants then we
667 // can turn this into a getelementptr ConstantExpr.
669 void SCCP::visitGetElementPtrInst(GetElementPtrInst &I) {
670 InstVal &IV = ValueState[&I];
671 if (IV.isOverdefined()) return;
673 std::vector<Constant*> Operands;
674 Operands.reserve(I.getNumOperands());
676 for (unsigned i = 0, e = I.getNumOperands(); i != e; ++i) {
677 InstVal &State = getValueState(I.getOperand(i));
678 if (State.isUndefined())
679 return; // Operands are not resolved yet...
680 else if (State.isOverdefined()) {
681 markOverdefined(IV, &I);
684 assert(State.isConstant() && "Unknown state!");
685 Operands.push_back(State.getConstant());
688 Constant *Ptr = Operands[0];
689 Operands.erase(Operands.begin()); // Erase the pointer from idx list...
691 markConstant(IV, &I, ConstantExpr::getGetElementPtr(Ptr, Operands));
694 /// GetGEPGlobalInitializer - Given a constant and a getelementptr constantexpr,
695 /// return the constant value being addressed by the constant expression, or
696 /// null if something is funny.
698 static Constant *GetGEPGlobalInitializer(Constant *C, ConstantExpr *CE) {
699 if (CE->getOperand(1) != Constant::getNullValue(Type::LongTy))
700 return 0; // Do not allow stepping over the value!
702 // Loop over all of the operands, tracking down which value we are
704 for (unsigned i = 2, e = CE->getNumOperands(); i != e; ++i)
705 if (ConstantUInt *CU = dyn_cast<ConstantUInt>(CE->getOperand(i))) {
706 ConstantStruct *CS = cast<ConstantStruct>(C);
707 if (CU->getValue() >= CS->getValues().size()) return 0;
708 C = cast<Constant>(CS->getValues()[CU->getValue()]);
709 } else if (ConstantSInt *CS = dyn_cast<ConstantSInt>(CE->getOperand(i))) {
710 ConstantArray *CA = cast<ConstantArray>(C);
711 if ((uint64_t)CS->getValue() >= CA->getValues().size()) return 0;
712 C = cast<Constant>(CA->getValues()[CS->getValue()]);
718 // Handle load instructions. If the operand is a constant pointer to a constant
719 // global, we can replace the load with the loaded constant value!
720 void SCCP::visitLoadInst(LoadInst &I) {
721 InstVal &IV = ValueState[&I];
722 if (IV.isOverdefined()) return;
724 InstVal &PtrVal = getValueState(I.getOperand(0));
725 if (PtrVal.isUndefined()) return; // The pointer is not resolved yet!
726 if (PtrVal.isConstant() && !I.isVolatile()) {
727 Value *Ptr = PtrVal.getConstant();
728 if (ConstantPointerRef *CPR = dyn_cast<ConstantPointerRef>(Ptr))
729 Ptr = CPR->getValue();
731 // Transform load (constant global) into the value loaded.
732 if (GlobalVariable *GV = dyn_cast<GlobalVariable>(Ptr))
733 if (GV->isConstant() && !GV->isExternal()) {
734 markConstant(IV, &I, GV->getInitializer());
738 // Transform load (constantexpr_GEP global, 0, ...) into the value loaded.
739 if (ConstantExpr *CE = dyn_cast<ConstantExpr>(Ptr))
740 if (CE->getOpcode() == Instruction::GetElementPtr)
741 if (ConstantPointerRef *G
742 = dyn_cast<ConstantPointerRef>(CE->getOperand(0)))
743 if (GlobalVariable *GV = dyn_cast<GlobalVariable>(G->getValue()))
744 if (GV->isConstant() && !GV->isExternal())
746 GetGEPGlobalInitializer(GV->getInitializer(), CE)) {
747 markConstant(IV, &I, V);
752 // Otherwise we cannot say for certain what value this load will produce.
754 markOverdefined(IV, &I);