1 //===- SCCP.cpp - Sparse Conditional Constant Propogation -----------------===//
3 // This file implements sparse conditional constant propogation 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 constant, and unconditionalizes them
10 // * Folds multiple identical constants in the constant pool together
13 // * This pass has a habit of making definitions be dead. It is a good idea
14 // to to run a DCE pass sometime after running this pass.
16 //===----------------------------------------------------------------------===//
18 #include "llvm/Transforms/Scalar.h"
19 #include "llvm/ConstantHandling.h"
20 #include "llvm/Function.h"
21 #include "llvm/BasicBlock.h"
22 #include "llvm/iPHINode.h"
23 #include "llvm/iMemory.h"
24 #include "llvm/iTerminators.h"
25 #include "llvm/iOther.h"
26 #include "llvm/Pass.h"
27 #include "llvm/Support/InstVisitor.h"
28 #include "Support/STLExtras.h"
29 #include "Support/StatisticReporter.h"
35 static Statistic<> NumInstRemoved("sccp\t\t- Number of instructions removed");
37 #if 0 // Enable this to get SCCP debug output
38 #define DEBUG_SCCP(X) X
43 // InstVal class - This class represents the different lattice values that an
44 // instruction may occupy. It is a simple class with value semantics.
49 undefined, // This instruction has no known value
50 constant, // This instruction has a constant value
51 // Range, // This instruction is known to fall within a range
52 overdefined // This instruction has an unknown value
53 } LatticeValue; // The current lattice position
54 Constant *ConstantVal; // If Constant value, the current value
56 inline InstVal() : LatticeValue(undefined), ConstantVal(0) {}
58 // markOverdefined - Return true if this is a new status to be in...
59 inline bool markOverdefined() {
60 if (LatticeValue != overdefined) {
61 LatticeValue = overdefined;
67 // markConstant - Return true if this is a new status for us...
68 inline bool markConstant(Constant *V) {
69 if (LatticeValue != constant) {
70 LatticeValue = constant;
74 assert(ConstantVal == V && "Marking constant with different value");
79 inline bool isUndefined() const { return LatticeValue == undefined; }
80 inline bool isConstant() const { return LatticeValue == constant; }
81 inline bool isOverdefined() const { return LatticeValue == overdefined; }
83 inline Constant *getConstant() const { return ConstantVal; }
86 } // end anonymous namespace
89 //===----------------------------------------------------------------------===//
92 // This class does all of the work of Sparse Conditional Constant Propogation.
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
103 const char *getPassName() const {
104 return "Sparse Conditional Constant Propogation";
107 // runOnFunction - Run the Sparse Conditional Constant Propogation algorithm,
108 // and return true if the function was modified.
110 bool runOnFunction(Function *F);
112 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
117 //===--------------------------------------------------------------------===//
118 // The implementation of this class
121 friend class InstVisitor<SCCP>; // Allow callbacks from visitor
123 // markValueOverdefined - Make a value be marked as "constant". If the value
124 // is not already a constant, add it to the instruction work list so that
125 // the users of the instruction are updated later.
127 inline bool markConstant(Instruction *I, Constant *V) {
128 DEBUG_SCCP(cerr << "markConstant: " << V << " = " << I);
130 if (ValueState[I].markConstant(V)) {
131 InstWorkList.push_back(I);
137 // markValueOverdefined - Make a value be marked as "overdefined". If the
138 // value is not already overdefined, add it to the instruction work list so
139 // that the users of the instruction are updated later.
141 inline bool markOverdefined(Value *V) {
142 if (ValueState[V].markOverdefined()) {
143 if (Instruction *I = dyn_cast<Instruction>(V)) {
144 DEBUG_SCCP(cerr << "markOverdefined: " << V);
145 InstWorkList.push_back(I); // Only instructions go on the work list
152 // getValueState - Return the InstVal object that corresponds to the value.
153 // This function is neccesary because not all values should start out in the
154 // underdefined state... Argument's should be overdefined, and
155 // constants should be marked as constants. If a value is not known to be an
156 // Instruction object, then use this accessor to get its value from the map.
158 inline InstVal &getValueState(Value *V) {
159 std::map<Value*, InstVal>::iterator I = ValueState.find(V);
160 if (I != ValueState.end()) return I->second; // Common case, in the map
162 if (Constant *CPV = dyn_cast<Constant>(V)) { // Constants are constant
163 ValueState[CPV].markConstant(CPV);
164 } else if (isa<Argument>(V)) { // Arguments are overdefined
165 ValueState[V].markOverdefined();
167 // All others are underdefined by default...
168 return ValueState[V];
171 // markExecutable - Mark a basic block as executable, adding it to the BB
172 // work list if it is not already executable...
174 void markExecutable(BasicBlock *BB) {
175 if (BBExecutable.count(BB)) return;
176 DEBUG_SCCP(cerr << "Marking BB Executable: " << BB);
177 BBExecutable.insert(BB); // Basic block is executable!
178 BBWorkList.push_back(BB); // Add the block to the work list!
180 // Visit all of the PHI nodes that merge values from this block... Because
181 // this block is newly executable, PHI nodes that used to be constant now
182 // may not be. Note that we only mark PHI nodes that live in blocks that
185 for (Value::use_iterator I = BB->use_begin(), E = BB->use_end(); I != E;++I)
186 if (PHINode *PN = dyn_cast<PHINode>(*I))
187 if (BBExecutable.count(PN->getParent()))
192 // visit implementations - Something changed in this instruction... Either an
193 // operand made a transition, or the instruction is newly executable. Change
194 // the value type of I to reflect these changes if appropriate.
196 void visitPHINode(PHINode *I);
199 void visitReturnInst(ReturnInst *I) { /*does not have an effect*/ }
200 void visitTerminatorInst(TerminatorInst *TI);
202 void visitUnaryOperator(Instruction *I);
203 void visitCastInst(CastInst *I) { visitUnaryOperator(I); }
204 void visitBinaryOperator(Instruction *I);
205 void visitShiftInst(ShiftInst *I) { visitBinaryOperator(I); }
207 // Instructions that cannot be folded away...
208 void visitStoreInst (Instruction *I) { /*returns void*/ }
209 void visitMemAccessInst (Instruction *I) { markOverdefined(I); }
210 void visitCallInst (Instruction *I) { markOverdefined(I); }
211 void visitInvokeInst (Instruction *I) { markOverdefined(I); }
212 void visitAllocationInst(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 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 *I, 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())) return;// Inst not executable yet!
242 } // end anonymous namespace
245 // createSCCPPass - This is the public interface to this file...
247 Pass *createSCCPPass() {
253 //===----------------------------------------------------------------------===//
254 // SCCP Class Implementation
257 // runOnFunction() - Run the Sparse Conditional Constant Propogation algorithm,
258 // and return true if the function was modified.
260 bool SCCP::runOnFunction(Function *F) {
261 // Mark the first block of the function as being executable...
262 markExecutable(F->front());
264 // Process the work lists until their are empty!
265 while (!BBWorkList.empty() || !InstWorkList.empty()) {
266 // Process the instruction work list...
267 while (!InstWorkList.empty()) {
268 Instruction *I = InstWorkList.back();
269 InstWorkList.pop_back();
271 DEBUG_SCCP(cerr << "\nPopped off I-WL: " << I);
274 // "I" got into the work list because it either made the transition from
275 // bottom to constant, or to Overdefined.
277 // Update all of the users of this instruction's value...
279 for_each(I->use_begin(), I->use_end(),
280 bind_obj(this, &SCCP::OperandChangedState));
283 // Process the basic block work list...
284 while (!BBWorkList.empty()) {
285 BasicBlock *BB = BBWorkList.back();
286 BBWorkList.pop_back();
288 DEBUG_SCCP(cerr << "\nPopped off BBWL: " << BB);
290 // If this block only has a single successor, mark it as executable as
291 // well... if not, terminate the do loop.
293 if (BB->getTerminator()->getNumSuccessors() == 1)
294 markExecutable(BB->getTerminator()->getSuccessor(0));
296 // Notify all instructions in this basic block that they are newly
303 for (Function::iterator BBI = F->begin(), BBEnd = F->end();
305 if (!BBExecutable.count(*BBI))
306 cerr << "BasicBlock Dead:" << *BBI;
310 // Iterate over all of the instructions in a function, replacing them with
311 // constants if we have found them to be of constant values.
313 bool MadeChanges = false;
314 for (Function::iterator FI = F->begin(), FE = F->end(); FI != FE; ++FI) {
315 BasicBlock *BB = *FI;
316 for (BasicBlock::iterator BI = BB->begin(); BI != BB->end();) {
317 Instruction *Inst = *BI;
318 InstVal &IV = ValueState[Inst];
319 if (IV.isConstant()) {
320 Constant *Const = IV.getConstant();
321 DEBUG_SCCP(cerr << "Constant: " << Const << " = " << Inst);
323 // Replaces all of the uses of a variable with uses of the constant.
324 Inst->replaceAllUsesWith(Const);
326 // Remove the operator from the list of definitions... and delete it.
327 delete BB->getInstList().remove(BI);
329 // Hey, we just changed something!
338 // Reset state so that the next invocation will have empty data structures
339 BBExecutable.clear();
346 // getFeasibleSuccessors - Return a vector of booleans to indicate which
347 // successors are reachable from a given terminator instruction.
349 void SCCP::getFeasibleSuccessors(TerminatorInst *TI, std::vector<bool> &Succs) {
350 assert(Succs.size() == TI->getNumSuccessors() && "Succs vector wrong size!");
351 if (BranchInst *BI = dyn_cast<BranchInst>(TI)) {
352 if (BI->isUnconditional()) {
355 InstVal &BCValue = getValueState(BI->getCondition());
356 if (BCValue.isOverdefined()) {
357 // Overdefined condition variables mean the branch could go either way.
358 Succs[0] = Succs[1] = true;
359 } else if (BCValue.isConstant()) {
360 // Constant condition variables mean the branch can only go a single way
361 Succs[BCValue.getConstant() == ConstantBool::False] = true;
364 } else if (InvokeInst *II = dyn_cast<InvokeInst>(TI)) {
365 // Invoke instructions successors are always executable.
366 Succs[0] = Succs[1] = true;
367 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
368 InstVal &SCValue = getValueState(SI->getCondition());
369 if (SCValue.isOverdefined()) { // Overdefined condition?
370 // All destinations are executable!
371 Succs.assign(TI->getNumSuccessors(), true);
372 } else if (SCValue.isConstant()) {
373 Constant *CPV = SCValue.getConstant();
374 // Make sure to skip the "default value" which isn't a value
375 for (unsigned i = 1, E = SI->getNumSuccessors(); i != E; ++i) {
376 if (SI->getSuccessorValue(i) == CPV) {// Found the right branch...
382 // Constant value not equal to any of the branches... must execute
383 // default branch then...
387 cerr << "SCCP: Don't know how to handle: " << TI;
388 Succs.assign(TI->getNumSuccessors(), true);
393 // isEdgeFeasible - Return true if the control flow edge from the 'From' basic
394 // block to the 'To' basic block is currently feasible...
396 bool SCCP::isEdgeFeasible(BasicBlock *From, BasicBlock *To) {
397 assert(BBExecutable.count(To) && "Dest should always be alive!");
399 // Make sure the source basic block is executable!!
400 if (!BBExecutable.count(From)) return false;
402 // Check to make sure this edge itself is actually feasible now...
403 TerminatorInst *FT = From->getTerminator();
404 std::vector<bool> SuccFeasible(FT->getNumSuccessors());
405 getFeasibleSuccessors(FT, SuccFeasible);
407 // Check all edges from From to To. If any are feasible, return true.
408 for (unsigned i = 0, e = SuccFeasible.size(); i != e; ++i)
409 if (FT->getSuccessor(i) == To && SuccFeasible[i])
412 // Otherwise, none of the edges are actually feasible at this time...
416 // visit Implementations - Something changed in this instruction... Either an
417 // operand made a transition, or the instruction is newly executable. Change
418 // the value type of I to reflect these changes if appropriate. This method
419 // makes sure to do the following actions:
421 // 1. If a phi node merges two constants in, and has conflicting value coming
422 // from different branches, or if the PHI node merges in an overdefined
423 // value, then the PHI node becomes overdefined.
424 // 2. If a phi node merges only constants in, and they all agree on value, the
425 // PHI node becomes a constant value equal to that.
426 // 3. If V <- x (op) y && isConstant(x) && isConstant(y) V = Constant
427 // 4. If V <- x (op) y && (isOverdefined(x) || isOverdefined(y)) V = Overdefined
428 // 5. If V <- MEM or V <- CALL or V <- (unknown) then V = Overdefined
429 // 6. If a conditional branch has a value that is constant, make the selected
430 // destination executable
431 // 7. If a conditional branch has a value that is overdefined, make all
432 // successors executable.
435 void SCCP::visitPHINode(PHINode *PN) {
436 unsigned NumValues = PN->getNumIncomingValues(), i;
437 InstVal *OperandIV = 0;
439 // Look at all of the executable operands of the PHI node. If any of them
440 // are overdefined, the PHI becomes overdefined as well. If they are all
441 // constant, and they agree with each other, the PHI becomes the identical
442 // constant. If they are constant and don't agree, the PHI is overdefined.
443 // If there are no executable operands, the PHI remains undefined.
445 for (i = 0; i < NumValues; ++i) {
446 if (isEdgeFeasible(PN->getIncomingBlock(i), PN->getParent())) {
447 InstVal &IV = getValueState(PN->getIncomingValue(i));
448 if (IV.isUndefined()) continue; // Doesn't influence PHI node.
449 if (IV.isOverdefined()) { // PHI node becomes overdefined!
454 if (OperandIV == 0) { // Grab the first value...
456 } else { // Another value is being merged in!
457 // There is already a reachable operand. If we conflict with it,
458 // then the PHI node becomes overdefined. If we agree with it, we
461 // Check to see if there are two different constants merging...
462 if (IV.getConstant() != OperandIV->getConstant()) {
463 // Yes there is. This means the PHI node is not constant.
464 // You must be overdefined poor PHI.
466 markOverdefined(PN); // The PHI node now becomes overdefined
467 return; // I'm done analyzing you
473 // If we exited the loop, this means that the PHI node only has constant
474 // arguments that agree with each other(and OperandIV is a pointer to one
475 // of their InstVal's) or OperandIV is null because there are no defined
476 // incoming arguments. If this is the case, the PHI remains undefined.
479 assert(OperandIV->isConstant() && "Should only be here for constants!");
480 markConstant(PN, OperandIV->getConstant()); // Aquire operand value
484 void SCCP::visitTerminatorInst(TerminatorInst *TI) {
485 std::vector<bool> SuccFeasible(TI->getNumSuccessors());
486 getFeasibleSuccessors(TI, SuccFeasible);
488 // Mark all feasible successors executable...
489 for (unsigned i = 0, e = SuccFeasible.size(); i != e; ++i)
491 markExecutable(TI->getSuccessor(i));
494 void SCCP::visitUnaryOperator(Instruction *I) {
495 Value *V = I->getOperand(0);
496 InstVal &VState = getValueState(V);
497 if (VState.isOverdefined()) { // Inherit overdefinedness of operand
499 } else if (VState.isConstant()) { // Propogate constant value
500 Constant *Result = isa<CastInst>(I)
501 ? ConstantFoldCastInstruction(VState.getConstant(), I->getType())
502 : ConstantFoldUnaryInstruction(I->getOpcode(), VState.getConstant());
505 // This instruction constant folds!
506 markConstant(I, Result);
508 markOverdefined(I); // Don't know how to fold this instruction. :(
513 // Handle BinaryOperators and Shift Instructions...
514 void SCCP::visitBinaryOperator(Instruction *I) {
515 InstVal &V1State = getValueState(I->getOperand(0));
516 InstVal &V2State = getValueState(I->getOperand(1));
517 if (V1State.isOverdefined() || V2State.isOverdefined()) {
519 } else if (V1State.isConstant() && V2State.isConstant()) {
520 Constant *Result = 0;
521 if (isa<BinaryOperator>(I))
522 Result = ConstantFoldBinaryInstruction(I->getOpcode(),
523 V1State.getConstant(),
524 V2State.getConstant());
525 else if (isa<ShiftInst>(I))
526 Result = ConstantFoldShiftInstruction(I->getOpcode(),
527 V1State.getConstant(),
528 V2State.getConstant());
530 markConstant(I, Result); // This instruction constant folds!
532 markOverdefined(I); // Don't know how to fold this instruction. :(