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/Optimizations/ConstantProp.h"
19 #include "llvm/Optimizations/ConstantHandling.h"
20 #include "llvm/Method.h"
21 #include "llvm/BasicBlock.h"
22 #include "llvm/ConstPoolVals.h"
23 #include "llvm/ConstantPool.h"
24 #include "llvm/InstrTypes.h"
25 #include "llvm/iOther.h"
26 #include "llvm/iMemory.h"
27 #include "llvm/iTerminators.h"
28 #include "llvm/Support/STLExtras.h"
29 #include "llvm/Assembly/Writer.h"
34 // InstVal class - This class represents the different lattice values that an
35 // instruction may occupy. It is a simple class with value semantics. The
36 // potential constant value that is pointed to is owned by the constant pool
37 // for the method being optimized.
41 Undefined, // This instruction has no known value
42 Constant, // This instruction has a constant value
43 // Range, // This instruction is known to fall within a range
44 Overdefined // This instruction has an unknown value
45 } LatticeValue; // The current lattice position
46 ConstPoolVal *ConstantVal; // If Constant value, the current value
48 inline InstVal() : LatticeValue(Undefined), ConstantVal(0) {}
50 // markOverdefined - Return true if this is a new status to be in...
51 inline bool markOverdefined() {
52 if (LatticeValue != Overdefined) {
53 LatticeValue = Overdefined;
59 // markConstant - Return true if this is a new status for us...
60 inline bool markConstant(ConstPoolVal *V) {
61 if (LatticeValue != Constant) {
62 LatticeValue = Constant;
66 assert(ConstantVal->equals(V) && "Marking constant with different value");
71 inline bool isUndefined() const { return LatticeValue == Undefined; }
72 inline bool isConstant() const { return LatticeValue == Constant; }
73 inline bool isOverdefined() const { return LatticeValue == Overdefined; }
75 inline ConstPoolVal *getConstant() const { return ConstantVal; }
80 //===----------------------------------------------------------------------===//
83 // This class does all of the work of Sparse Conditional Constant Propogation.
84 // It's public interface consists of a constructor and a doSCCP() method.
87 Method *M; // The method that we are working on...
89 set<BasicBlock*> BBExecutable; // The basic blocks that are executable
90 map<Value*, InstVal> ValueState; // The state each value is in...
92 vector<Instruction*> InstWorkList; // The instruction work list
93 vector<BasicBlock*> BBWorkList; // The BasicBlock work list
95 //===--------------------------------------------------------------------===//
96 // The public interface for this class
100 // SCCP Ctor - Save the method to operate on...
101 inline SCCP(Method *m) : M(m) {}
103 // doSCCP() - Run the Sparse Conditional Constant Propogation algorithm, and
104 // return true if the method was modified.
107 //===--------------------------------------------------------------------===//
108 // The implementation of this class
112 // markValueOverdefined - Make a value be marked as "constant". If the value
113 // is not already a constant, add it to the instruction work list so that
114 // the users of the instruction are updated later.
116 inline bool markConstant(Instruction *I, ConstPoolVal *V) {
117 //cerr << "markConstant: " << V << " = " << I;
118 if (ValueState[I].markConstant(V)) {
119 InstWorkList.push_back(I);
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 bool markOverdefined(Value *V) {
130 if (ValueState[V].markOverdefined()) {
131 if (Instruction *I = V->castInstruction()) {
132 //cerr << "markOverdefined: " << V;
133 InstWorkList.push_back(I); // Only instructions go on the work list
140 // getValueState - Return the InstVal object that corresponds to the value.
141 // This function is neccesary because not all values should start out in the
142 // underdefined state... MethodArgument's should be overdefined, and constants
143 // should be marked as constants. If a value is not known to be an
144 // Instruction object, then use this accessor to get its value from the map.
146 inline InstVal &getValueState(Value *V) {
147 map<Value*, InstVal>::iterator I = ValueState.find(V);
148 if (I != ValueState.end()) return I->second; // Common case, in the map
150 if (ConstPoolVal *CPV = V->castConstant()) { // Constants are constant
151 ValueState[CPV].markConstant(CPV);
152 } else if (V->isMethodArgument()) { // MethodArgs are overdefined
153 ValueState[V].markOverdefined();
155 // All others are underdefined by default...
156 return ValueState[V];
159 // markExecutable - Mark a basic block as executable, adding it to the BB
160 // work list if it is not already executable...
162 void markExecutable(BasicBlock *BB) {
163 if (BBExecutable.count(BB)) return;
164 //cerr << "Marking BB Executable: " << BB;
165 BBExecutable.insert(BB); // Basic block is executable!
166 BBWorkList.push_back(BB); // Add the block to the work list!
170 // UpdateInstruction - Something changed in this instruction... Either an
171 // operand made a transition, or the instruction is newly executable. Change
172 // the value type of I to reflect these changes if appropriate.
174 void UpdateInstruction(Instruction *I);
176 // OperandChangedState - This method is invoked on all of the users of an
177 // instruction that was just changed state somehow.... Based on this
178 // information, we need to update the specified user of this instruction.
180 void OperandChangedState(User *U);
184 //===----------------------------------------------------------------------===//
185 // SCCP Class Implementation
188 // doSCCP() - Run the Sparse Conditional Constant Propogation algorithm, and
189 // return true if the method was modified.
191 bool SCCP::doSCCP() {
192 // Mark the first block of the method as being executable...
193 markExecutable(M->front());
195 // Process the work lists until their are empty!
196 while (!BBWorkList.empty() || !InstWorkList.empty()) {
197 // Process the instruction work list...
198 while (!InstWorkList.empty()) {
199 Instruction *I = InstWorkList.back();
200 InstWorkList.pop_back();
202 //cerr << "\nPopped off I-WL: " << I;
205 // "I" got into the work list because it either made the transition from
206 // bottom to constant, or to Overdefined.
208 // Update all of the users of this instruction's value...
210 for_each(I->use_begin(), I->use_end(),
211 bind_obj(this, &SCCP::OperandChangedState));
214 // Process the basic block work list...
215 while (!BBWorkList.empty()) {
216 BasicBlock *BB = BBWorkList.back();
217 BBWorkList.pop_back();
219 //cerr << "\nPopped off BBWL: " << BB;
221 // If this block only has a single successor, mark it as executable as
222 // well... if not, terminate the do loop.
224 if (BB->getTerminator()->getNumSuccessors() == 1)
225 markExecutable(BB->getTerminator()->getSuccessor(0));
227 // Loop over all of the instructions and notify them that they are newly
229 for_each(BB->begin(), BB->end(),
230 bind_obj(this, &SCCP::UpdateInstruction));
235 for (Method::iterator BBI = M->begin(), BBEnd = M->end(); BBI != BBEnd; ++BBI)
236 if (!BBExecutable.count(*BBI))
237 cerr << "BasicBlock Dead:" << *BBI;
241 // Iterate over all of the instructions in a method, replacing them with
242 // constants if we have found them to be of constant values.
244 bool MadeChanges = false;
245 for (Method::inst_iterator II = M->inst_begin(); II != M->inst_end(); ) {
246 Instruction *Inst = *II;
247 InstVal &IV = ValueState[Inst];
248 if (IV.isConstant()) {
249 ConstPoolVal *Const = IV.getConstant();
250 // cerr << "Constant: " << Inst << " is: " << Const;
252 // Replaces all of the uses of a variable with uses of the constant.
253 Inst->replaceAllUsesWith(Const);
255 // Remove the operator from the list of definitions...
256 Inst->getParent()->getInstList().remove(II.getInstructionIterator());
258 // The new constant inherits the old name of the operator...
259 if (Inst->hasName() && !Const->hasName())
260 Const->setName(Inst->getName());
262 // Delete the operator now...
265 // Incrementing the iterator in an unchecked manner could mess up the
266 // internals of 'II'. To make sure everything is happy, tell it we might
268 II.resyncInstructionIterator();
270 // Hey, we just changed something!
272 continue; // Skip the ++II at the end of the loop here...
273 } else if (Inst->isTerminator()) {
274 MadeChanges |= opt::ConstantFoldTerminator((TerminatorInst*)Inst);
280 // Merge identical constants last: this is important because we may have just
281 // introduced constants that already exist, and we don't want to pollute later
282 // stages with extraneous constants.
284 return MadeChanges | opt::DoConstantPoolMerging(M->getConstantPool());
288 // UpdateInstruction - Something changed in this instruction... Either an
289 // operand made a transition, or the instruction is newly executable. Change
290 // the value type of I to reflect these changes if appropriate. This method
291 // makes sure to do the following actions:
293 // 1. If a phi node merges two constants in, and has conflicting value coming
294 // from different branches, or if the PHI node merges in an overdefined
295 // value, then the PHI node becomes overdefined.
296 // 2. If a phi node merges only constants in, and they all agree on value, the
297 // PHI node becomes a constant value equal to that.
298 // 3. If V <- x (op) y && isConstant(x) && isConstant(y) V = Constant
299 // 4. If V <- x (op) y && (isOverdefined(x) || isOverdefined(y)) V = Overdefined
300 // 5. If V <- MEM or V <- CALL or V <- (unknown) then V = Overdefined
301 // 6. If a conditional branch has a value that is constant, make the selected
302 // destination executable
303 // 7. If a conditional branch has a value that is overdefined, make all
304 // successors executable.
306 void SCCP::UpdateInstruction(Instruction *I) {
307 InstVal &IValue = ValueState[I];
308 if (IValue.isOverdefined())
309 return; // If already overdefined, we aren't going to effect anything
311 switch (I->getOpcode()) {
312 //===-----------------------------------------------------------------===//
313 // Handle PHI nodes...
315 case Instruction::PHINode: {
316 PHINode *PN = (PHINode*)I;
317 unsigned NumValues = PN->getNumIncomingValues(), i;
318 InstVal *OperandIV = 0;
320 // Look at all of the executable operands of the PHI node. If any of them
321 // are overdefined, the PHI becomes overdefined as well. If they are all
322 // constant, and they agree with each other, the PHI becomes the identical
323 // constant. If they are constant and don't agree, the PHI is overdefined.
324 // If there are no executable operands, the PHI remains undefined.
326 for (i = 0; i < NumValues; ++i) {
327 if (BBExecutable.count(PN->getIncomingBlock(i))) {
328 InstVal &IV = getValueState(PN->getIncomingValue(i));
329 if (IV.isUndefined()) continue; // Doesn't influence PHI node.
330 if (IV.isOverdefined()) { // PHI node becomes overdefined!
335 if (OperandIV == 0) { // Grab the first value...
337 } else { // Another value is being merged in!
338 // There is already a reachable operand. If we conflict with it,
339 // then the PHI node becomes overdefined. If we agree with it, we
342 // Check to see if there are two different constants merging...
343 if (!IV.getConstant()->equals(OperandIV->getConstant())) {
344 // Yes there is. This means the PHI node is not constant.
345 // You must be overdefined poor PHI.
347 markOverdefined(I); // The PHI node now becomes overdefined
348 return; // I'm done analyzing you
354 // If we exited the loop, this means that the PHI node only has constant
355 // arguments that agree with each other(and OperandIV is a pointer to one
356 // of their InstVal's) or OperandIV is null because there are no defined
357 // incoming arguments. If this is the case, the PHI remains undefined.
360 assert(OperandIV->isConstant() && "Should only be here for constants!");
361 markConstant(I, OperandIV->getConstant()); // Aquire operand value
366 //===-----------------------------------------------------------------===//
367 // Handle instructions that unconditionally provide overdefined values...
369 case Instruction::Malloc:
370 case Instruction::Free:
371 case Instruction::Alloca:
372 case Instruction::Load:
373 case Instruction::Store:
374 // TODO: getfield/putfield?
375 case Instruction::Call:
376 markOverdefined(I); // Memory and call's are all overdefined
379 //===-----------------------------------------------------------------===//
380 // Handle Terminator instructions...
382 case Instruction::Ret: return; // Method return doesn't affect anything
383 case Instruction::Br: { // Handle conditional branches...
384 BranchInst *BI = (BranchInst*)I;
385 if (BI->isUnconditional())
386 return; // Unconditional branches are already handled!
388 InstVal &BCValue = getValueState(BI->getCondition());
389 if (BCValue.isOverdefined()) {
390 // Overdefined condition variables mean the branch could go either way.
391 markExecutable(BI->getSuccessor(0));
392 markExecutable(BI->getSuccessor(1));
393 } else if (BCValue.isConstant()) {
394 // Constant condition variables mean the branch can only go a single way.
395 ConstPoolBool *CPB = (ConstPoolBool*)BCValue.getConstant();
396 if (CPB->getValue()) // If the branch condition is TRUE...
397 markExecutable(BI->getSuccessor(0));
398 else // Else if the br cond is FALSE...
399 markExecutable(BI->getSuccessor(1));
404 case Instruction::Switch: {
405 SwitchInst *SI = (SwitchInst*)I;
406 InstVal &SCValue = getValueState(SI->getCondition());
407 if (SCValue.isOverdefined()) { // Overdefined condition? All dests are exe
408 for(unsigned i = 0; BasicBlock *Succ = SI->getSuccessor(i); ++i)
409 markExecutable(Succ);
410 } else if (SCValue.isConstant()) {
411 ConstPoolVal *CPV = SCValue.getConstant();
412 // Make sure to skip the "default value" which isn't a value
413 for (unsigned i = 1, E = SI->getNumSuccessors(); i != E; ++i) {
414 if (SI->getSuccessorValue(i)->equals(CPV)) {// Found the right branch...
415 markExecutable(SI->getSuccessor(i));
420 // Constant value not equal to any of the branches... must execute
421 // default branch then...
422 markExecutable(SI->getDefaultDest());
427 default: break; // Handle math operators as groups.
428 } // end switch(I->getOpcode())
431 //===-------------------------------------------------------------------===//
432 // Handle Unary instructions...
433 // Also treated as unary here, are cast instructions and getelementptr
434 // instructions on struct* operands.
436 if (I->isUnaryOp() || I->getOpcode() == Instruction::Cast ||
437 (I->getOpcode() == Instruction::GetElementPtr &&
438 ((GetElementPtrInst*)I)->isStructSelector())) {
440 Value *V = I->getOperand(0);
441 InstVal &VState = getValueState(V);
442 if (VState.isOverdefined()) { // Inherit overdefinedness of operand
444 } else if (VState.isConstant()) { // Propogate constant value
445 ConstPoolVal *Result =
446 opt::ConstantFoldUnaryInstruction(I->getOpcode(),
447 VState.getConstant());
450 // This instruction constant folds! The only problem is that the value
451 // returned is newly allocated. Make sure to stick it into the methods
453 M->getConstantPool().insert(Result);
454 markConstant(I, Result);
456 markOverdefined(I); // Don't know how to fold this instruction. :(
462 //===-----------------------------------------------------------------===//
463 // Handle Binary instructions...
465 if (I->isBinaryOp() || I->getOpcode() == Instruction::Shl ||
466 I->getOpcode() == Instruction::Shr) {
467 Value *V1 = I->getOperand(0);
468 Value *V2 = I->getOperand(1);
470 InstVal &V1State = getValueState(V1);
471 InstVal &V2State = getValueState(V2);
472 if (V1State.isOverdefined() || V2State.isOverdefined()) {
474 } else if (V1State.isConstant() && V2State.isConstant()) {
475 ConstPoolVal *Result =
476 opt::ConstantFoldBinaryInstruction(I->getOpcode(),
477 V1State.getConstant(),
478 V2State.getConstant());
480 // This instruction constant folds! The only problem is that the value
481 // returned is newly allocated. Make sure to stick it into the methods
483 M->getConstantPool().insert(Result);
484 markConstant(I, Result);
486 markOverdefined(I); // Don't know how to fold this instruction. :(
492 // Shouldn't get here... either the switch statement or one of the group
493 // handlers should have kicked in...
495 cerr << "SCCP: Don't know how to handle: " << I;
496 markOverdefined(I); // Just in case
501 // OperandChangedState - This method is invoked on all of the users of an
502 // instruction that was just changed state somehow.... Based on this
503 // information, we need to update the specified user of this instruction.
505 void SCCP::OperandChangedState(User *U) {
506 // Only instructions use other variable values!
507 Instruction *I = U->castInstructionAsserting();
508 if (!BBExecutable.count(I->getParent())) return; // Inst not executable yet!
510 UpdateInstruction(I);
514 // DoSparseConditionalConstantProp - Use Sparse Conditional Constant Propogation
515 // to prove whether a value is constant and whether blocks are used.
517 bool opt::DoSCCP(Method *M) {
518 if (M->isExternal()) return false;