SCCP.cpp revision a41f50dea8573e4a610b5aa5e45b5c368559b084
1//===- SCCP.cpp - Sparse Conditional Constant Propogation -----------------===// 2// 3// This file implements sparse conditional constant propogation and merging: 4// 5// Specifically, this: 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 11// 12// Notice that: 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. 15// 16//===----------------------------------------------------------------------===// 17 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/iTerminators.h" 27#include "llvm/Tools/STLExtras.h" 28//#include "llvm/Assembly/Writer.h" 29#include <algorithm> 30#include <map> 31#include <set> 32 33// InstVal class - This class represents the different lattice values that an 34// instruction may occupy. It is a simple class with value semantics. The 35// potential constant value that is pointed to is owned by the constant pool 36// for the method being optimized. 37// 38class InstVal { 39 enum { 40 Undefined, // This instruction has no known value 41 Constant, // This instruction has a constant value 42 // Range, // This instruction is known to fall within a range 43 Overdefined // This instruction has an unknown value 44 } LatticeValue; // The current lattice position 45 ConstPoolVal *ConstantVal; // If Constant value, the current value 46public: 47 inline InstVal() : LatticeValue(Undefined), ConstantVal(0) {} 48 49 // markOverdefined - Return true if this is a new status to be in... 50 inline bool markOverdefined() { 51 if (LatticeValue != Overdefined) { 52 LatticeValue = Overdefined; 53 return true; 54 } 55 return false; 56 } 57 58 // markConstant - Return true if this is a new status for us... 59 inline bool markConstant(ConstPoolVal *V) { 60 if (LatticeValue != Constant) { 61 LatticeValue = Constant; 62 ConstantVal = V; 63 return true; 64 } else { 65 assert(ConstantVal->equals(V) && "Marking constant with different value"); 66 } 67 return false; 68 } 69 70 inline bool isUndefined() const { return LatticeValue == Undefined; } 71 inline bool isConstant() const { return LatticeValue == Constant; } 72 inline bool isOverdefined() const { return LatticeValue == Overdefined; } 73 74 inline ConstPoolVal *getConstant() const { return ConstantVal; } 75}; 76 77 78 79//===----------------------------------------------------------------------===// 80// SCCP Class 81// 82// This class does all of the work of Sparse Conditional Constant Propogation. 83// It's public interface consists of a constructor and a doSCCP() method. 84// 85class SCCP { 86 Method *M; // The method that we are working on... 87 88 set<BasicBlock*> BBExecutable; // The basic blocks that are executable 89 map<Value*, InstVal> ValueState; // The state each value is in... 90 91 vector<Instruction*> InstWorkList; // The instruction work list 92 vector<BasicBlock*> BBWorkList; // The BasicBlock work list 93 94 //===--------------------------------------------------------------------===// 95 // The public interface for this class 96 // 97public: 98 99 // SCCP Ctor - Save the method to operate on... 100 inline SCCP(Method *m) : M(m) {} 101 102 // doSCCP() - Run the Sparse Conditional Constant Propogation algorithm, and 103 // return true if the method was modified. 104 bool doSCCP(); 105 106 //===--------------------------------------------------------------------===// 107 // The implementation of this class 108 // 109private: 110 111 // markValueOverdefined - Make a value be marked as "constant". If the value 112 // is not already a constant, add it to the instruction work list so that 113 // the users of the instruction are updated later. 114 // 115 inline bool markConstant(Instruction *I, ConstPoolVal *V) { 116 //cerr << "markConstant: " << V << " = " << I; 117 if (ValueState[I].markConstant(V)) { 118 InstWorkList.push_back(I); 119 return true; 120 } 121 return false; 122 } 123 124 // markValueOverdefined - Make a value be marked as "overdefined". If the 125 // value is not already overdefined, add it to the instruction work list so 126 // that the users of the instruction are updated later. 127 // 128 inline bool markOverdefined(Value *V) { 129 if (ValueState[V].markOverdefined()) { 130 if (Instruction *I = V->castInstruction()) { 131 //cerr << "markOverdefined: " << V; 132 InstWorkList.push_back(I); // Only instructions go on the work list 133 } 134 return true; 135 } 136 return false; 137 } 138 139 // getValueState - Return the InstVal object that corresponds to the value. 140 // This function is neccesary because not all values should start out in the 141 // underdefined state... MethodArgument's should be overdefined, and constants 142 // should be marked as constants. If a value is not known to be an 143 // Instruction object, then use this accessor to get its value from the map. 144 // 145 inline InstVal &getValueState(Value *V) { 146 map<Value*, InstVal>::iterator I = ValueState.find(V); 147 if (I != ValueState.end()) return I->second; // Common case, in the map 148 149 if (ConstPoolVal *CPV = V->castConstant()) { // Constants are constant 150 ValueState[CPV].markConstant(CPV); 151 } else if (V->isMethodArgument()) { // MethodArgs are overdefined 152 ValueState[V].markOverdefined(); 153 } 154 // All others are underdefined by default... 155 return ValueState[V]; 156 } 157 158 // markExecutable - Mark a basic block as executable, adding it to the BB 159 // work list if it is not already executable... 160 // 161 void markExecutable(BasicBlock *BB) { 162 if (BBExecutable.count(BB)) return; 163 //cerr << "Marking BB Executable: " << BB; 164 BBExecutable.insert(BB); // Basic block is executable! 165 BBWorkList.push_back(BB); // Add the block to the work list! 166 } 167 168 169 // UpdateInstruction - Something changed in this instruction... Either an 170 // operand made a transition, or the instruction is newly executable. Change 171 // the value type of I to reflect these changes if appropriate. 172 // 173 void UpdateInstruction(Instruction *I); 174 175 // OperandChangedState - This method is invoked on all of the users of an 176 // instruction that was just changed state somehow.... Based on this 177 // information, we need to update the specified user of this instruction. 178 // 179 void OperandChangedState(User *U); 180}; 181 182 183//===----------------------------------------------------------------------===// 184// SCCP Class Implementation 185 186 187// doSCCP() - Run the Sparse Conditional Constant Propogation algorithm, and 188// return true if the method was modified. 189// 190bool SCCP::doSCCP() { 191 // Mark the first block of the method as being executable... 192 markExecutable(M->front()); 193 194 // Process the work lists until their are empty! 195 while (!BBWorkList.empty() || !InstWorkList.empty()) { 196 // Process the instruction work list... 197 while (!InstWorkList.empty()) { 198 Instruction *I = InstWorkList.back(); 199 InstWorkList.pop_back(); 200 201 //cerr << "\nPopped off I-WL: " << I; 202 203 204 // "I" got into the work list because it either made the transition from 205 // bottom to constant, or to Overdefined. 206 // 207 // Update all of the users of this instruction's value... 208 // 209 for_each(I->use_begin(), I->use_end(), 210 bind_obj(this, &SCCP::OperandChangedState)); 211 } 212 213 // Process the basic block work list... 214 while (!BBWorkList.empty()) { 215 BasicBlock *BB = BBWorkList.back(); 216 BBWorkList.pop_back(); 217 218 //cerr << "\nPopped off BBWL: " << BB; 219 220 // If this block only has a single successor, mark it as executable as 221 // well... if not, terminate the do loop. 222 // 223 if (BB->getTerminator()->getNumSuccessors() == 1) 224 markExecutable(BB->getTerminator()->getSuccessor(0)); 225 226 // Loop over all of the instructions and notify them that they are newly 227 // executable... 228 for_each(BB->begin(), BB->end(), 229 bind_obj(this, &SCCP::UpdateInstruction)); 230 } 231 } 232 233#if 0 234 for (Method::iterator BBI = M->begin(), BBEnd = M->end(); BBI != BBEnd; ++BBI) 235 if (!BBExecutable.count(*BBI)) 236 cerr << "BasicBlock Dead:" << *BBI; 237#endif 238 239 240 // Iterate over all of the instructions in a method, replacing them with 241 // constants if we have found them to be of constant values. 242 // 243 bool MadeChanges = false; 244 for (Method::inst_iterator II = M->inst_begin(); II != M->inst_end(); ) { 245 Instruction *Inst = *II; 246 InstVal &IV = ValueState[Inst]; 247 if (IV.isConstant()) { 248 ConstPoolVal *Const = IV.getConstant(); 249 // cerr << "Constant: " << Inst << " is: " << Const; 250 251 // Replaces all of the uses of a variable with uses of the constant. 252 Inst->replaceAllUsesWith(Const); 253 254 // Remove the operator from the list of definitions... 255 Inst->getParent()->getInstList().remove(II.getInstructionIterator()); 256 257 // The new constant inherits the old name of the operator... 258 if (Inst->hasName() && !Const->hasName()) 259 Const->setName(Inst->getName()); 260 261 // Delete the operator now... 262 delete Inst; 263 264 // Incrementing the iterator in an unchecked manner could mess up the 265 // internals of 'II'. To make sure everything is happy, tell it we might 266 // have broken it. 267 II.resyncInstructionIterator(); 268 269 // Hey, we just changed something! 270 MadeChanges = true; 271 continue; // Skip the ++II at the end of the loop here... 272 } else if (Inst->isTerminator()) { 273 MadeChanges |= opt::ConstantFoldTerminator((TerminatorInst*)Inst); 274 } 275 276 ++II; 277 } 278 279 // Merge identical constants last: this is important because we may have just 280 // introduced constants that already exist, and we don't want to pollute later 281 // stages with extraneous constants. 282 // 283 return MadeChanges | opt::DoConstantPoolMerging(M->getConstantPool()); 284} 285 286 287// UpdateInstruction - Something changed in this instruction... Either an 288// operand made a transition, or the instruction is newly executable. Change 289// the value type of I to reflect these changes if appropriate. This method 290// makes sure to do the following actions: 291// 292// 1. If a phi node merges two constants in, and has conflicting value coming 293// from different branches, or if the PHI node merges in an overdefined 294// value, then the PHI node becomes overdefined. 295// 2. If a phi node merges only constants in, and they all agree on value, the 296// PHI node becomes a constant value equal to that. 297// 3. If V <- x (op) y && isConstant(x) && isConstant(y) V = Constant 298// 4. If V <- x (op) y && (isOverdefined(x) || isOverdefined(y)) V = Overdefined 299// 5. If V <- MEM or V <- CALL or V <- (unknown) then V = Overdefined 300// 6. If a conditional branch has a value that is constant, make the selected 301// destination executable 302// 7. If a conditional branch has a value that is overdefined, make all 303// successors executable. 304// 305void SCCP::UpdateInstruction(Instruction *I) { 306 InstVal &IValue = ValueState[I]; 307 if (IValue.isOverdefined()) 308 return; // If already overdefined, we aren't going to effect anything 309 310 switch (I->getOpcode()) { 311 //===-----------------------------------------------------------------===// 312 // Handle PHI nodes... 313 // 314 case Instruction::PHINode: { 315 PHINode *PN = (PHINode*)I; 316 unsigned NumValues = PN->getNumIncomingValues(), i; 317 InstVal *OperandIV = 0; 318 319 // Look at all of the executable operands of the PHI node. If any of them 320 // are overdefined, the PHI becomes overdefined as well. If they are all 321 // constant, and they agree with each other, the PHI becomes the identical 322 // constant. If they are constant and don't agree, the PHI is overdefined. 323 // If there are no executable operands, the PHI remains undefined. 324 // 325 for (i = 0; i < NumValues; ++i) { 326 if (BBExecutable.count(PN->getIncomingBlock(i))) { 327 InstVal &IV = getValueState(PN->getIncomingValue(i)); 328 if (IV.isUndefined()) continue; // Doesn't influence PHI node. 329 if (IV.isOverdefined()) { // PHI node becomes overdefined! 330 markOverdefined(PN); 331 return; 332 } 333 334 if (OperandIV == 0) { // Grab the first value... 335 OperandIV = &IV; 336 } else { // Another value is being merged in! 337 // There is already a reachable operand. If we conflict with it, 338 // then the PHI node becomes overdefined. If we agree with it, we 339 // can continue on. 340 341 // Check to see if there are two different constants merging... 342 if (!IV.getConstant()->equals(OperandIV->getConstant())) { 343 // Yes there is. This means the PHI node is not constant. 344 // You must be overdefined poor PHI. 345 // 346 markOverdefined(I); // The PHI node now becomes overdefined 347 return; // I'm done analyzing you 348 } 349 } 350 } 351 } 352 353 // If we exited the loop, this means that the PHI node only has constant 354 // arguments that agree with each other(and OperandIV is a pointer to one 355 // of their InstVal's) or OperandIV is null because there are no defined 356 // incoming arguments. If this is the case, the PHI remains undefined. 357 // 358 if (OperandIV) { 359 assert(OperandIV->isConstant() && "Should only be here for constants!"); 360 markConstant(I, OperandIV->getConstant()); // Aquire operand value 361 } 362 return; 363 } 364 365 //===-----------------------------------------------------------------===// 366 // Handle instructions that unconditionally provide overdefined values... 367 // 368 case Instruction::Malloc: 369 case Instruction::Free: 370 case Instruction::Alloca: 371 case Instruction::Load: 372 case Instruction::Store: 373 // TODO: getfield/putfield? 374 case Instruction::Call: 375 markOverdefined(I); // Memory and call's are all overdefined 376 return; 377 378 //===-----------------------------------------------------------------===// 379 // Handle Terminator instructions... 380 // 381 case Instruction::Ret: return; // Method return doesn't affect anything 382 case Instruction::Br: { // Handle conditional branches... 383 BranchInst *BI = (BranchInst*)I; 384 if (BI->isUnconditional()) 385 return; // Unconditional branches are already handled! 386 387 InstVal &BCValue = getValueState(BI->getCondition()); 388 if (BCValue.isOverdefined()) { 389 // Overdefined condition variables mean the branch could go either way. 390 markExecutable(BI->getSuccessor(0)); 391 markExecutable(BI->getSuccessor(1)); 392 } else if (BCValue.isConstant()) { 393 // Constant condition variables mean the branch can only go a single way. 394 ConstPoolBool *CPB = (ConstPoolBool*)BCValue.getConstant(); 395 if (CPB->getValue()) // If the branch condition is TRUE... 396 markExecutable(BI->getSuccessor(0)); 397 else // Else if the br cond is FALSE... 398 markExecutable(BI->getSuccessor(1)); 399 } 400 return; 401 } 402 403 case Instruction::Switch: { 404 SwitchInst *SI = (SwitchInst*)I; 405 InstVal &SCValue = getValueState(SI->getCondition()); 406 if (SCValue.isOverdefined()) { // Overdefined condition? All dests are exe 407 for(unsigned i = 0; BasicBlock *Succ = SI->getSuccessor(i); ++i) 408 markExecutable(Succ); 409 } else if (SCValue.isConstant()) { 410 ConstPoolVal *CPV = SCValue.getConstant(); 411 // Make sure to skip the "default value" which isn't a value 412 for (unsigned i = 1, E = SI->getNumSuccessors(); i != E; ++i) { 413 if (SI->getSuccessorValue(i)->equals(CPV)) {// Found the right branch... 414 markExecutable(SI->getSuccessor(i)); 415 return; 416 } 417 } 418 419 // Constant value not equal to any of the branches... must execute 420 // default branch then... 421 markExecutable(SI->getDefaultDest()); 422 } 423 return; 424 } 425 426 default: break; // Handle math operators as groups. 427 } // end switch(I->getOpcode()) 428 429 430 //===-------------------------------------------------------------------===// 431 // Handle Unary instructions... 432 // 433 if (I->isUnaryOp()) { 434 Value *V = I->getOperand(0); 435 InstVal &VState = getValueState(V); 436 if (VState.isOverdefined()) { // Inherit overdefinedness of operand 437 markOverdefined(I); 438 } else if (VState.isConstant()) { // Propogate constant value 439 ConstPoolVal *Result = 440 opt::ConstantFoldUnaryInstruction(I->getOpcode(), 441 VState.getConstant()); 442 443 if (Result) { 444 // This instruction constant folds! The only problem is that the value 445 // returned is newly allocated. Make sure to stick it into the methods 446 // constant pool... 447 M->getConstantPool().insert(Result); 448 markConstant(I, Result); 449 } else { 450 markOverdefined(I); // Don't know how to fold this instruction. :( 451 } 452 } 453 return; 454 } 455 456 //===-----------------------------------------------------------------===// 457 // Handle Binary instructions... 458 // 459 if (I->isBinaryOp()) { 460 Value *V1 = I->getOperand(0); 461 Value *V2 = I->getOperand(1); 462 463 InstVal &V1State = getValueState(V1); 464 InstVal &V2State = getValueState(V2); 465 if (V1State.isOverdefined() || V2State.isOverdefined()) { 466 markOverdefined(I); 467 } else if (V1State.isConstant() && V2State.isConstant()) { 468 ConstPoolVal *Result = 469 opt::ConstantFoldBinaryInstruction(I->getOpcode(), 470 V1State.getConstant(), 471 V2State.getConstant()); 472 if (Result) { 473 // This instruction constant folds! The only problem is that the value 474 // returned is newly allocated. Make sure to stick it into the methods 475 // constant pool... 476 M->getConstantPool().insert(Result); 477 markConstant(I, Result); 478 } else { 479 markOverdefined(I); // Don't know how to fold this instruction. :( 480 } 481 } 482 return; 483 } 484 485 // Shouldn't get here... either the switch statement or one of the group 486 // handlers should have kicked in... 487 // 488 cerr << "SCCP: Don't know how to handle: " << I; 489 markOverdefined(I); // Just in case 490} 491 492 493 494// OperandChangedState - This method is invoked on all of the users of an 495// instruction that was just changed state somehow.... Based on this 496// information, we need to update the specified user of this instruction. 497// 498void SCCP::OperandChangedState(User *U) { 499 // Only instructions use other variable values! 500 Instruction *I = U->castInstructionAsserting(); 501 if (!BBExecutable.count(I->getParent())) return; // Inst not executable yet! 502 503 UpdateInstruction(I); 504} 505 506 507// DoSparseConditionalConstantProp - Use Sparse Conditional Constant Propogation 508// to prove whether a value is constant and whether blocks are used. 509// 510bool opt::DoSCCP(Method *M) { 511 SCCP S(M); 512 return S.doSCCP(); 513} 514