IPConstantPropagation.cpp revision b2710041fbed34ae888a6fed4172aea92bced1b5
1//===-- IPConstantPropagation.cpp - Propagate constants through calls -----===// 2// 3// The LLVM Compiler Infrastructure 4// 5// This file is distributed under the University of Illinois Open Source 6// License. See LICENSE.TXT for details. 7// 8//===----------------------------------------------------------------------===// 9// 10// This pass implements an _extremely_ simple interprocedural constant 11// propagation pass. It could certainly be improved in many different ways, 12// like using a worklist. This pass makes arguments dead, but does not remove 13// them. The existing dead argument elimination pass should be run after this 14// to clean up the mess. 15// 16//===----------------------------------------------------------------------===// 17 18#define DEBUG_TYPE "ipconstprop" 19#include "llvm/Transforms/IPO.h" 20#include "llvm/Constants.h" 21#include "llvm/Instructions.h" 22#include "llvm/LLVMContext.h" 23#include "llvm/Module.h" 24#include "llvm/Pass.h" 25#include "llvm/Analysis/ValueTracking.h" 26#include "llvm/Support/CallSite.h" 27#include "llvm/ADT/Statistic.h" 28#include "llvm/ADT/SmallVector.h" 29using namespace llvm; 30 31STATISTIC(NumArgumentsProped, "Number of args turned into constants"); 32STATISTIC(NumReturnValProped, "Number of return values turned into constants"); 33 34namespace { 35 /// IPCP - The interprocedural constant propagation pass 36 /// 37 struct IPCP : public ModulePass { 38 static char ID; // Pass identification, replacement for typeid 39 IPCP() : ModulePass(&ID) {} 40 41 bool runOnModule(Module &M); 42 private: 43 bool PropagateConstantsIntoArguments(Function &F); 44 bool PropagateConstantReturn(Function &F); 45 }; 46} 47 48char IPCP::ID = 0; 49static RegisterPass<IPCP> 50X("ipconstprop", "Interprocedural constant propagation"); 51 52ModulePass *llvm::createIPConstantPropagationPass() { return new IPCP(); } 53 54bool IPCP::runOnModule(Module &M) { 55 bool Changed = false; 56 bool LocalChange = true; 57 58 // FIXME: instead of using smart algorithms, we just iterate until we stop 59 // making changes. 60 while (LocalChange) { 61 LocalChange = false; 62 for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I) 63 if (!I->isDeclaration()) { 64 // Delete any klingons. 65 I->removeDeadConstantUsers(); 66 if (I->hasLocalLinkage()) 67 LocalChange |= PropagateConstantsIntoArguments(*I); 68 Changed |= PropagateConstantReturn(*I); 69 } 70 Changed |= LocalChange; 71 } 72 return Changed; 73} 74 75/// PropagateConstantsIntoArguments - Look at all uses of the specified 76/// function. If all uses are direct call sites, and all pass a particular 77/// constant in for an argument, propagate that constant in as the argument. 78/// 79bool IPCP::PropagateConstantsIntoArguments(Function &F) { 80 if (F.arg_empty() || F.use_empty()) return false; // No arguments? Early exit. 81 82 // For each argument, keep track of its constant value and whether it is a 83 // constant or not. The bool is driven to true when found to be non-constant. 84 SmallVector<std::pair<Constant*, bool>, 16> ArgumentConstants; 85 ArgumentConstants.resize(F.arg_size()); 86 87 unsigned NumNonconstant = 0; 88 for (Value::use_iterator UI = F.use_begin(), E = F.use_end(); UI != E; ++UI) { 89 // Ignore blockaddress uses. 90 if (isa<BlockAddress>(*UI)) continue; 91 92 // Used by a non-instruction, or not the callee of a function, do not 93 // transform. 94 if (!isa<CallInst>(*UI) && !isa<InvokeInst>(*UI)) 95 return false; 96 97 CallSite CS = CallSite::get(cast<Instruction>(*UI)); 98 if (!CS.isCallee(UI)) 99 return false; 100 101 // Check out all of the potentially constant arguments. Note that we don't 102 // inspect varargs here. 103 CallSite::arg_iterator AI = CS.arg_begin(); 104 Function::arg_iterator Arg = F.arg_begin(); 105 for (unsigned i = 0, e = ArgumentConstants.size(); i != e; 106 ++i, ++AI, ++Arg) { 107 108 // If this argument is known non-constant, ignore it. 109 if (ArgumentConstants[i].second) 110 continue; 111 112 Constant *C = dyn_cast<Constant>(*AI); 113 if (C && ArgumentConstants[i].first == 0) { 114 ArgumentConstants[i].first = C; // First constant seen. 115 } else if (C && ArgumentConstants[i].first == C) { 116 // Still the constant value we think it is. 117 } else if (*AI == &*Arg) { 118 // Ignore recursive calls passing argument down. 119 } else { 120 // Argument became non-constant. If all arguments are non-constant now, 121 // give up on this function. 122 if (++NumNonconstant == ArgumentConstants.size()) 123 return false; 124 ArgumentConstants[i].second = true; 125 } 126 } 127 } 128 129 // If we got to this point, there is a constant argument! 130 assert(NumNonconstant != ArgumentConstants.size()); 131 bool MadeChange = false; 132 Function::arg_iterator AI = F.arg_begin(); 133 for (unsigned i = 0, e = ArgumentConstants.size(); i != e; ++i, ++AI) { 134 // Do we have a constant argument? 135 if (ArgumentConstants[i].second || AI->use_empty() || 136 (AI->hasByValAttr() && !F.onlyReadsMemory())) 137 continue; 138 139 Value *V = ArgumentConstants[i].first; 140 if (V == 0) V = UndefValue::get(AI->getType()); 141 AI->replaceAllUsesWith(V); 142 ++NumArgumentsProped; 143 MadeChange = true; 144 } 145 return MadeChange; 146} 147 148 149// Check to see if this function returns one or more constants. If so, replace 150// all callers that use those return values with the constant value. This will 151// leave in the actual return values and instructions, but deadargelim will 152// clean that up. 153// 154// Additionally if a function always returns one of its arguments directly, 155// callers will be updated to use the value they pass in directly instead of 156// using the return value. 157bool IPCP::PropagateConstantReturn(Function &F) { 158 if (F.getReturnType() == Type::getVoidTy(F.getContext())) 159 return false; // No return value. 160 161 // If this function could be overridden later in the link stage, we can't 162 // propagate information about its results into callers. 163 if (F.mayBeOverridden()) 164 return false; 165 166 LLVMContext &Context = F.getContext(); 167 168 // Check to see if this function returns a constant. 169 SmallVector<Value *,4> RetVals; 170 const StructType *STy = dyn_cast<StructType>(F.getReturnType()); 171 if (STy) 172 for (unsigned i = 0, e = STy->getNumElements(); i < e; ++i) 173 RetVals.push_back(UndefValue::get(STy->getElementType(i))); 174 else 175 RetVals.push_back(UndefValue::get(F.getReturnType())); 176 177 unsigned NumNonConstant = 0; 178 for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB) 179 if (ReturnInst *RI = dyn_cast<ReturnInst>(BB->getTerminator())) { 180 for (unsigned i = 0, e = RetVals.size(); i != e; ++i) { 181 // Already found conflicting return values? 182 Value *RV = RetVals[i]; 183 if (!RV) 184 continue; 185 186 // Find the returned value 187 Value *V; 188 if (!STy) 189 V = RI->getOperand(i); 190 else 191 V = FindInsertedValue(RI->getOperand(0), i, Context); 192 193 if (V) { 194 // Ignore undefs, we can change them into anything 195 if (isa<UndefValue>(V)) 196 continue; 197 198 // Try to see if all the rets return the same constant or argument. 199 if (isa<Constant>(V) || isa<Argument>(V)) { 200 if (isa<UndefValue>(RV)) { 201 // No value found yet? Try the current one. 202 RetVals[i] = V; 203 continue; 204 } 205 // Returning the same value? Good. 206 if (RV == V) 207 continue; 208 } 209 } 210 // Different or no known return value? Don't propagate this return 211 // value. 212 RetVals[i] = 0; 213 // All values non constant? Stop looking. 214 if (++NumNonConstant == RetVals.size()) 215 return false; 216 } 217 } 218 219 // If we got here, the function returns at least one constant value. Loop 220 // over all users, replacing any uses of the return value with the returned 221 // constant. 222 bool MadeChange = false; 223 for (Value::use_iterator UI = F.use_begin(), E = F.use_end(); UI != E; ++UI) { 224 CallSite CS = CallSite::get(*UI); 225 Instruction* Call = CS.getInstruction(); 226 227 // Not a call instruction or a call instruction that's not calling F 228 // directly? 229 if (!Call || !CS.isCallee(UI)) 230 continue; 231 232 // Call result not used? 233 if (Call->use_empty()) 234 continue; 235 236 MadeChange = true; 237 238 if (STy == 0) { 239 Value* New = RetVals[0]; 240 if (Argument *A = dyn_cast<Argument>(New)) 241 // Was an argument returned? Then find the corresponding argument in 242 // the call instruction and use that. 243 New = CS.getArgument(A->getArgNo()); 244 Call->replaceAllUsesWith(New); 245 continue; 246 } 247 248 for (Value::use_iterator I = Call->use_begin(), E = Call->use_end(); 249 I != E;) { 250 Instruction *Ins = cast<Instruction>(*I); 251 252 // Increment now, so we can remove the use 253 ++I; 254 255 // Find the index of the retval to replace with 256 int index = -1; 257 if (ExtractValueInst *EV = dyn_cast<ExtractValueInst>(Ins)) 258 if (EV->hasIndices()) 259 index = *EV->idx_begin(); 260 261 // If this use uses a specific return value, and we have a replacement, 262 // replace it. 263 if (index != -1) { 264 Value *New = RetVals[index]; 265 if (New) { 266 if (Argument *A = dyn_cast<Argument>(New)) 267 // Was an argument returned? Then find the corresponding argument in 268 // the call instruction and use that. 269 New = CS.getArgument(A->getArgNo()); 270 Ins->replaceAllUsesWith(New); 271 Ins->eraseFromParent(); 272 } 273 } 274 } 275 } 276 277 if (MadeChange) ++NumReturnValProped; 278 return MadeChange; 279} 280