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/STLExtras.h"
24 #include "Support/Statistic.h"
28 // InstVal class - This class represents the different lattice values that an
29 // instruction may occupy. It is a simple class with value semantics.
32 Statistic<> NumInstRemoved("sccp", "Number of instructions removed");
36 undefined, // This instruction has no known value
37 constant, // This instruction has a constant value
38 overdefined // This instruction has an unknown value
39 } LatticeValue; // The current lattice position
40 Constant *ConstantVal; // If Constant value, the current value
42 inline InstVal() : LatticeValue(undefined), ConstantVal(0) {}
44 // markOverdefined - Return true if this is a new status to be in...
45 inline bool markOverdefined() {
46 if (LatticeValue != overdefined) {
47 LatticeValue = overdefined;
53 // markConstant - Return true if this is a new status for us...
54 inline bool markConstant(Constant *V) {
55 if (LatticeValue != constant) {
56 LatticeValue = constant;
60 assert(ConstantVal == V && "Marking constant with different value");
65 inline bool isUndefined() const { return LatticeValue == undefined; }
66 inline bool isConstant() const { return LatticeValue == constant; }
67 inline bool isOverdefined() const { return LatticeValue == overdefined; }
69 inline Constant *getConstant() const { return ConstantVal; }
72 } // end anonymous namespace
75 //===----------------------------------------------------------------------===//
78 // This class does all of the work of Sparse Conditional Constant Propagation.
81 class SCCP : public FunctionPass, public InstVisitor<SCCP> {
82 std::set<BasicBlock*> BBExecutable;// The basic blocks that are executable
83 std::map<Value*, InstVal> ValueState; // The state each value is in...
85 std::vector<Instruction*> InstWorkList;// The instruction work list
86 std::vector<BasicBlock*> BBWorkList; // The BasicBlock work list
89 // runOnFunction - Run the Sparse Conditional Constant Propagation algorithm,
90 // and return true if the function was modified.
92 bool runOnFunction(Function &F);
94 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
99 //===--------------------------------------------------------------------===//
100 // The implementation of this class
103 friend class InstVisitor<SCCP>; // Allow callbacks from visitor
105 // markValueOverdefined - Make a value be marked as "constant". If the value
106 // is not already a constant, add it to the instruction work list so that
107 // the users of the instruction are updated later.
109 inline bool markConstant(Instruction *I, Constant *V) {
110 if (ValueState[I].markConstant(V)) {
111 DEBUG(std::cerr << "markConstant: " << V << " = " << I);
112 InstWorkList.push_back(I);
118 // markValueOverdefined - Make a value be marked as "overdefined". If the
119 // value is not already overdefined, add it to the instruction work list so
120 // that the users of the instruction are updated later.
122 inline bool markOverdefined(Value *V) {
123 if (ValueState[V].markOverdefined()) {
124 if (Instruction *I = dyn_cast<Instruction>(V)) {
125 DEBUG(std::cerr << "markOverdefined: " << V);
126 InstWorkList.push_back(I); // Only instructions go on the work list
133 // getValueState - Return the InstVal object that corresponds to the value.
134 // This function is neccesary because not all values should start out in the
135 // underdefined state... Argument's should be overdefined, and
136 // constants should be marked as constants. If a value is not known to be an
137 // Instruction object, then use this accessor to get its value from the map.
139 inline InstVal &getValueState(Value *V) {
140 std::map<Value*, InstVal>::iterator I = ValueState.find(V);
141 if (I != ValueState.end()) return I->second; // Common case, in the map
143 if (Constant *CPV = dyn_cast<Constant>(V)) { // Constants are constant
144 ValueState[CPV].markConstant(CPV);
145 } else if (isa<Argument>(V)) { // Arguments are overdefined
146 ValueState[V].markOverdefined();
147 } else if (GlobalValue *GV = dyn_cast<GlobalValue>(V)) {
148 // The address of a global is a constant...
149 ValueState[V].markConstant(ConstantPointerRef::get(GV));
151 // All others are underdefined by default...
152 return ValueState[V];
155 // markExecutable - Mark a basic block as executable, adding it to the BB
156 // work list if it is not already executable...
158 void markExecutable(BasicBlock *BB) {
159 if (BBExecutable.count(BB)) {
160 // BB is already executable, but we may have just made an edge feasible
161 // that wasn't before. Add the PHI nodes to the work list so that they
163 for (BasicBlock::iterator I = BB->begin();
164 PHINode *PN = dyn_cast<PHINode>(I); ++I)
168 DEBUG(std::cerr << "Marking BB Executable: " << *BB);
169 BBExecutable.insert(BB); // Basic block is executable!
170 BBWorkList.push_back(BB); // Add the block to the work list!
175 // visit implementations - Something changed in this instruction... Either an
176 // operand made a transition, or the instruction is newly executable. Change
177 // the value type of I to reflect these changes if appropriate.
179 void visitPHINode(PHINode &I);
182 void visitReturnInst(ReturnInst &I) { /*does not have an effect*/ }
183 void visitTerminatorInst(TerminatorInst &TI);
185 void visitCastInst(CastInst &I);
186 void visitBinaryOperator(Instruction &I);
187 void visitShiftInst(ShiftInst &I) { visitBinaryOperator(I); }
189 // Instructions that cannot be folded away...
190 void visitStoreInst (Instruction &I) { /*returns void*/ }
191 void visitLoadInst (Instruction &I) { markOverdefined(&I); }
192 void visitGetElementPtrInst(GetElementPtrInst &I);
193 void visitCallInst (Instruction &I) { markOverdefined(&I); }
194 void visitInvokeInst (Instruction &I) { markOverdefined(&I); }
195 void visitAllocationInst(Instruction &I) { markOverdefined(&I); }
196 void visitVarArgInst (Instruction &I) { markOverdefined(&I); }
197 void visitFreeInst (Instruction &I) { /*returns void*/ }
199 void visitInstruction(Instruction &I) {
200 // If a new instruction is added to LLVM that we don't handle...
201 std::cerr << "SCCP: Don't know how to handle: " << I;
202 markOverdefined(&I); // Just in case
205 // getFeasibleSuccessors - Return a vector of booleans to indicate which
206 // successors are reachable from a given terminator instruction.
208 void getFeasibleSuccessors(TerminatorInst &TI, std::vector<bool> &Succs);
210 // isEdgeFeasible - Return true if the control flow edge from the 'From' basic
211 // block to the 'To' basic block is currently feasible...
213 bool isEdgeFeasible(BasicBlock *From, BasicBlock *To);
215 // OperandChangedState - This method is invoked on all of the users of an
216 // instruction that was just changed state somehow.... Based on this
217 // information, we need to update the specified user of this instruction.
219 void OperandChangedState(User *U) {
220 // Only instructions use other variable values!
221 Instruction &I = cast<Instruction>(*U);
222 if (BBExecutable.count(I.getParent())) // Inst is executable?
227 RegisterOpt<SCCP> X("sccp", "Sparse Conditional Constant Propagation");
228 } // end anonymous namespace
231 // createSCCPPass - This is the public interface to this file...
233 Pass *createSCCPPass() {
238 //===----------------------------------------------------------------------===//
239 // SCCP Class Implementation
242 // runOnFunction() - Run the Sparse Conditional Constant Propagation algorithm,
243 // and return true if the function was modified.
245 bool SCCP::runOnFunction(Function &F) {
246 // Mark the first block of the function as being executable...
247 markExecutable(&F.front());
249 // Process the work lists until their are empty!
250 while (!BBWorkList.empty() || !InstWorkList.empty()) {
251 // Process the instruction work list...
252 while (!InstWorkList.empty()) {
253 Instruction *I = InstWorkList.back();
254 InstWorkList.pop_back();
256 DEBUG(std::cerr << "\nPopped off I-WL: " << I);
258 // "I" got into the work list because it either made the transition from
259 // bottom to constant, or to Overdefined.
261 // Update all of the users of this instruction's value...
263 for_each(I->use_begin(), I->use_end(),
264 bind_obj(this, &SCCP::OperandChangedState));
267 // Process the basic block work list...
268 while (!BBWorkList.empty()) {
269 BasicBlock *BB = BBWorkList.back();
270 BBWorkList.pop_back();
272 DEBUG(std::cerr << "\nPopped off BBWL: " << BB);
274 // Notify all instructions in this basic block that they are newly
281 for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I)
282 if (!BBExecutable.count(I))
283 std::cerr << "BasicBlock Dead:" << *I;
286 // Iterate over all of the instructions in a function, replacing them with
287 // constants if we have found them to be of constant values.
289 bool MadeChanges = false;
290 for (Function::iterator BB = F.begin(), BBE = F.end(); BB != BBE; ++BB)
291 for (BasicBlock::iterator BI = BB->begin(); BI != BB->end();) {
292 Instruction &Inst = *BI;
293 InstVal &IV = ValueState[&Inst];
294 if (IV.isConstant()) {
295 Constant *Const = IV.getConstant();
296 DEBUG(std::cerr << "Constant: " << Const << " = " << Inst);
298 // Replaces all of the uses of a variable with uses of the constant.
299 Inst.replaceAllUsesWith(Const);
301 // Remove the operator from the list of definitions... and delete it.
302 BI = BB->getInstList().erase(BI);
304 // Hey, we just changed something!
312 // Reset state so that the next invocation will have empty data structures
313 BBExecutable.clear();
315 std::vector<Instruction*>().swap(InstWorkList);
316 std::vector<BasicBlock*>().swap(BBWorkList);
322 // getFeasibleSuccessors - Return a vector of booleans to indicate which
323 // successors are reachable from a given terminator instruction.
325 void SCCP::getFeasibleSuccessors(TerminatorInst &TI, std::vector<bool> &Succs) {
326 Succs.resize(TI.getNumSuccessors());
327 if (BranchInst *BI = dyn_cast<BranchInst>(&TI)) {
328 if (BI->isUnconditional()) {
331 InstVal &BCValue = getValueState(BI->getCondition());
332 if (BCValue.isOverdefined()) {
333 // Overdefined condition variables mean the branch could go either way.
334 Succs[0] = Succs[1] = true;
335 } else if (BCValue.isConstant()) {
336 // Constant condition variables mean the branch can only go a single way
337 Succs[BCValue.getConstant() == ConstantBool::False] = true;
340 } else if (InvokeInst *II = dyn_cast<InvokeInst>(&TI)) {
341 // Invoke instructions successors are always executable.
342 Succs[0] = Succs[1] = true;
343 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(&TI)) {
344 InstVal &SCValue = getValueState(SI->getCondition());
345 if (SCValue.isOverdefined()) { // Overdefined condition?
346 // All destinations are executable!
347 Succs.assign(TI.getNumSuccessors(), true);
348 } else if (SCValue.isConstant()) {
349 Constant *CPV = SCValue.getConstant();
350 // Make sure to skip the "default value" which isn't a value
351 for (unsigned i = 1, E = SI->getNumSuccessors(); i != E; ++i) {
352 if (SI->getSuccessorValue(i) == CPV) {// Found the right branch...
358 // Constant value not equal to any of the branches... must execute
359 // default branch then...
363 std::cerr << "SCCP: Don't know how to handle: " << TI;
364 Succs.assign(TI.getNumSuccessors(), true);
369 // isEdgeFeasible - Return true if the control flow edge from the 'From' basic
370 // block to the 'To' basic block is currently feasible...
372 bool SCCP::isEdgeFeasible(BasicBlock *From, BasicBlock *To) {
373 assert(BBExecutable.count(To) && "Dest should always be alive!");
375 // Make sure the source basic block is executable!!
376 if (!BBExecutable.count(From)) return false;
378 // Check to make sure this edge itself is actually feasible now...
379 TerminatorInst *FT = From->getTerminator();
380 std::vector<bool> SuccFeasible;
381 getFeasibleSuccessors(*FT, SuccFeasible);
383 // Check all edges from From to To. If any are feasible, return true.
384 for (unsigned i = 0, e = SuccFeasible.size(); i != e; ++i)
385 if (FT->getSuccessor(i) == To && SuccFeasible[i])
388 // Otherwise, none of the edges are actually feasible at this time...
392 // visit Implementations - Something changed in this instruction... Either an
393 // operand made a transition, or the instruction is newly executable. Change
394 // the value type of I to reflect these changes if appropriate. This method
395 // makes sure to do the following actions:
397 // 1. If a phi node merges two constants in, and has conflicting value coming
398 // from different branches, or if the PHI node merges in an overdefined
399 // value, then the PHI node becomes overdefined.
400 // 2. If a phi node merges only constants in, and they all agree on value, the
401 // PHI node becomes a constant value equal to that.
402 // 3. If V <- x (op) y && isConstant(x) && isConstant(y) V = Constant
403 // 4. If V <- x (op) y && (isOverdefined(x) || isOverdefined(y)) V = Overdefined
404 // 5. If V <- MEM or V <- CALL or V <- (unknown) then V = Overdefined
405 // 6. If a conditional branch has a value that is constant, make the selected
406 // destination executable
407 // 7. If a conditional branch has a value that is overdefined, make all
408 // successors executable.
410 void SCCP::visitPHINode(PHINode &PN) {
411 if (getValueState(&PN).isOverdefined()) return; // Quick exit
413 // Look at all of the executable operands of the PHI node. If any of them
414 // are overdefined, the PHI becomes overdefined as well. If they are all
415 // constant, and they agree with each other, the PHI becomes the identical
416 // constant. If they are constant and don't agree, the PHI is overdefined.
417 // If there are no executable operands, the PHI remains undefined.
419 Constant *OperandVal = 0;
420 for (unsigned i = 0, e = PN.getNumIncomingValues(); i != e; ++i) {
421 InstVal &IV = getValueState(PN.getIncomingValue(i));
422 if (IV.isUndefined()) continue; // Doesn't influence PHI node.
424 if (isEdgeFeasible(PN.getIncomingBlock(i), PN.getParent())) {
425 if (IV.isOverdefined()) { // PHI node becomes overdefined!
426 markOverdefined(&PN);
430 if (OperandVal == 0) { // Grab the first value...
431 OperandVal = IV.getConstant();
432 } else { // Another value is being merged in!
433 // There is already a reachable operand. If we conflict with it,
434 // then the PHI node becomes overdefined. If we agree with it, we
437 // Check to see if there are two different constants merging...
438 if (IV.getConstant() != OperandVal) {
439 // Yes there is. This means the PHI node is not constant.
440 // You must be overdefined poor PHI.
442 markOverdefined(&PN); // The PHI node now becomes overdefined
443 return; // I'm done analyzing you
449 // If we exited the loop, this means that the PHI node only has constant
450 // arguments that agree with each other(and OperandVal is the constant) or
451 // OperandVal is null because there are no defined incoming arguments. If
452 // this is the case, the PHI remains undefined.
455 markConstant(&PN, OperandVal); // Aquire operand value
458 void SCCP::visitTerminatorInst(TerminatorInst &TI) {
459 std::vector<bool> SuccFeasible;
460 getFeasibleSuccessors(TI, SuccFeasible);
462 // Mark all feasible successors executable...
463 for (unsigned i = 0, e = SuccFeasible.size(); i != e; ++i)
464 if (SuccFeasible[i]) {
465 BasicBlock *Succ = TI.getSuccessor(i);
466 markExecutable(Succ);
470 void SCCP::visitCastInst(CastInst &I) {
471 Value *V = I.getOperand(0);
472 InstVal &VState = getValueState(V);
473 if (VState.isOverdefined()) { // Inherit overdefinedness of operand
475 } else if (VState.isConstant()) { // Propagate constant value
477 ConstantFoldCastInstruction(VState.getConstant(), I.getType());
480 // This instruction constant folds!
481 markConstant(&I, Result);
483 markOverdefined(&I); // Don't know how to fold this instruction. :(
488 // Handle BinaryOperators and Shift Instructions...
489 void SCCP::visitBinaryOperator(Instruction &I) {
490 InstVal &V1State = getValueState(I.getOperand(0));
491 InstVal &V2State = getValueState(I.getOperand(1));
492 if (V1State.isOverdefined() || V2State.isOverdefined()) {
494 } else if (V1State.isConstant() && V2State.isConstant()) {
495 Constant *Result = 0;
496 if (isa<BinaryOperator>(I))
497 Result = ConstantFoldBinaryInstruction(I.getOpcode(),
498 V1State.getConstant(),
499 V2State.getConstant());
500 else if (isa<ShiftInst>(I))
501 Result = ConstantFoldShiftInstruction(I.getOpcode(),
502 V1State.getConstant(),
503 V2State.getConstant());
505 markConstant(&I, Result); // This instruction constant folds!
507 markOverdefined(&I); // Don't know how to fold this instruction. :(
511 // Handle getelementptr instructions... if all operands are constants then we
512 // can turn this into a getelementptr ConstantExpr.
514 void SCCP::visitGetElementPtrInst(GetElementPtrInst &I) {
515 std::vector<Constant*> Operands;
516 Operands.reserve(I.getNumOperands());
518 for (unsigned i = 0, e = I.getNumOperands(); i != e; ++i) {
519 InstVal &State = getValueState(I.getOperand(i));
520 if (State.isUndefined())
521 return; // Operands are not resolved yet...
522 else if (State.isOverdefined()) {
526 assert(State.isConstant() && "Unknown state!");
527 Operands.push_back(State.getConstant());
530 Constant *Ptr = Operands[0];
531 Operands.erase(Operands.begin()); // Erase the pointer from idx list...
533 markConstant(&I, ConstantExpr::getGetElementPtr(Ptr, Operands));