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/ConstantProp.h"
19 #include "llvm/Transforms/Scalar/ConstantHandling.h"
20 #include "llvm/Method.h"
21 #include "llvm/BasicBlock.h"
22 #include "llvm/ConstantVals.h"
23 #include "llvm/InstrTypes.h"
24 #include "llvm/iPHINode.h"
25 #include "llvm/iMemory.h"
26 #include "llvm/iTerminators.h"
27 #include "llvm/iOther.h"
28 #include "llvm/Assembly/Writer.h"
29 #include "Support/STLExtras.h"
36 // InstVal class - This class represents the different lattice values that an
37 // instruction may occupy. It is a simple class with value semantics. The
38 // potential constant value that is pointed to is owned by the constant pool
39 // for the method being optimized.
43 undefined, // This instruction has no known value
44 constant, // This instruction has a constant value
45 // Range, // This instruction is known to fall within a range
46 overdefined // This instruction has an unknown value
47 } LatticeValue; // The current lattice position
48 Constant *ConstantVal; // If Constant value, the current value
50 inline InstVal() : LatticeValue(undefined), ConstantVal(0) {}
52 // markOverdefined - Return true if this is a new status to be in...
53 inline bool markOverdefined() {
54 if (LatticeValue != overdefined) {
55 LatticeValue = overdefined;
61 // markConstant - Return true if this is a new status for us...
62 inline bool markConstant(Constant *V) {
63 if (LatticeValue != constant) {
64 LatticeValue = constant;
68 assert(ConstantVal == V && "Marking constant with different value");
73 inline bool isUndefined() const { return LatticeValue == undefined; }
74 inline bool isConstant() const { return LatticeValue == constant; }
75 inline bool isOverdefined() const { return LatticeValue == overdefined; }
77 inline Constant *getConstant() const { return ConstantVal; }
82 //===----------------------------------------------------------------------===//
85 // This class does all of the work of Sparse Conditional Constant Propogation.
86 // It's public interface consists of a constructor and a doSCCP() method.
89 Method *M; // The method that we are working on...
91 std::set<BasicBlock*> BBExecutable;// The basic blocks that are executable
92 std::map<Value*, InstVal> ValueState; // The state each value is in...
94 std::vector<Instruction*> InstWorkList;// The instruction work list
95 std::vector<BasicBlock*> BBWorkList; // The BasicBlock work list
97 //===--------------------------------------------------------------------===//
98 // The public interface for this class
102 // SCCP Ctor - Save the method to operate on...
103 inline SCCP(Method *m) : M(m) {}
105 // doSCCP() - Run the Sparse Conditional Constant Propogation algorithm, and
106 // return true if the method was modified.
109 //===--------------------------------------------------------------------===//
110 // The implementation of this class
114 // markValueOverdefined - Make a value be marked as "constant". If the value
115 // is not already a constant, add it to the instruction work list so that
116 // the users of the instruction are updated later.
118 inline bool markConstant(Instruction *I, Constant *V) {
119 //cerr << "markConstant: " << V << " = " << I;
120 if (ValueState[I].markConstant(V)) {
121 InstWorkList.push_back(I);
127 // markValueOverdefined - Make a value be marked as "overdefined". If the
128 // value is not already overdefined, add it to the instruction work list so
129 // that the users of the instruction are updated later.
131 inline bool markOverdefined(Value *V) {
132 if (ValueState[V].markOverdefined()) {
133 if (Instruction *I = dyn_cast<Instruction>(V)) {
134 //cerr << "markOverdefined: " << V;
135 InstWorkList.push_back(I); // Only instructions go on the work list
142 // getValueState - Return the InstVal object that corresponds to the value.
143 // This function is neccesary because not all values should start out in the
144 // underdefined state... MethodArgument's should be overdefined, and constants
145 // should be marked as constants. If a value is not known to be an
146 // Instruction object, then use this accessor to get its value from the map.
148 inline InstVal &getValueState(Value *V) {
149 std::map<Value*, InstVal>::iterator I = ValueState.find(V);
150 if (I != ValueState.end()) return I->second; // Common case, in the map
152 if (Constant *CPV = dyn_cast<Constant>(V)) { // Constants are constant
153 ValueState[CPV].markConstant(CPV);
154 } else if (isa<MethodArgument>(V)) { // MethodArgs are overdefined
155 ValueState[V].markOverdefined();
157 // All others are underdefined by default...
158 return ValueState[V];
161 // markExecutable - Mark a basic block as executable, adding it to the BB
162 // work list if it is not already executable...
164 void markExecutable(BasicBlock *BB) {
165 if (BBExecutable.count(BB)) return;
166 //cerr << "Marking BB Executable: " << BB;
167 BBExecutable.insert(BB); // Basic block is executable!
168 BBWorkList.push_back(BB); // Add the block to the work list!
172 // UpdateInstruction - Something changed in this instruction... Either an
173 // operand made a transition, or the instruction is newly executable. Change
174 // the value type of I to reflect these changes if appropriate.
176 void UpdateInstruction(Instruction *I);
178 // OperandChangedState - This method is invoked on all of the users of an
179 // instruction that was just changed state somehow.... Based on this
180 // information, we need to update the specified user of this instruction.
182 void OperandChangedState(User *U);
186 //===----------------------------------------------------------------------===//
187 // SCCP Class Implementation
190 // doSCCP() - Run the Sparse Conditional Constant Propogation algorithm, and
191 // return true if the method was modified.
193 bool SCCP::doSCCP() {
194 // Mark the first block of the method as being executable...
195 markExecutable(M->front());
197 // Process the work lists until their are empty!
198 while (!BBWorkList.empty() || !InstWorkList.empty()) {
199 // Process the instruction work list...
200 while (!InstWorkList.empty()) {
201 Instruction *I = InstWorkList.back();
202 InstWorkList.pop_back();
204 //cerr << "\nPopped off I-WL: " << I;
207 // "I" got into the work list because it either made the transition from
208 // bottom to constant, or to Overdefined.
210 // Update all of the users of this instruction's value...
212 for_each(I->use_begin(), I->use_end(),
213 bind_obj(this, &SCCP::OperandChangedState));
216 // Process the basic block work list...
217 while (!BBWorkList.empty()) {
218 BasicBlock *BB = BBWorkList.back();
219 BBWorkList.pop_back();
221 //cerr << "\nPopped off BBWL: " << BB;
223 // If this block only has a single successor, mark it as executable as
224 // well... if not, terminate the do loop.
226 if (BB->getTerminator()->getNumSuccessors() == 1)
227 markExecutable(BB->getTerminator()->getSuccessor(0));
229 // Loop over all of the instructions and notify them that they are newly
231 for_each(BB->begin(), BB->end(),
232 bind_obj(this, &SCCP::UpdateInstruction));
237 for (Method::iterator BBI = M->begin(), BBEnd = M->end(); BBI != BBEnd; ++BBI)
238 if (!BBExecutable.count(*BBI))
239 cerr << "BasicBlock Dead:" << *BBI;
243 // Iterate over all of the instructions in a method, replacing them with
244 // constants if we have found them to be of constant values.
246 bool MadeChanges = false;
247 for (Method::inst_iterator II = M->inst_begin(); II != M->inst_end(); ) {
248 Instruction *Inst = *II;
249 InstVal &IV = ValueState[Inst];
250 if (IV.isConstant()) {
251 Constant *Const = IV.getConstant();
252 // cerr << "Constant: " << Inst << " is: " << Const;
254 // Replaces all of the uses of a variable with uses of the constant.
255 Inst->replaceAllUsesWith(Const);
257 // Remove the operator from the list of definitions...
258 Inst->getParent()->getInstList().remove(II.getInstructionIterator());
260 // The new constant inherits the old name of the operator...
261 if (Inst->hasName() && !Const->hasName())
262 Const->setName(Inst->getName(), M->getSymbolTableSure());
264 // Delete the operator now...
267 // Incrementing the iterator in an unchecked manner could mess up the
268 // internals of 'II'. To make sure everything is happy, tell it we might
270 II.resyncInstructionIterator();
272 // Hey, we just changed something!
274 continue; // Skip the ++II at the end of the loop here...
275 } else if (Inst->isTerminator()) {
276 MadeChanges |= ConstantFoldTerminator(cast<TerminatorInst>(Inst));
282 // Merge identical constants last: this is important because we may have just
283 // introduced constants that already exist, and we don't want to pollute later
284 // stages with extraneous constants.
290 // UpdateInstruction - Something changed in this instruction... Either an
291 // operand made a transition, or the instruction is newly executable. Change
292 // the value type of I to reflect these changes if appropriate. This method
293 // makes sure to do the following actions:
295 // 1. If a phi node merges two constants in, and has conflicting value coming
296 // from different branches, or if the PHI node merges in an overdefined
297 // value, then the PHI node becomes overdefined.
298 // 2. If a phi node merges only constants in, and they all agree on value, the
299 // PHI node becomes a constant value equal to that.
300 // 3. If V <- x (op) y && isConstant(x) && isConstant(y) V = Constant
301 // 4. If V <- x (op) y && (isOverdefined(x) || isOverdefined(y)) V = Overdefined
302 // 5. If V <- MEM or V <- CALL or V <- (unknown) then V = Overdefined
303 // 6. If a conditional branch has a value that is constant, make the selected
304 // destination executable
305 // 7. If a conditional branch has a value that is overdefined, make all
306 // successors executable.
308 void SCCP::UpdateInstruction(Instruction *I) {
309 InstVal &IValue = ValueState[I];
310 if (IValue.isOverdefined())
311 return; // If already overdefined, we aren't going to effect anything
313 switch (I->getOpcode()) {
314 //===-----------------------------------------------------------------===//
315 // Handle PHI nodes...
317 case Instruction::PHINode: {
318 PHINode *PN = cast<PHINode>(I);
319 unsigned NumValues = PN->getNumIncomingValues(), i;
320 InstVal *OperandIV = 0;
322 // Look at all of the executable operands of the PHI node. If any of them
323 // are overdefined, the PHI becomes overdefined as well. If they are all
324 // constant, and they agree with each other, the PHI becomes the identical
325 // constant. If they are constant and don't agree, the PHI is overdefined.
326 // If there are no executable operands, the PHI remains undefined.
328 for (i = 0; i < NumValues; ++i) {
329 if (BBExecutable.count(PN->getIncomingBlock(i))) {
330 InstVal &IV = getValueState(PN->getIncomingValue(i));
331 if (IV.isUndefined()) continue; // Doesn't influence PHI node.
332 if (IV.isOverdefined()) { // PHI node becomes overdefined!
337 if (OperandIV == 0) { // Grab the first value...
339 } else { // Another value is being merged in!
340 // There is already a reachable operand. If we conflict with it,
341 // then the PHI node becomes overdefined. If we agree with it, we
344 // Check to see if there are two different constants merging...
345 if (IV.getConstant() != OperandIV->getConstant()) {
346 // Yes there is. This means the PHI node is not constant.
347 // You must be overdefined poor PHI.
349 markOverdefined(I); // The PHI node now becomes overdefined
350 return; // I'm done analyzing you
356 // If we exited the loop, this means that the PHI node only has constant
357 // arguments that agree with each other(and OperandIV is a pointer to one
358 // of their InstVal's) or OperandIV is null because there are no defined
359 // incoming arguments. If this is the case, the PHI remains undefined.
362 assert(OperandIV->isConstant() && "Should only be here for constants!");
363 markConstant(I, OperandIV->getConstant()); // Aquire operand value
368 //===-----------------------------------------------------------------===//
369 // Handle instructions that unconditionally provide overdefined values...
371 case Instruction::Malloc:
372 case Instruction::Free:
373 case Instruction::Alloca:
374 case Instruction::Load:
375 case Instruction::Store:
377 case Instruction::Call:
378 case Instruction::Invoke:
379 markOverdefined(I); // Memory and call's are all overdefined
382 //===-----------------------------------------------------------------===//
383 // Handle Terminator instructions...
385 case Instruction::Ret: return; // Method return doesn't affect anything
386 case Instruction::Br: { // Handle conditional branches...
387 BranchInst *BI = cast<BranchInst>(I);
388 if (BI->isUnconditional())
389 return; // Unconditional branches are already handled!
391 InstVal &BCValue = getValueState(BI->getCondition());
392 if (BCValue.isOverdefined()) {
393 // Overdefined condition variables mean the branch could go either way.
394 markExecutable(BI->getSuccessor(0));
395 markExecutable(BI->getSuccessor(1));
396 } else if (BCValue.isConstant()) {
397 // Constant condition variables mean the branch can only go a single way.
398 ConstantBool *CPB = cast<ConstantBool>(BCValue.getConstant());
399 if (CPB->getValue()) // If the branch condition is TRUE...
400 markExecutable(BI->getSuccessor(0));
401 else // Else if the br cond is FALSE...
402 markExecutable(BI->getSuccessor(1));
407 case Instruction::Switch: {
408 SwitchInst *SI = cast<SwitchInst>(I);
409 InstVal &SCValue = getValueState(SI->getCondition());
410 if (SCValue.isOverdefined()) { // Overdefined condition? All dests are exe
411 for(unsigned i = 0; BasicBlock *Succ = SI->getSuccessor(i); ++i)
412 markExecutable(Succ);
413 } else if (SCValue.isConstant()) {
414 Constant *CPV = SCValue.getConstant();
415 // Make sure to skip the "default value" which isn't a value
416 for (unsigned i = 1, E = SI->getNumSuccessors(); i != E; ++i) {
417 if (SI->getSuccessorValue(i) == CPV) {// Found the right branch...
418 markExecutable(SI->getSuccessor(i));
423 // Constant value not equal to any of the branches... must execute
424 // default branch then...
425 markExecutable(SI->getDefaultDest());
430 default: break; // Handle math operators as groups.
431 } // end switch(I->getOpcode())
434 //===-------------------------------------------------------------------===//
435 // Handle Unary instructions...
436 // Also treated as unary here, are cast instructions and getelementptr
437 // instructions on struct* operands.
439 if (isa<UnaryOperator>(I) || isa<CastInst>(I) ||
440 (isa<GetElementPtrInst>(I) &&
441 cast<GetElementPtrInst>(I)->isStructSelector())) {
443 Value *V = I->getOperand(0);
444 InstVal &VState = getValueState(V);
445 if (VState.isOverdefined()) { // Inherit overdefinedness of operand
447 } else if (VState.isConstant()) { // Propogate constant value
448 Constant *Result = isa<CastInst>(I)
449 ? ConstantFoldCastInstruction(VState.getConstant(), I->getType())
450 : ConstantFoldUnaryInstruction(I->getOpcode(), VState.getConstant());
453 // This instruction constant folds!
454 markConstant(I, Result);
456 markOverdefined(I); // Don't know how to fold this instruction. :(
462 //===-----------------------------------------------------------------===//
463 // Handle Binary instructions...
465 if (isa<BinaryOperator>(I) || isa<ShiftInst>(I)) {
466 Value *V1 = I->getOperand(0);
467 Value *V2 = I->getOperand(1);
469 InstVal &V1State = getValueState(V1);
470 InstVal &V2State = getValueState(V2);
471 if (V1State.isOverdefined() || V2State.isOverdefined()) {
473 } else if (V1State.isConstant() && V2State.isConstant()) {
475 ConstantFoldBinaryInstruction(I->getOpcode(),
476 V1State.getConstant(),
477 V2State.getConstant());
479 // This instruction constant folds!
480 markConstant(I, Result);
482 markOverdefined(I); // Don't know how to fold this instruction. :(
488 // Shouldn't get here... either the switch statement or one of the group
489 // handlers should have kicked in...
491 cerr << "SCCP: Don't know how to handle: " << I;
492 markOverdefined(I); // Just in case
497 // OperandChangedState - This method is invoked on all of the users of an
498 // instruction that was just changed state somehow.... Based on this
499 // information, we need to update the specified user of this instruction.
501 void SCCP::OperandChangedState(User *U) {
502 // Only instructions use other variable values!
503 Instruction *I = cast<Instruction>(U);
504 if (!BBExecutable.count(I->getParent())) return; // Inst not executable yet!
506 UpdateInstruction(I);
510 // DoSparseConditionalConstantProp - Use Sparse Conditional Constant Propogation
511 // to prove whether a value is constant and whether blocks are used.
513 bool SCCPPass::doSCCP(Method *M) {
514 if (M->isExternal()) return false;