1 //===- SCCP.cpp - Sparse Conditional Constant Propagation -----------------===//
3 // This file implements sparse conditional constant propagation and merging:
6 // * Assumes values are constant unless proven otherwise
7 // * Assumes BasicBlocks are dead unless proven otherwise
8 // * Proves values to be constant, and replaces them with constants
9 // * Proves conditional branches to be unconditional
12 // * This pass has a habit of making definitions be dead. It is a good idea
13 // to to run a DCE pass sometime after running this pass.
15 //===----------------------------------------------------------------------===//
17 #include "llvm/Transforms/Scalar.h"
18 #include "llvm/ConstantHandling.h"
19 #include "llvm/Function.h"
20 #include "llvm/Instructions.h"
21 #include "llvm/Pass.h"
22 #include "llvm/Support/InstVisitor.h"
23 #include "Support/Debug.h"
24 #include "Support/Statistic.h"
25 #include "Support/STLExtras.h"
29 // InstVal class - This class represents the different lattice values that an
30 // instruction may occupy. It is a simple class with value semantics.
33 Statistic<> NumInstRemoved("sccp", "Number of instructions removed");
37 undefined, // This instruction has no known value
38 constant, // This instruction has a constant value
39 overdefined // This instruction has an unknown value
40 } LatticeValue; // The current lattice position
41 Constant *ConstantVal; // If Constant value, the current value
43 inline InstVal() : LatticeValue(undefined), ConstantVal(0) {}
45 // markOverdefined - Return true if this is a new status to be in...
46 inline bool markOverdefined() {
47 if (LatticeValue != overdefined) {
48 LatticeValue = overdefined;
54 // markConstant - Return true if this is a new status for us...
55 inline bool markConstant(Constant *V) {
56 if (LatticeValue != constant) {
57 LatticeValue = constant;
61 assert(ConstantVal == V && "Marking constant with different value");
66 inline bool isUndefined() const { return LatticeValue == undefined; }
67 inline bool isConstant() const { return LatticeValue == constant; }
68 inline bool isOverdefined() const { return LatticeValue == overdefined; }
70 inline Constant *getConstant() const { return ConstantVal; }
73 } // end anonymous namespace
76 //===----------------------------------------------------------------------===//
79 // This class does all of the work of Sparse Conditional Constant Propagation.
82 class SCCP : public FunctionPass, public InstVisitor<SCCP> {
83 std::set<BasicBlock*> BBExecutable;// The basic blocks that are executable
84 std::map<Value*, InstVal> ValueState; // The state each value is in...
86 std::vector<Instruction*> InstWorkList;// The instruction work list
87 std::vector<BasicBlock*> BBWorkList; // The BasicBlock work list
89 /// KnownFeasibleEdges - Entries in this set are edges which have already had
90 /// PHI nodes retriggered.
91 typedef std::pair<BasicBlock*,BasicBlock*> Edge;
92 std::set<Edge> KnownFeasibleEdges;
95 // runOnFunction - Run the Sparse Conditional Constant Propagation algorithm,
96 // and return true if the function was modified.
98 bool runOnFunction(Function &F);
100 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
101 AU.setPreservesCFG();
105 //===--------------------------------------------------------------------===//
106 // The implementation of this class
109 friend class InstVisitor<SCCP>; // Allow callbacks from visitor
111 // markValueOverdefined - Make a value be marked as "constant". If the value
112 // is not already a constant, add it to the instruction work list so that
113 // the users of the instruction are updated later.
115 inline void markConstant(InstVal &IV, Instruction *I, Constant *C) {
116 if (IV.markConstant(C)) {
117 DEBUG(std::cerr << "markConstant: " << *C << ": " << *I);
118 InstWorkList.push_back(I);
121 inline void markConstant(Instruction *I, Constant *C) {
122 markConstant(ValueState[I], I, C);
125 // markValueOverdefined - Make a value be marked as "overdefined". If the
126 // value is not already overdefined, add it to the instruction work list so
127 // that the users of the instruction are updated later.
129 inline void markOverdefined(InstVal &IV, Instruction *I) {
130 if (IV.markOverdefined()) {
131 DEBUG(std::cerr << "markOverdefined: " << *I);
132 InstWorkList.push_back(I); // Only instructions go on the work list
135 inline void markOverdefined(Instruction *I) {
136 markOverdefined(ValueState[I], I);
139 // getValueState - Return the InstVal object that corresponds to the value.
140 // This function is necessary because not all values should start out in the
141 // underdefined state... Argument's should be overdefined, and
142 // constants should be marked as constants. If a value is not known to be an
143 // Instruction object, then use this accessor to get its value from the map.
145 inline InstVal &getValueState(Value *V) {
146 std::map<Value*, InstVal>::iterator I = ValueState.find(V);
147 if (I != ValueState.end()) return I->second; // Common case, in the map
149 if (Constant *CPV = dyn_cast<Constant>(V)) { // Constants are constant
150 ValueState[CPV].markConstant(CPV);
151 } else if (isa<Argument>(V)) { // Arguments are overdefined
152 ValueState[V].markOverdefined();
153 } else if (GlobalValue *GV = dyn_cast<GlobalValue>(V)) {
154 // The address of a global is a constant...
155 ValueState[V].markConstant(ConstantPointerRef::get(GV));
157 // All others are underdefined by default...
158 return ValueState[V];
161 // markEdgeExecutable - Mark a basic block as executable, adding it to the BB
162 // work list if it is not already executable...
164 void markEdgeExecutable(BasicBlock *Source, BasicBlock *Dest) {
165 if (!KnownFeasibleEdges.insert(Edge(Source, Dest)).second)
166 return; // This edge is already known to be executable!
168 if (BBExecutable.count(Dest)) {
169 DEBUG(std::cerr << "Marking Edge Executable: " << Source->getName()
170 << " -> " << Dest->getName() << "\n");
172 // The destination is already executable, but we just made an edge
173 // feasible that wasn't before. Add the PHI nodes to the work list so
174 // that they can be rechecked.
175 for (BasicBlock::iterator I = Dest->begin();
176 PHINode *PN = dyn_cast<PHINode>(I); ++I)
180 DEBUG(std::cerr << "Marking Block Executable: " << Dest->getName()<<"\n");
181 BBExecutable.insert(Dest); // Basic block is executable!
182 BBWorkList.push_back(Dest); // Add the block to the work list!
187 // visit implementations - Something changed in this instruction... Either an
188 // operand made a transition, or the instruction is newly executable. Change
189 // the value type of I to reflect these changes if appropriate.
191 void visitPHINode(PHINode &I);
194 void visitReturnInst(ReturnInst &I) { /*does not have an effect*/ }
195 void visitTerminatorInst(TerminatorInst &TI);
197 void visitCastInst(CastInst &I);
198 void visitBinaryOperator(Instruction &I);
199 void visitShiftInst(ShiftInst &I) { visitBinaryOperator(I); }
201 // Instructions that cannot be folded away...
202 void visitStoreInst (Instruction &I) { /*returns void*/ }
203 void visitLoadInst (Instruction &I) { markOverdefined(&I); }
204 void visitGetElementPtrInst(GetElementPtrInst &I);
205 void visitCallInst (Instruction &I) { markOverdefined(&I); }
206 void visitInvokeInst (TerminatorInst &I) {
207 if (I.getType() != Type::VoidTy) markOverdefined(&I);
208 visitTerminatorInst(I);
210 void visitUnwindInst (TerminatorInst &I) { /*returns void*/ }
211 void visitAllocationInst(Instruction &I) { markOverdefined(&I); }
212 void visitVarArgInst (Instruction &I) { markOverdefined(&I); }
213 void visitFreeInst (Instruction &I) { /*returns void*/ }
215 void visitInstruction(Instruction &I) {
216 // If a new instruction is added to LLVM that we don't handle...
217 std::cerr << "SCCP: Don't know how to handle: " << I;
218 markOverdefined(&I); // Just in case
221 // getFeasibleSuccessors - Return a vector of booleans to indicate which
222 // successors are reachable from a given terminator instruction.
224 void getFeasibleSuccessors(TerminatorInst &TI, std::vector<bool> &Succs);
226 // isEdgeFeasible - Return true if the control flow edge from the 'From' basic
227 // block to the 'To' basic block is currently feasible...
229 bool isEdgeFeasible(BasicBlock *From, BasicBlock *To);
231 // OperandChangedState - This method is invoked on all of the users of an
232 // instruction that was just changed state somehow.... Based on this
233 // information, we need to update the specified user of this instruction.
235 void OperandChangedState(User *U) {
236 // Only instructions use other variable values!
237 Instruction &I = cast<Instruction>(*U);
238 if (BBExecutable.count(I.getParent())) // Inst is executable?
243 RegisterOpt<SCCP> X("sccp", "Sparse Conditional Constant Propagation");
244 } // end anonymous namespace
247 // createSCCPPass - This is the public interface to this file...
249 Pass *createSCCPPass() {
254 //===----------------------------------------------------------------------===//
255 // SCCP Class Implementation
258 // runOnFunction() - Run the Sparse Conditional Constant Propagation algorithm,
259 // and return true if the function was modified.
261 bool SCCP::runOnFunction(Function &F) {
262 // Mark the first block of the function as being executable...
263 BBExecutable.insert(F.begin()); // Basic block is executable!
264 BBWorkList.push_back(F.begin()); // Add the block to the work list!
266 // Process the work lists until their are empty!
267 while (!BBWorkList.empty() || !InstWorkList.empty()) {
268 // Process the instruction work list...
269 while (!InstWorkList.empty()) {
270 Instruction *I = InstWorkList.back();
271 InstWorkList.pop_back();
273 DEBUG(std::cerr << "\nPopped off I-WL: " << I);
275 // "I" got into the work list because it either made the transition from
276 // bottom to constant, or to Overdefined.
278 // Update all of the users of this instruction's value...
280 for_each(I->use_begin(), I->use_end(),
281 bind_obj(this, &SCCP::OperandChangedState));
284 // Process the basic block work list...
285 while (!BBWorkList.empty()) {
286 BasicBlock *BB = BBWorkList.back();
287 BBWorkList.pop_back();
289 DEBUG(std::cerr << "\nPopped off BBWL: " << BB);
291 // Notify all instructions in this basic block that they are newly
298 for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I)
299 if (!BBExecutable.count(I))
300 std::cerr << "BasicBlock Dead:" << *I;
303 // Iterate over all of the instructions in a function, replacing them with
304 // constants if we have found them to be of constant values.
306 bool MadeChanges = false;
307 for (Function::iterator BB = F.begin(), BBE = F.end(); BB != BBE; ++BB)
308 for (BasicBlock::iterator BI = BB->begin(); BI != BB->end();) {
309 Instruction &Inst = *BI;
310 InstVal &IV = ValueState[&Inst];
311 if (IV.isConstant()) {
312 Constant *Const = IV.getConstant();
313 DEBUG(std::cerr << "Constant: " << Const << " = " << Inst);
315 // Replaces all of the uses of a variable with uses of the constant.
316 Inst.replaceAllUsesWith(Const);
318 // Remove the operator from the list of definitions... and delete it.
319 BI = BB->getInstList().erase(BI);
321 // Hey, we just changed something!
329 // Reset state so that the next invocation will have empty data structures
330 BBExecutable.clear();
332 std::vector<Instruction*>().swap(InstWorkList);
333 std::vector<BasicBlock*>().swap(BBWorkList);
339 // getFeasibleSuccessors - Return a vector of booleans to indicate which
340 // successors are reachable from a given terminator instruction.
342 void SCCP::getFeasibleSuccessors(TerminatorInst &TI, std::vector<bool> &Succs) {
343 Succs.resize(TI.getNumSuccessors());
344 if (BranchInst *BI = dyn_cast<BranchInst>(&TI)) {
345 if (BI->isUnconditional()) {
348 InstVal &BCValue = getValueState(BI->getCondition());
349 if (BCValue.isOverdefined()) {
350 // Overdefined condition variables mean the branch could go either way.
351 Succs[0] = Succs[1] = true;
352 } else if (BCValue.isConstant()) {
353 // Constant condition variables mean the branch can only go a single way
354 Succs[BCValue.getConstant() == ConstantBool::False] = true;
357 } else if (InvokeInst *II = dyn_cast<InvokeInst>(&TI)) {
358 // Invoke instructions successors are always executable.
359 Succs[0] = Succs[1] = true;
360 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(&TI)) {
361 InstVal &SCValue = getValueState(SI->getCondition());
362 if (SCValue.isOverdefined()) { // Overdefined condition?
363 // All destinations are executable!
364 Succs.assign(TI.getNumSuccessors(), true);
365 } else if (SCValue.isConstant()) {
366 Constant *CPV = SCValue.getConstant();
367 // Make sure to skip the "default value" which isn't a value
368 for (unsigned i = 1, E = SI->getNumSuccessors(); i != E; ++i) {
369 if (SI->getSuccessorValue(i) == CPV) {// Found the right branch...
375 // Constant value not equal to any of the branches... must execute
376 // default branch then...
380 std::cerr << "SCCP: Don't know how to handle: " << TI;
381 Succs.assign(TI.getNumSuccessors(), true);
386 // isEdgeFeasible - Return true if the control flow edge from the 'From' basic
387 // block to the 'To' basic block is currently feasible...
389 bool SCCP::isEdgeFeasible(BasicBlock *From, BasicBlock *To) {
390 assert(BBExecutable.count(To) && "Dest should always be alive!");
392 // Make sure the source basic block is executable!!
393 if (!BBExecutable.count(From)) return false;
395 // Check to make sure this edge itself is actually feasible now...
396 TerminatorInst *TI = From->getTerminator();
397 if (BranchInst *BI = dyn_cast<BranchInst>(TI)) {
398 if (BI->isUnconditional())
401 InstVal &BCValue = getValueState(BI->getCondition());
402 if (BCValue.isOverdefined()) {
403 // Overdefined condition variables mean the branch could go either way.
405 } else if (BCValue.isConstant()) {
406 // Constant condition variables mean the branch can only go a single way
407 return BI->getSuccessor(BCValue.getConstant() ==
408 ConstantBool::False) == To;
412 } else if (InvokeInst *II = dyn_cast<InvokeInst>(TI)) {
413 // Invoke instructions successors are always executable.
415 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
416 InstVal &SCValue = getValueState(SI->getCondition());
417 if (SCValue.isOverdefined()) { // Overdefined condition?
418 // All destinations are executable!
420 } else if (SCValue.isConstant()) {
421 Constant *CPV = SCValue.getConstant();
422 // Make sure to skip the "default value" which isn't a value
423 for (unsigned i = 1, E = SI->getNumSuccessors(); i != E; ++i)
424 if (SI->getSuccessorValue(i) == CPV) // Found the taken branch...
425 return SI->getSuccessor(i) == To;
427 // Constant value not equal to any of the branches... must execute
428 // default branch then...
429 return SI->getDefaultDest() == To;
433 std::cerr << "Unknown terminator instruction: " << *TI;
438 // visit Implementations - Something changed in this instruction... Either an
439 // operand made a transition, or the instruction is newly executable. Change
440 // the value type of I to reflect these changes if appropriate. This method
441 // makes sure to do the following actions:
443 // 1. If a phi node merges two constants in, and has conflicting value coming
444 // from different branches, or if the PHI node merges in an overdefined
445 // value, then the PHI node becomes overdefined.
446 // 2. If a phi node merges only constants in, and they all agree on value, the
447 // PHI node becomes a constant value equal to that.
448 // 3. If V <- x (op) y && isConstant(x) && isConstant(y) V = Constant
449 // 4. If V <- x (op) y && (isOverdefined(x) || isOverdefined(y)) V = Overdefined
450 // 5. If V <- MEM or V <- CALL or V <- (unknown) then V = Overdefined
451 // 6. If a conditional branch has a value that is constant, make the selected
452 // destination executable
453 // 7. If a conditional branch has a value that is overdefined, make all
454 // successors executable.
456 void SCCP::visitPHINode(PHINode &PN) {
457 InstVal &PNIV = getValueState(&PN);
458 if (PNIV.isOverdefined()) return; // Quick exit
460 // Look at all of the executable operands of the PHI node. If any of them
461 // are overdefined, the PHI becomes overdefined as well. If they are all
462 // constant, and they agree with each other, the PHI becomes the identical
463 // constant. If they are constant and don't agree, the PHI is overdefined.
464 // If there are no executable operands, the PHI remains undefined.
466 Constant *OperandVal = 0;
467 for (unsigned i = 0, e = PN.getNumIncomingValues(); i != e; ++i) {
468 InstVal &IV = getValueState(PN.getIncomingValue(i));
469 if (IV.isUndefined()) continue; // Doesn't influence PHI node.
471 if (isEdgeFeasible(PN.getIncomingBlock(i), PN.getParent())) {
472 if (IV.isOverdefined()) { // PHI node becomes overdefined!
473 markOverdefined(PNIV, &PN);
477 if (OperandVal == 0) { // Grab the first value...
478 OperandVal = IV.getConstant();
479 } else { // Another value is being merged in!
480 // There is already a reachable operand. If we conflict with it,
481 // then the PHI node becomes overdefined. If we agree with it, we
484 // Check to see if there are two different constants merging...
485 if (IV.getConstant() != OperandVal) {
486 // Yes there is. This means the PHI node is not constant.
487 // You must be overdefined poor PHI.
489 markOverdefined(PNIV, &PN); // The PHI node now becomes overdefined
490 return; // I'm done analyzing you
496 // If we exited the loop, this means that the PHI node only has constant
497 // arguments that agree with each other(and OperandVal is the constant) or
498 // OperandVal is null because there are no defined incoming arguments. If
499 // this is the case, the PHI remains undefined.
502 markConstant(PNIV, &PN, OperandVal); // Aquire operand value
505 void SCCP::visitTerminatorInst(TerminatorInst &TI) {
506 std::vector<bool> SuccFeasible;
507 getFeasibleSuccessors(TI, SuccFeasible);
509 BasicBlock *BB = TI.getParent();
511 // Mark all feasible successors executable...
512 for (unsigned i = 0, e = SuccFeasible.size(); i != e; ++i)
514 markEdgeExecutable(BB, TI.getSuccessor(i));
517 void SCCP::visitCastInst(CastInst &I) {
518 Value *V = I.getOperand(0);
519 InstVal &VState = getValueState(V);
520 if (VState.isOverdefined()) { // Inherit overdefinedness of operand
522 } else if (VState.isConstant()) { // Propagate constant value
524 ConstantFoldCastInstruction(VState.getConstant(), I.getType());
526 if (Result) // If this instruction constant folds!
527 markConstant(&I, Result);
529 markOverdefined(&I); // Don't know how to fold this instruction. :(
533 // Handle BinaryOperators and Shift Instructions...
534 void SCCP::visitBinaryOperator(Instruction &I) {
535 InstVal &V1State = getValueState(I.getOperand(0));
536 InstVal &V2State = getValueState(I.getOperand(1));
537 if (V1State.isOverdefined() || V2State.isOverdefined()) {
539 } else if (V1State.isConstant() && V2State.isConstant()) {
540 Constant *Result = 0;
541 if (isa<BinaryOperator>(I))
542 Result = ConstantFoldBinaryInstruction(I.getOpcode(),
543 V1State.getConstant(),
544 V2State.getConstant());
545 else if (isa<ShiftInst>(I))
546 Result = ConstantFoldShiftInstruction(I.getOpcode(),
547 V1State.getConstant(),
548 V2State.getConstant());
550 markConstant(&I, Result); // This instruction constant folds!
552 markOverdefined(&I); // Don't know how to fold this instruction. :(
556 // Handle getelementptr instructions... if all operands are constants then we
557 // can turn this into a getelementptr ConstantExpr.
559 void SCCP::visitGetElementPtrInst(GetElementPtrInst &I) {
560 std::vector<Constant*> Operands;
561 Operands.reserve(I.getNumOperands());
563 for (unsigned i = 0, e = I.getNumOperands(); i != e; ++i) {
564 InstVal &State = getValueState(I.getOperand(i));
565 if (State.isUndefined())
566 return; // Operands are not resolved yet...
567 else if (State.isOverdefined()) {
571 assert(State.isConstant() && "Unknown state!");
572 Operands.push_back(State.getConstant());
575 Constant *Ptr = Operands[0];
576 Operands.erase(Operands.begin()); // Erase the pointer from idx list...
578 markConstant(&I, ConstantExpr::getGetElementPtr(Ptr, Operands));