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!
182 // visit implementations - Something changed in this instruction... Either an
183 // operand made a transition, or the instruction is newly executable. Change
184 // the value type of I to reflect these changes if appropriate.
186 void visitPHINode(PHINode *I);
189 void visitReturnInst(ReturnInst *I) { /*does not have an effect*/ }
190 void visitTerminatorInst(TerminatorInst *TI);
192 void visitUnaryOperator(Instruction *I);
193 void visitCastInst(CastInst *I) { visitUnaryOperator(I); }
194 void visitBinaryOperator(Instruction *I);
195 void visitShiftInst(ShiftInst *I) { visitBinaryOperator(I); }
197 // Instructions that cannot be folded away...
198 void visitStoreInst (Instruction *I) { /*returns void*/ }
199 void visitMemAccessInst (Instruction *I) { markOverdefined(I); }
200 void visitCallInst (Instruction *I) { markOverdefined(I); }
201 void visitInvokeInst (Instruction *I) { markOverdefined(I); }
202 void visitAllocationInst(Instruction *I) { markOverdefined(I); }
203 void visitFreeInst (Instruction *I) { /*returns void*/ }
205 void visitInstruction(Instruction *I) {
206 // If a new instruction is added to LLVM that we don't handle...
207 cerr << "SCCP: Don't know how to handle: " << I;
208 markOverdefined(I); // Just in case
211 // getFeasibleSuccessors - Return a vector of booleans to indicate which
212 // successors are reachable from a given terminator instruction.
214 void getFeasibleSuccessors(TerminatorInst *I, std::vector<bool> &Succs);
216 // isEdgeFeasible - Return true if the control flow edge from the 'From' basic
217 // block to the 'To' basic block is currently feasible...
219 bool isEdgeFeasible(BasicBlock *From, BasicBlock *To);
221 // OperandChangedState - This method is invoked on all of the users of an
222 // instruction that was just changed state somehow.... Based on this
223 // information, we need to update the specified user of this instruction.
225 void OperandChangedState(User *U) {
226 // Only instructions use other variable values!
227 Instruction *I = cast<Instruction>(U);
228 if (!BBExecutable.count(I->getParent())) return;// Inst not executable yet!
232 } // end anonymous namespace
235 // createSCCPPass - This is the public interface to this file...
237 Pass *createSCCPPass() {
243 //===----------------------------------------------------------------------===//
244 // SCCP Class Implementation
247 // runOnFunction() - Run the Sparse Conditional Constant Propogation algorithm,
248 // and return true if the function was modified.
250 bool SCCP::runOnFunction(Function *F) {
251 // Mark the first block of the function as being executable...
252 markExecutable(F->front());
254 // Process the work lists until their are empty!
255 while (!BBWorkList.empty() || !InstWorkList.empty()) {
256 // Process the instruction work list...
257 while (!InstWorkList.empty()) {
258 Instruction *I = InstWorkList.back();
259 InstWorkList.pop_back();
261 DEBUG_SCCP(cerr << "\nPopped off I-WL: " << I);
264 // "I" got into the work list because it either made the transition from
265 // bottom to constant, or to Overdefined.
267 // Update all of the users of this instruction's value...
269 for_each(I->use_begin(), I->use_end(),
270 bind_obj(this, &SCCP::OperandChangedState));
273 // Process the basic block work list...
274 while (!BBWorkList.empty()) {
275 BasicBlock *BB = BBWorkList.back();
276 BBWorkList.pop_back();
278 DEBUG_SCCP(cerr << "\nPopped off BBWL: " << BB);
280 // If this block only has a single successor, mark it as executable as
281 // well... if not, terminate the do loop.
283 if (BB->getTerminator()->getNumSuccessors() == 1)
284 markExecutable(BB->getTerminator()->getSuccessor(0));
286 // Notify all instructions in this basic block that they are newly
293 for (Function::iterator BBI = F->begin(), BBEnd = F->end();
295 if (!BBExecutable.count(*BBI))
296 cerr << "BasicBlock Dead:" << *BBI;
300 // Iterate over all of the instructions in a function, replacing them with
301 // constants if we have found them to be of constant values.
303 bool MadeChanges = false;
304 for (Function::iterator FI = F->begin(), FE = F->end(); FI != FE; ++FI) {
305 BasicBlock *BB = *FI;
306 for (BasicBlock::iterator BI = BB->begin(); BI != BB->end();) {
307 Instruction *Inst = *BI;
308 InstVal &IV = ValueState[Inst];
309 if (IV.isConstant()) {
310 Constant *Const = IV.getConstant();
311 DEBUG_SCCP(cerr << "Constant: " << Const << " = " << Inst);
313 // Replaces all of the uses of a variable with uses of the constant.
314 Inst->replaceAllUsesWith(Const);
316 // Remove the operator from the list of definitions... and delete it.
317 delete BB->getInstList().remove(BI);
319 // Hey, we just changed something!
328 // Reset state so that the next invocation will have empty data structures
329 BBExecutable.clear();
336 // getFeasibleSuccessors - Return a vector of booleans to indicate which
337 // successors are reachable from a given terminator instruction.
339 void SCCP::getFeasibleSuccessors(TerminatorInst *TI, std::vector<bool> &Succs) {
340 assert(Succs.size() == TI->getNumSuccessors() && "Succs vector wrong size!");
341 if (BranchInst *BI = dyn_cast<BranchInst>(TI)) {
342 if (BI->isUnconditional()) {
345 InstVal &BCValue = getValueState(BI->getCondition());
346 if (BCValue.isOverdefined()) {
347 // Overdefined condition variables mean the branch could go either way.
348 Succs[0] = Succs[1] = true;
349 } else if (BCValue.isConstant()) {
350 // Constant condition variables mean the branch can only go a single way
351 Succs[BCValue.getConstant() == ConstantBool::False] = true;
354 } else if (InvokeInst *II = dyn_cast<InvokeInst>(TI)) {
355 // Invoke instructions successors are always executable.
356 Succs[0] = Succs[1] = true;
357 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
358 InstVal &SCValue = getValueState(SI->getCondition());
359 if (SCValue.isOverdefined()) { // Overdefined condition?
360 // All destinations are executable!
361 Succs.assign(TI->getNumSuccessors(), true);
362 } else if (SCValue.isConstant()) {
363 Constant *CPV = SCValue.getConstant();
364 // Make sure to skip the "default value" which isn't a value
365 for (unsigned i = 1, E = SI->getNumSuccessors(); i != E; ++i) {
366 if (SI->getSuccessorValue(i) == CPV) {// Found the right branch...
372 // Constant value not equal to any of the branches... must execute
373 // default branch then...
377 cerr << "SCCP: Don't know how to handle: " << TI;
378 Succs.assign(TI->getNumSuccessors(), true);
383 // isEdgeFeasible - Return true if the control flow edge from the 'From' basic
384 // block to the 'To' basic block is currently feasible...
386 bool SCCP::isEdgeFeasible(BasicBlock *From, BasicBlock *To) {
387 assert(BBExecutable.count(To) && "Dest should always be alive!");
389 // Make sure the source basic block is executable!!
390 if (!BBExecutable.count(From)) return false;
392 // Check to make sure this edge itself is actually feasible now...
393 TerminatorInst *FT = From->getTerminator();
394 std::vector<bool> SuccFeasible(FT->getNumSuccessors());
395 getFeasibleSuccessors(FT, SuccFeasible);
397 // Check all edges from From to To. If any are feasible, return true.
398 for (unsigned i = 0, e = SuccFeasible.size(); i != e; ++i)
399 if (FT->getSuccessor(i) == To && SuccFeasible[i])
402 // Otherwise, none of the edges are actually feasible at this time...
406 // visit Implementations - Something changed in this instruction... Either an
407 // operand made a transition, or the instruction is newly executable. Change
408 // the value type of I to reflect these changes if appropriate. This method
409 // makes sure to do the following actions:
411 // 1. If a phi node merges two constants in, and has conflicting value coming
412 // from different branches, or if the PHI node merges in an overdefined
413 // value, then the PHI node becomes overdefined.
414 // 2. If a phi node merges only constants in, and they all agree on value, the
415 // PHI node becomes a constant value equal to that.
416 // 3. If V <- x (op) y && isConstant(x) && isConstant(y) V = Constant
417 // 4. If V <- x (op) y && (isOverdefined(x) || isOverdefined(y)) V = Overdefined
418 // 5. If V <- MEM or V <- CALL or V <- (unknown) then V = Overdefined
419 // 6. If a conditional branch has a value that is constant, make the selected
420 // destination executable
421 // 7. If a conditional branch has a value that is overdefined, make all
422 // successors executable.
425 void SCCP::visitPHINode(PHINode *PN) {
426 unsigned NumValues = PN->getNumIncomingValues(), i;
427 InstVal *OperandIV = 0;
429 // Look at all of the executable operands of the PHI node. If any of them
430 // are overdefined, the PHI becomes overdefined as well. If they are all
431 // constant, and they agree with each other, the PHI becomes the identical
432 // constant. If they are constant and don't agree, the PHI is overdefined.
433 // If there are no executable operands, the PHI remains undefined.
435 for (i = 0; i < NumValues; ++i) {
436 if (isEdgeFeasible(PN->getIncomingBlock(i), PN->getParent())) {
437 InstVal &IV = getValueState(PN->getIncomingValue(i));
438 if (IV.isUndefined()) continue; // Doesn't influence PHI node.
439 if (IV.isOverdefined()) { // PHI node becomes overdefined!
444 if (OperandIV == 0) { // Grab the first value...
446 } else { // Another value is being merged in!
447 // There is already a reachable operand. If we conflict with it,
448 // then the PHI node becomes overdefined. If we agree with it, we
451 // Check to see if there are two different constants merging...
452 if (IV.getConstant() != OperandIV->getConstant()) {
453 // Yes there is. This means the PHI node is not constant.
454 // You must be overdefined poor PHI.
456 markOverdefined(PN); // The PHI node now becomes overdefined
457 return; // I'm done analyzing you
463 // If we exited the loop, this means that the PHI node only has constant
464 // arguments that agree with each other(and OperandIV is a pointer to one
465 // of their InstVal's) or OperandIV is null because there are no defined
466 // incoming arguments. If this is the case, the PHI remains undefined.
469 assert(OperandIV->isConstant() && "Should only be here for constants!");
470 markConstant(PN, OperandIV->getConstant()); // Aquire operand value
474 void SCCP::visitTerminatorInst(TerminatorInst *TI) {
475 std::vector<bool> SuccFeasible(TI->getNumSuccessors());
476 getFeasibleSuccessors(TI, SuccFeasible);
478 // Mark all feasible successors executable...
479 for (unsigned i = 0, e = SuccFeasible.size(); i != e; ++i)
480 if (SuccFeasible[i]) {
481 BasicBlock *Succ = TI->getSuccessor(i);
482 markExecutable(Succ);
484 // Visit all of the PHI nodes that merge values from this block...
485 // Because this edge may be new executable, and PHI nodes that used to be
486 // constant now may not be.
488 for (BasicBlock::iterator I = Succ->begin();
489 PHINode *PN = dyn_cast<PHINode>(*I); ++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. :(