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
90 // runOnFunction - Run the Sparse Conditional Constant Propagation algorithm,
91 // and return true if the function was modified.
93 bool runOnFunction(Function &F);
95 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
100 //===--------------------------------------------------------------------===//
101 // The implementation of this class
104 friend class InstVisitor<SCCP>; // Allow callbacks from visitor
106 // markValueOverdefined - Make a value be marked as "constant". If the value
107 // is not already a constant, add it to the instruction work list so that
108 // the users of the instruction are updated later.
110 inline bool markConstant(Instruction *I, Constant *V) {
111 if (ValueState[I].markConstant(V)) {
112 DEBUG(std::cerr << "markConstant: " << V << " = " << I);
113 InstWorkList.push_back(I);
119 // markValueOverdefined - Make a value be marked as "overdefined". If the
120 // value is not already overdefined, add it to the instruction work list so
121 // that the users of the instruction are updated later.
123 inline bool markOverdefined(Value *V) {
124 if (ValueState[V].markOverdefined()) {
125 if (Instruction *I = dyn_cast<Instruction>(V)) {
126 DEBUG(std::cerr << "markOverdefined: " << V);
127 InstWorkList.push_back(I); // Only instructions go on the work list
134 // getValueState - Return the InstVal object that corresponds to the value.
135 // This function is neccesary because not all values should start out in the
136 // underdefined state... Argument's should be overdefined, and
137 // constants should be marked as constants. If a value is not known to be an
138 // Instruction object, then use this accessor to get its value from the map.
140 inline InstVal &getValueState(Value *V) {
141 std::map<Value*, InstVal>::iterator I = ValueState.find(V);
142 if (I != ValueState.end()) return I->second; // Common case, in the map
144 if (Constant *CPV = dyn_cast<Constant>(V)) { // Constants are constant
145 ValueState[CPV].markConstant(CPV);
146 } else if (isa<Argument>(V)) { // Arguments are overdefined
147 ValueState[V].markOverdefined();
148 } else if (GlobalValue *GV = dyn_cast<GlobalValue>(V)) {
149 // The address of a global is a constant...
150 ValueState[V].markConstant(ConstantPointerRef::get(GV));
152 // All others are underdefined by default...
153 return ValueState[V];
156 // markExecutable - Mark a basic block as executable, adding it to the BB
157 // work list if it is not already executable...
159 void markExecutable(BasicBlock *BB) {
160 if (BBExecutable.count(BB)) {
161 // BB is already executable, but we may have just made an edge feasible
162 // that wasn't before. Add the PHI nodes to the work list so that they
164 for (BasicBlock::iterator I = BB->begin();
165 PHINode *PN = dyn_cast<PHINode>(I); ++I)
169 DEBUG(std::cerr << "Marking BB Executable: " << *BB);
170 BBExecutable.insert(BB); // Basic block is executable!
171 BBWorkList.push_back(BB); // Add the block to the work list!
176 // visit implementations - Something changed in this instruction... Either an
177 // operand made a transition, or the instruction is newly executable. Change
178 // the value type of I to reflect these changes if appropriate.
180 void visitPHINode(PHINode &I);
183 void visitReturnInst(ReturnInst &I) { /*does not have an effect*/ }
184 void visitTerminatorInst(TerminatorInst &TI);
186 void visitCastInst(CastInst &I);
187 void visitBinaryOperator(Instruction &I);
188 void visitShiftInst(ShiftInst &I) { visitBinaryOperator(I); }
190 // Instructions that cannot be folded away...
191 void visitStoreInst (Instruction &I) { /*returns void*/ }
192 void visitLoadInst (Instruction &I) { markOverdefined(&I); }
193 void visitGetElementPtrInst(GetElementPtrInst &I);
194 void visitCallInst (Instruction &I) { markOverdefined(&I); }
195 void visitInvokeInst (Instruction &I) { markOverdefined(&I); }
196 void visitAllocationInst(Instruction &I) { markOverdefined(&I); }
197 void visitVarArgInst (Instruction &I) { markOverdefined(&I); }
198 void visitFreeInst (Instruction &I) { /*returns void*/ }
200 void visitInstruction(Instruction &I) {
201 // If a new instruction is added to LLVM that we don't handle...
202 std::cerr << "SCCP: Don't know how to handle: " << I;
203 markOverdefined(&I); // Just in case
206 // getFeasibleSuccessors - Return a vector of booleans to indicate which
207 // successors are reachable from a given terminator instruction.
209 void getFeasibleSuccessors(TerminatorInst &TI, std::vector<bool> &Succs);
211 // isEdgeFeasible - Return true if the control flow edge from the 'From' basic
212 // block to the 'To' basic block is currently feasible...
214 bool isEdgeFeasible(BasicBlock *From, BasicBlock *To);
216 // OperandChangedState - This method is invoked on all of the users of an
217 // instruction that was just changed state somehow.... Based on this
218 // information, we need to update the specified user of this instruction.
220 void OperandChangedState(User *U) {
221 // Only instructions use other variable values!
222 Instruction &I = cast<Instruction>(*U);
223 if (BBExecutable.count(I.getParent())) // Inst is executable?
228 RegisterOpt<SCCP> X("sccp", "Sparse Conditional Constant Propagation");
229 } // end anonymous namespace
232 // createSCCPPass - This is the public interface to this file...
234 Pass *createSCCPPass() {
239 //===----------------------------------------------------------------------===//
240 // SCCP Class Implementation
243 // runOnFunction() - Run the Sparse Conditional Constant Propagation algorithm,
244 // and return true if the function was modified.
246 bool SCCP::runOnFunction(Function &F) {
247 // Mark the first block of the function as being executable...
248 markExecutable(&F.front());
250 // Process the work lists until their are empty!
251 while (!BBWorkList.empty() || !InstWorkList.empty()) {
252 // Process the instruction work list...
253 while (!InstWorkList.empty()) {
254 Instruction *I = InstWorkList.back();
255 InstWorkList.pop_back();
257 DEBUG(std::cerr << "\nPopped off I-WL: " << I);
259 // "I" got into the work list because it either made the transition from
260 // bottom to constant, or to Overdefined.
262 // Update all of the users of this instruction's value...
264 for_each(I->use_begin(), I->use_end(),
265 bind_obj(this, &SCCP::OperandChangedState));
268 // Process the basic block work list...
269 while (!BBWorkList.empty()) {
270 BasicBlock *BB = BBWorkList.back();
271 BBWorkList.pop_back();
273 DEBUG(std::cerr << "\nPopped off BBWL: " << BB);
275 // Notify all instructions in this basic block that they are newly
282 for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I)
283 if (!BBExecutable.count(I))
284 std::cerr << "BasicBlock Dead:" << *I;
287 // Iterate over all of the instructions in a function, replacing them with
288 // constants if we have found them to be of constant values.
290 bool MadeChanges = false;
291 for (Function::iterator BB = F.begin(), BBE = F.end(); BB != BBE; ++BB)
292 for (BasicBlock::iterator BI = BB->begin(); BI != BB->end();) {
293 Instruction &Inst = *BI;
294 InstVal &IV = ValueState[&Inst];
295 if (IV.isConstant()) {
296 Constant *Const = IV.getConstant();
297 DEBUG(std::cerr << "Constant: " << Const << " = " << Inst);
299 // Replaces all of the uses of a variable with uses of the constant.
300 Inst.replaceAllUsesWith(Const);
302 // Remove the operator from the list of definitions... and delete it.
303 BI = BB->getInstList().erase(BI);
305 // Hey, we just changed something!
313 // Reset state so that the next invocation will have empty data structures
314 BBExecutable.clear();
316 std::vector<Instruction*>().swap(InstWorkList);
317 std::vector<BasicBlock*>().swap(BBWorkList);
323 // getFeasibleSuccessors - Return a vector of booleans to indicate which
324 // successors are reachable from a given terminator instruction.
326 void SCCP::getFeasibleSuccessors(TerminatorInst &TI, std::vector<bool> &Succs) {
327 Succs.resize(TI.getNumSuccessors());
328 if (BranchInst *BI = dyn_cast<BranchInst>(&TI)) {
329 if (BI->isUnconditional()) {
332 InstVal &BCValue = getValueState(BI->getCondition());
333 if (BCValue.isOverdefined()) {
334 // Overdefined condition variables mean the branch could go either way.
335 Succs[0] = Succs[1] = true;
336 } else if (BCValue.isConstant()) {
337 // Constant condition variables mean the branch can only go a single way
338 Succs[BCValue.getConstant() == ConstantBool::False] = true;
341 } else if (InvokeInst *II = dyn_cast<InvokeInst>(&TI)) {
342 // Invoke instructions successors are always executable.
343 Succs[0] = Succs[1] = true;
344 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(&TI)) {
345 InstVal &SCValue = getValueState(SI->getCondition());
346 if (SCValue.isOverdefined()) { // Overdefined condition?
347 // All destinations are executable!
348 Succs.assign(TI.getNumSuccessors(), true);
349 } else if (SCValue.isConstant()) {
350 Constant *CPV = SCValue.getConstant();
351 // Make sure to skip the "default value" which isn't a value
352 for (unsigned i = 1, E = SI->getNumSuccessors(); i != E; ++i) {
353 if (SI->getSuccessorValue(i) == CPV) {// Found the right branch...
359 // Constant value not equal to any of the branches... must execute
360 // default branch then...
364 std::cerr << "SCCP: Don't know how to handle: " << TI;
365 Succs.assign(TI.getNumSuccessors(), true);
370 // isEdgeFeasible - Return true if the control flow edge from the 'From' basic
371 // block to the 'To' basic block is currently feasible...
373 bool SCCP::isEdgeFeasible(BasicBlock *From, BasicBlock *To) {
374 assert(BBExecutable.count(To) && "Dest should always be alive!");
376 // Make sure the source basic block is executable!!
377 if (!BBExecutable.count(From)) return false;
379 // Check to make sure this edge itself is actually feasible now...
380 TerminatorInst *FT = From->getTerminator();
381 std::vector<bool> SuccFeasible;
382 getFeasibleSuccessors(*FT, SuccFeasible);
384 // Check all edges from From to To. If any are feasible, return true.
385 for (unsigned i = 0, e = SuccFeasible.size(); i != e; ++i)
386 if (FT->getSuccessor(i) == To && SuccFeasible[i])
389 // Otherwise, none of the edges are actually feasible at this time...
393 // visit Implementations - Something changed in this instruction... Either an
394 // operand made a transition, or the instruction is newly executable. Change
395 // the value type of I to reflect these changes if appropriate. This method
396 // makes sure to do the following actions:
398 // 1. If a phi node merges two constants in, and has conflicting value coming
399 // from different branches, or if the PHI node merges in an overdefined
400 // value, then the PHI node becomes overdefined.
401 // 2. If a phi node merges only constants in, and they all agree on value, the
402 // PHI node becomes a constant value equal to that.
403 // 3. If V <- x (op) y && isConstant(x) && isConstant(y) V = Constant
404 // 4. If V <- x (op) y && (isOverdefined(x) || isOverdefined(y)) V = Overdefined
405 // 5. If V <- MEM or V <- CALL or V <- (unknown) then V = Overdefined
406 // 6. If a conditional branch has a value that is constant, make the selected
407 // destination executable
408 // 7. If a conditional branch has a value that is overdefined, make all
409 // successors executable.
411 void SCCP::visitPHINode(PHINode &PN) {
412 if (getValueState(&PN).isOverdefined()) return; // Quick exit
414 // Look at all of the executable operands of the PHI node. If any of them
415 // are overdefined, the PHI becomes overdefined as well. If they are all
416 // constant, and they agree with each other, the PHI becomes the identical
417 // constant. If they are constant and don't agree, the PHI is overdefined.
418 // If there are no executable operands, the PHI remains undefined.
420 Constant *OperandVal = 0;
421 for (unsigned i = 0, e = PN.getNumIncomingValues(); i != e; ++i) {
422 InstVal &IV = getValueState(PN.getIncomingValue(i));
423 if (IV.isUndefined()) continue; // Doesn't influence PHI node.
425 if (isEdgeFeasible(PN.getIncomingBlock(i), PN.getParent())) {
426 if (IV.isOverdefined()) { // PHI node becomes overdefined!
427 markOverdefined(&PN);
431 if (OperandVal == 0) { // Grab the first value...
432 OperandVal = IV.getConstant();
433 } else { // Another value is being merged in!
434 // There is already a reachable operand. If we conflict with it,
435 // then the PHI node becomes overdefined. If we agree with it, we
438 // Check to see if there are two different constants merging...
439 if (IV.getConstant() != OperandVal) {
440 // Yes there is. This means the PHI node is not constant.
441 // You must be overdefined poor PHI.
443 markOverdefined(&PN); // The PHI node now becomes overdefined
444 return; // I'm done analyzing you
450 // If we exited the loop, this means that the PHI node only has constant
451 // arguments that agree with each other(and OperandVal is the constant) or
452 // OperandVal is null because there are no defined incoming arguments. If
453 // this is the case, the PHI remains undefined.
456 markConstant(&PN, OperandVal); // Aquire operand value
459 void SCCP::visitTerminatorInst(TerminatorInst &TI) {
460 std::vector<bool> SuccFeasible;
461 getFeasibleSuccessors(TI, SuccFeasible);
463 // Mark all feasible successors executable...
464 for (unsigned i = 0, e = SuccFeasible.size(); i != e; ++i)
465 if (SuccFeasible[i]) {
466 BasicBlock *Succ = TI.getSuccessor(i);
467 markExecutable(Succ);
471 void SCCP::visitCastInst(CastInst &I) {
472 Value *V = I.getOperand(0);
473 InstVal &VState = getValueState(V);
474 if (VState.isOverdefined()) { // Inherit overdefinedness of operand
476 } else if (VState.isConstant()) { // Propagate constant value
478 ConstantFoldCastInstruction(VState.getConstant(), I.getType());
481 // This instruction constant folds!
482 markConstant(&I, Result);
484 markOverdefined(&I); // Don't know how to fold this instruction. :(
489 // Handle BinaryOperators and Shift Instructions...
490 void SCCP::visitBinaryOperator(Instruction &I) {
491 InstVal &V1State = getValueState(I.getOperand(0));
492 InstVal &V2State = getValueState(I.getOperand(1));
493 if (V1State.isOverdefined() || V2State.isOverdefined()) {
495 } else if (V1State.isConstant() && V2State.isConstant()) {
496 Constant *Result = 0;
497 if (isa<BinaryOperator>(I))
498 Result = ConstantFoldBinaryInstruction(I.getOpcode(),
499 V1State.getConstant(),
500 V2State.getConstant());
501 else if (isa<ShiftInst>(I))
502 Result = ConstantFoldShiftInstruction(I.getOpcode(),
503 V1State.getConstant(),
504 V2State.getConstant());
506 markConstant(&I, Result); // This instruction constant folds!
508 markOverdefined(&I); // Don't know how to fold this instruction. :(
512 // Handle getelementptr instructions... if all operands are constants then we
513 // can turn this into a getelementptr ConstantExpr.
515 void SCCP::visitGetElementPtrInst(GetElementPtrInst &I) {
516 std::vector<Constant*> Operands;
517 Operands.reserve(I.getNumOperands());
519 for (unsigned i = 0, e = I.getNumOperands(); i != e; ++i) {
520 InstVal &State = getValueState(I.getOperand(i));
521 if (State.isUndefined())
522 return; // Operands are not resolved yet...
523 else if (State.isOverdefined()) {
527 assert(State.isConstant() && "Unknown state!");
528 Operands.push_back(State.getConstant());
531 Constant *Ptr = Operands[0];
532 Operands.erase(Operands.begin()); // Erase the pointer from idx list...
534 markConstant(&I, ConstantExpr::getGetElementPtr(Ptr, Operands));