SCCP.cpp revision 7e70829632f82de15db187845666aaca6e04b792
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/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" 30#include <algorithm> 31#include <set> 32#include <iostream> 33using std::cerr; 34 35static Statistic<> NumInstRemoved("sccp\t\t- Number of instructions removed"); 36 37// InstVal class - This class represents the different lattice values that an 38// instruction may occupy. It is a simple class with value semantics. 39// 40namespace { 41class InstVal { 42 enum { 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 49public: 50 inline InstVal() : LatticeValue(undefined), ConstantVal(0) {} 51 52 // markOverdefined - Return true if this is a new status to be in... 53 inline bool markOverdefined() { 54 if (LatticeValue != overdefined) { 55 LatticeValue = overdefined; 56 return true; 57 } 58 return false; 59 } 60 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; 65 ConstantVal = V; 66 return true; 67 } else { 68 assert(ConstantVal == V && "Marking constant with different value"); 69 } 70 return false; 71 } 72 73 inline bool isUndefined() const { return LatticeValue == undefined; } 74 inline bool isConstant() const { return LatticeValue == constant; } 75 inline bool isOverdefined() const { return LatticeValue == overdefined; } 76 77 inline Constant *getConstant() const { return ConstantVal; } 78}; 79 80} // end anonymous namespace 81 82 83//===----------------------------------------------------------------------===// 84// SCCP Class 85// 86// This class does all of the work of Sparse Conditional Constant Propogation. 87// 88namespace { 89class SCCP : public FunctionPass, public InstVisitor<SCCP> { 90 std::set<BasicBlock*> BBExecutable;// The basic blocks that are executable 91 std::map<Value*, InstVal> ValueState; // The state each value is in... 92 93 std::vector<Instruction*> InstWorkList;// The instruction work list 94 std::vector<BasicBlock*> BBWorkList; // The BasicBlock work list 95public: 96 97 const char *getPassName() const { 98 return "Sparse Conditional Constant Propogation"; 99 } 100 101 // runOnFunction - Run the Sparse Conditional Constant Propogation algorithm, 102 // and return true if the function was modified. 103 // 104 bool runOnFunction(Function &F); 105 106 virtual void getAnalysisUsage(AnalysisUsage &AU) const { 107 AU.preservesCFG(); 108 } 109 110 111 //===--------------------------------------------------------------------===// 112 // The implementation of this class 113 // 114private: 115 friend class InstVisitor<SCCP>; // Allow callbacks from visitor 116 117 // markValueOverdefined - Make a value be marked as "constant". If the value 118 // is not already a constant, add it to the instruction work list so that 119 // the users of the instruction are updated later. 120 // 121 inline bool markConstant(Instruction *I, Constant *V) { 122 DEBUG(cerr << "markConstant: " << V << " = " << I); 123 124 if (ValueState[I].markConstant(V)) { 125 InstWorkList.push_back(I); 126 return true; 127 } 128 return false; 129 } 130 131 // markValueOverdefined - Make a value be marked as "overdefined". If the 132 // value is not already overdefined, add it to the instruction work list so 133 // that the users of the instruction are updated later. 134 // 135 inline bool markOverdefined(Value *V) { 136 if (ValueState[V].markOverdefined()) { 137 if (Instruction *I = dyn_cast<Instruction>(V)) { 138 DEBUG(cerr << "markOverdefined: " << V); 139 InstWorkList.push_back(I); // Only instructions go on the work list 140 } 141 return true; 142 } 143 return false; 144 } 145 146 // getValueState - Return the InstVal object that corresponds to the value. 147 // This function is neccesary because not all values should start out in the 148 // underdefined state... Argument's should be overdefined, and 149 // constants should be marked as constants. If a value is not known to be an 150 // Instruction object, then use this accessor to get its value from the map. 151 // 152 inline InstVal &getValueState(Value *V) { 153 std::map<Value*, InstVal>::iterator I = ValueState.find(V); 154 if (I != ValueState.end()) return I->second; // Common case, in the map 155 156 if (Constant *CPV = dyn_cast<Constant>(V)) { // Constants are constant 157 ValueState[CPV].markConstant(CPV); 158 } else if (isa<Argument>(V)) { // Arguments are overdefined 159 ValueState[V].markOverdefined(); 160 } 161 // All others are underdefined by default... 162 return ValueState[V]; 163 } 164 165 // markExecutable - Mark a basic block as executable, adding it to the BB 166 // work list if it is not already executable... 167 // 168 void markExecutable(BasicBlock *BB) { 169 if (BBExecutable.count(BB)) return; 170 DEBUG(cerr << "Marking BB Executable: " << *BB); 171 BBExecutable.insert(BB); // Basic block is executable! 172 BBWorkList.push_back(BB); // Add the block to the work list! 173 } 174 175 176 // visit implementations - Something changed in this instruction... Either an 177 // operand made a transition, or the instruction is newly executable. Change 178 // the value type of I to reflect these changes if appropriate. 179 // 180 void visitPHINode(PHINode &I); 181 182 // Terminators 183 void visitReturnInst(ReturnInst &I) { /*does not have an effect*/ } 184 void visitTerminatorInst(TerminatorInst &TI); 185 186 void visitUnaryOperator(Instruction &I); 187 void visitCastInst(CastInst &I) { visitUnaryOperator(I); } 188 void visitBinaryOperator(Instruction &I); 189 void visitShiftInst(ShiftInst &I) { visitBinaryOperator(I); } 190 191 // Instructions that cannot be folded away... 192 void visitStoreInst (Instruction &I) { /*returns void*/ } 193 void visitMemAccessInst (Instruction &I) { markOverdefined(&I); } 194 void visitCallInst (Instruction &I) { markOverdefined(&I); } 195 void visitInvokeInst (Instruction &I) { markOverdefined(&I); } 196 void visitAllocationInst(Instruction &I) { markOverdefined(&I); } 197 void visitFreeInst (Instruction &I) { /*returns void*/ } 198 199 void visitInstruction(Instruction &I) { 200 // If a new instruction is added to LLVM that we don't handle... 201 cerr << "SCCP: Don't know how to handle: " << I; 202 markOverdefined(&I); // Just in case 203 } 204 205 // getFeasibleSuccessors - Return a vector of booleans to indicate which 206 // successors are reachable from a given terminator instruction. 207 // 208 void getFeasibleSuccessors(TerminatorInst &TI, std::vector<bool> &Succs); 209 210 // isEdgeFeasible - Return true if the control flow edge from the 'From' basic 211 // block to the 'To' basic block is currently feasible... 212 // 213 bool isEdgeFeasible(BasicBlock *From, BasicBlock *To); 214 215 // OperandChangedState - This method is invoked on all of the users of an 216 // instruction that was just changed state somehow.... Based on this 217 // information, we need to update the specified user of this instruction. 218 // 219 void OperandChangedState(User *U) { 220 // Only instructions use other variable values! 221 Instruction &I = cast<Instruction>(*U); 222 if (!BBExecutable.count(I.getParent())) return;// Inst not executable yet! 223 visit(I); 224 } 225}; 226} // end anonymous namespace 227 228 229// createSCCPPass - This is the public interface to this file... 230// 231Pass *createSCCPPass() { 232 return new SCCP(); 233} 234 235 236 237//===----------------------------------------------------------------------===// 238// SCCP Class Implementation 239 240 241// runOnFunction() - Run the Sparse Conditional Constant Propogation algorithm, 242// and return true if the function was modified. 243// 244bool SCCP::runOnFunction(Function &F) { 245 // Mark the first block of the function as being executable... 246 markExecutable(&F.front()); 247 248 // Process the work lists until their are empty! 249 while (!BBWorkList.empty() || !InstWorkList.empty()) { 250 // Process the instruction work list... 251 while (!InstWorkList.empty()) { 252 Instruction *I = InstWorkList.back(); 253 InstWorkList.pop_back(); 254 255 DEBUG(cerr << "\nPopped off I-WL: " << I); 256 257 258 // "I" got into the work list because it either made the transition from 259 // bottom to constant, or to Overdefined. 260 // 261 // Update all of the users of this instruction's value... 262 // 263 for_each(I->use_begin(), I->use_end(), 264 bind_obj(this, &SCCP::OperandChangedState)); 265 } 266 267 // Process the basic block work list... 268 while (!BBWorkList.empty()) { 269 BasicBlock *BB = BBWorkList.back(); 270 BBWorkList.pop_back(); 271 272 DEBUG(cerr << "\nPopped off BBWL: " << BB); 273 274 // If this block only has a single successor, mark it as executable as 275 // well... if not, terminate the do loop. 276 // 277 if (BB->getTerminator()->getNumSuccessors() == 1) 278 markExecutable(BB->getTerminator()->getSuccessor(0)); 279 280 // Notify all instructions in this basic block that they are newly 281 // executable. 282 visit(BB); 283 } 284 } 285 286 if (DebugFlag) { 287 for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I) 288 if (!BBExecutable.count(I)) 289 cerr << "BasicBlock Dead:" << *I; 290 } 291 292 // Iterate over all of the instructions in a function, replacing them with 293 // constants if we have found them to be of constant values. 294 // 295 bool MadeChanges = false; 296 for (Function::iterator BB = F.begin(), BBE = F.end(); BB != BBE; ++BB) 297 for (BasicBlock::iterator BI = BB->begin(); BI != BB->end();) { 298 Instruction &Inst = *BI; 299 InstVal &IV = ValueState[&Inst]; 300 if (IV.isConstant()) { 301 Constant *Const = IV.getConstant(); 302 DEBUG(cerr << "Constant: " << Const << " = " << Inst); 303 304 // Replaces all of the uses of a variable with uses of the constant. 305 Inst.replaceAllUsesWith(Const); 306 307 // Remove the operator from the list of definitions... and delete it. 308 BI = BB->getInstList().erase(BI); 309 310 // Hey, we just changed something! 311 MadeChanges = true; 312 ++NumInstRemoved; 313 } else { 314 ++BI; 315 } 316 } 317 318 // Reset state so that the next invocation will have empty data structures 319 BBExecutable.clear(); 320 ValueState.clear(); 321 322 return MadeChanges; 323} 324 325 326// getFeasibleSuccessors - Return a vector of booleans to indicate which 327// successors are reachable from a given terminator instruction. 328// 329void SCCP::getFeasibleSuccessors(TerminatorInst &TI, std::vector<bool> &Succs) { 330 assert(Succs.size() == TI.getNumSuccessors() && "Succs vector wrong size!"); 331 if (BranchInst *BI = dyn_cast<BranchInst>(&TI)) { 332 if (BI->isUnconditional()) { 333 Succs[0] = true; 334 } else { 335 InstVal &BCValue = getValueState(BI->getCondition()); 336 if (BCValue.isOverdefined()) { 337 // Overdefined condition variables mean the branch could go either way. 338 Succs[0] = Succs[1] = true; 339 } else if (BCValue.isConstant()) { 340 // Constant condition variables mean the branch can only go a single way 341 Succs[BCValue.getConstant() == ConstantBool::False] = true; 342 } 343 } 344 } else if (InvokeInst *II = dyn_cast<InvokeInst>(&TI)) { 345 // Invoke instructions successors are always executable. 346 Succs[0] = Succs[1] = true; 347 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(&TI)) { 348 InstVal &SCValue = getValueState(SI->getCondition()); 349 if (SCValue.isOverdefined()) { // Overdefined condition? 350 // All destinations are executable! 351 Succs.assign(TI.getNumSuccessors(), true); 352 } else if (SCValue.isConstant()) { 353 Constant *CPV = SCValue.getConstant(); 354 // Make sure to skip the "default value" which isn't a value 355 for (unsigned i = 1, E = SI->getNumSuccessors(); i != E; ++i) { 356 if (SI->getSuccessorValue(i) == CPV) {// Found the right branch... 357 Succs[i] = true; 358 return; 359 } 360 } 361 362 // Constant value not equal to any of the branches... must execute 363 // default branch then... 364 Succs[0] = true; 365 } 366 } else { 367 cerr << "SCCP: Don't know how to handle: " << TI; 368 Succs.assign(TI.getNumSuccessors(), true); 369 } 370} 371 372 373// isEdgeFeasible - Return true if the control flow edge from the 'From' basic 374// block to the 'To' basic block is currently feasible... 375// 376bool SCCP::isEdgeFeasible(BasicBlock *From, BasicBlock *To) { 377 assert(BBExecutable.count(To) && "Dest should always be alive!"); 378 379 // Make sure the source basic block is executable!! 380 if (!BBExecutable.count(From)) return false; 381 382 // Check to make sure this edge itself is actually feasible now... 383 TerminatorInst *FT = From->getTerminator(); 384 std::vector<bool> SuccFeasible(FT->getNumSuccessors()); 385 getFeasibleSuccessors(*FT, SuccFeasible); 386 387 // Check all edges from From to To. If any are feasible, return true. 388 for (unsigned i = 0, e = SuccFeasible.size(); i != e; ++i) 389 if (FT->getSuccessor(i) == To && SuccFeasible[i]) 390 return true; 391 392 // Otherwise, none of the edges are actually feasible at this time... 393 return false; 394} 395 396// visit Implementations - Something changed in this instruction... Either an 397// operand made a transition, or the instruction is newly executable. Change 398// the value type of I to reflect these changes if appropriate. This method 399// makes sure to do the following actions: 400// 401// 1. If a phi node merges two constants in, and has conflicting value coming 402// from different branches, or if the PHI node merges in an overdefined 403// value, then the PHI node becomes overdefined. 404// 2. If a phi node merges only constants in, and they all agree on value, the 405// PHI node becomes a constant value equal to that. 406// 3. If V <- x (op) y && isConstant(x) && isConstant(y) V = Constant 407// 4. If V <- x (op) y && (isOverdefined(x) || isOverdefined(y)) V = Overdefined 408// 5. If V <- MEM or V <- CALL or V <- (unknown) then V = Overdefined 409// 6. If a conditional branch has a value that is constant, make the selected 410// destination executable 411// 7. If a conditional branch has a value that is overdefined, make all 412// successors executable. 413// 414 415void SCCP::visitPHINode(PHINode &PN) { 416 unsigned NumValues = PN.getNumIncomingValues(), i; 417 InstVal *OperandIV = 0; 418 419 // Look at all of the executable operands of the PHI node. If any of them 420 // are overdefined, the PHI becomes overdefined as well. If they are all 421 // constant, and they agree with each other, the PHI becomes the identical 422 // constant. If they are constant and don't agree, the PHI is overdefined. 423 // If there are no executable operands, the PHI remains undefined. 424 // 425 for (i = 0; i < NumValues; ++i) { 426 if (isEdgeFeasible(PN.getIncomingBlock(i), PN.getParent())) { 427 InstVal &IV = getValueState(PN.getIncomingValue(i)); 428 if (IV.isUndefined()) continue; // Doesn't influence PHI node. 429 if (IV.isOverdefined()) { // PHI node becomes overdefined! 430 markOverdefined(&PN); 431 return; 432 } 433 434 if (OperandIV == 0) { // Grab the first value... 435 OperandIV = &IV; 436 } else { // Another value is being merged in! 437 // There is already a reachable operand. If we conflict with it, 438 // then the PHI node becomes overdefined. If we agree with it, we 439 // can continue on. 440 441 // Check to see if there are two different constants merging... 442 if (IV.getConstant() != OperandIV->getConstant()) { 443 // Yes there is. This means the PHI node is not constant. 444 // You must be overdefined poor PHI. 445 // 446 markOverdefined(&PN); // The PHI node now becomes overdefined 447 return; // I'm done analyzing you 448 } 449 } 450 } 451 } 452 453 // If we exited the loop, this means that the PHI node only has constant 454 // arguments that agree with each other(and OperandIV is a pointer to one 455 // of their InstVal's) or OperandIV is null because there are no defined 456 // incoming arguments. If this is the case, the PHI remains undefined. 457 // 458 if (OperandIV) { 459 assert(OperandIV->isConstant() && "Should only be here for constants!"); 460 markConstant(&PN, OperandIV->getConstant()); // Aquire operand value 461 } 462} 463 464void SCCP::visitTerminatorInst(TerminatorInst &TI) { 465 std::vector<bool> SuccFeasible(TI.getNumSuccessors()); 466 getFeasibleSuccessors(TI, SuccFeasible); 467 468 // Mark all feasible successors executable... 469 for (unsigned i = 0, e = SuccFeasible.size(); i != e; ++i) 470 if (SuccFeasible[i]) { 471 BasicBlock *Succ = TI.getSuccessor(i); 472 markExecutable(Succ); 473 474 // Visit all of the PHI nodes that merge values from this block... 475 // Because this edge may be new executable, and PHI nodes that used to be 476 // constant now may not be. 477 // 478 for (BasicBlock::iterator I = Succ->begin(); 479 PHINode *PN = dyn_cast<PHINode>(&*I); ++I) 480 visitPHINode(*PN); 481 } 482} 483 484void SCCP::visitUnaryOperator(Instruction &I) { 485 Value *V = I.getOperand(0); 486 InstVal &VState = getValueState(V); 487 if (VState.isOverdefined()) { // Inherit overdefinedness of operand 488 markOverdefined(&I); 489 } else if (VState.isConstant()) { // Propogate constant value 490 Constant *Result = isa<CastInst>(I) 491 ? ConstantFoldCastInstruction(VState.getConstant(), I.getType()) 492 : ConstantFoldUnaryInstruction(I.getOpcode(), VState.getConstant()); 493 494 if (Result) { 495 // This instruction constant folds! 496 markConstant(&I, Result); 497 } else { 498 markOverdefined(&I); // Don't know how to fold this instruction. :( 499 } 500 } 501} 502 503// Handle BinaryOperators and Shift Instructions... 504void SCCP::visitBinaryOperator(Instruction &I) { 505 InstVal &V1State = getValueState(I.getOperand(0)); 506 InstVal &V2State = getValueState(I.getOperand(1)); 507 if (V1State.isOverdefined() || V2State.isOverdefined()) { 508 markOverdefined(&I); 509 } else if (V1State.isConstant() && V2State.isConstant()) { 510 Constant *Result = 0; 511 if (isa<BinaryOperator>(I)) 512 Result = ConstantFoldBinaryInstruction(I.getOpcode(), 513 V1State.getConstant(), 514 V2State.getConstant()); 515 else if (isa<ShiftInst>(I)) 516 Result = ConstantFoldShiftInstruction(I.getOpcode(), 517 V1State.getConstant(), 518 V2State.getConstant()); 519 if (Result) 520 markConstant(&I, Result); // This instruction constant folds! 521 else 522 markOverdefined(&I); // Don't know how to fold this instruction. :( 523 } 524} 525