IPConstantPropagation.cpp revision edc4d69917df7dc34543adf719d5c93249fd5e27
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/Module.h" 23#include "llvm/Pass.h" 24#include "llvm/Analysis/ValueTracking.h" 25#include "llvm/Support/CallSite.h" 26#include "llvm/Support/Compiler.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 VISIBILITY_HIDDEN 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 // Used by a non-instruction, or not the callee of a function, do not 90 // transform. 91 if (!isa<CallInst>(*UI) && !isa<InvokeInst>(*UI)) 92 return false; 93 94 CallSite CS = CallSite::get(cast<Instruction>(*UI)); 95 if (!CS.isCallee(UI)) 96 return false; 97 98 // Check out all of the potentially constant arguments. Note that we don't 99 // inspect varargs here. 100 CallSite::arg_iterator AI = CS.arg_begin(); 101 Function::arg_iterator Arg = F.arg_begin(); 102 for (unsigned i = 0, e = ArgumentConstants.size(); i != e; 103 ++i, ++AI, ++Arg) { 104 105 // If this argument is known non-constant, ignore it. 106 if (ArgumentConstants[i].second) 107 continue; 108 109 Constant *C = dyn_cast<Constant>(*AI); 110 if (C && ArgumentConstants[i].first == 0) { 111 ArgumentConstants[i].first = C; // First constant seen. 112 } else if (C && ArgumentConstants[i].first == C) { 113 // Still the constant value we think it is. 114 } else if (*AI == &*Arg) { 115 // Ignore recursive calls passing argument down. 116 } else { 117 // Argument became non-constant. If all arguments are non-constant now, 118 // give up on this function. 119 if (++NumNonconstant == ArgumentConstants.size()) 120 return false; 121 ArgumentConstants[i].second = true; 122 } 123 } 124 } 125 126 // If we got to this point, there is a constant argument! 127 assert(NumNonconstant != ArgumentConstants.size()); 128 bool MadeChange = false; 129 Function::arg_iterator AI = F.arg_begin(); 130 for (unsigned i = 0, e = ArgumentConstants.size(); i != e; ++i, ++AI) { 131 // Do we have a constant argument? 132 if (ArgumentConstants[i].second || AI->use_empty()) 133 continue; 134 135 Value *V = ArgumentConstants[i].first; 136 if (V == 0) V = UndefValue::get(AI->getType()); 137 AI->replaceAllUsesWith(V); 138 ++NumArgumentsProped; 139 MadeChange = true; 140 } 141 return MadeChange; 142} 143 144 145// Check to see if this function returns one or more constants. If so, replace 146// all callers that use those return values with the constant value. This will 147// leave in the actual return values and instructions, but deadargelim will 148// clean that up. 149// 150// Additionally if a function always returns one of its arguments directly, 151// callers will be updated to use the value they pass in directly instead of 152// using the return value. 153bool IPCP::PropagateConstantReturn(Function &F) { 154 if (F.getReturnType() == Type::VoidTy) 155 return false; // No return value. 156 157 // If this function could be overridden later in the link stage, we can't 158 // propagate information about its results into callers. 159 if (F.mayBeOverridden()) 160 return false; 161 162 // Check to see if this function returns a constant. 163 SmallVector<Value *,4> RetVals; 164 const StructType *STy = dyn_cast<StructType>(F.getReturnType()); 165 if (STy) 166 for (unsigned i = 0, e = STy->getNumElements(); i < e; ++i) 167 RetVals.push_back(UndefValue::get(STy->getElementType(i))); 168 else 169 RetVals.push_back(UndefValue::get(F.getReturnType())); 170 171 unsigned NumNonConstant = 0; 172 for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB) 173 if (ReturnInst *RI = dyn_cast<ReturnInst>(BB->getTerminator())) { 174 for (unsigned i = 0, e = RetVals.size(); i != e; ++i) { 175 // Already found conflicting return values? 176 Value *RV = RetVals[i]; 177 if (!RV) 178 continue; 179 180 // Find the returned value 181 Value *V; 182 if (!STy) 183 V = RI->getOperand(i); 184 else 185 V = FindInsertedValue(RI->getOperand(0), i); 186 187 if (V) { 188 // Ignore undefs, we can change them into anything 189 if (isa<UndefValue>(V)) 190 continue; 191 192 // Try to see if all the rets return the same constant or argument. 193 if (isa<Constant>(V) || isa<Argument>(V)) { 194 if (isa<UndefValue>(RV)) { 195 // No value found yet? Try the current one. 196 RetVals[i] = V; 197 continue; 198 } 199 // Returning the same value? Good. 200 if (RV == V) 201 continue; 202 } 203 } 204 // Different or no known return value? Don't propagate this return 205 // value. 206 RetVals[i] = 0; 207 // All values non constant? Stop looking. 208 if (++NumNonConstant == RetVals.size()) 209 return false; 210 } 211 } 212 213 // If we got here, the function returns at least one constant value. Loop 214 // over all users, replacing any uses of the return value with the returned 215 // constant. 216 bool MadeChange = false; 217 for (Value::use_iterator UI = F.use_begin(), E = F.use_end(); UI != E; ++UI) { 218 CallSite CS = CallSite::get(*UI); 219 Instruction* Call = CS.getInstruction(); 220 221 // Not a call instruction or a call instruction that's not calling F 222 // directly? 223 if (!Call || !CS.isCallee(UI)) 224 continue; 225 226 // Call result not used? 227 if (Call->use_empty()) 228 continue; 229 230 MadeChange = true; 231 232 if (STy == 0) { 233 Value* New = RetVals[0]; 234 if (Argument *A = dyn_cast<Argument>(New)) 235 // Was an argument returned? Then find the corresponding argument in 236 // the call instruction and use that. 237 New = CS.getArgument(A->getArgNo()); 238 Call->replaceAllUsesWith(New); 239 continue; 240 } 241 242 for (Value::use_iterator I = Call->use_begin(), E = Call->use_end(); 243 I != E;) { 244 Instruction *Ins = dyn_cast<Instruction>(*I); 245 246 // Increment now, so we can remove the use 247 ++I; 248 249 // Not an instruction? Ignore 250 if (!Ins) 251 continue; 252 253 // Find the index of the retval to replace with 254 int index = -1; 255 if (ExtractValueInst *EV = dyn_cast<ExtractValueInst>(Ins)) 256 if (EV->hasIndices()) 257 index = *EV->idx_begin(); 258 259 // If this use uses a specific return value, and we have a replacement, 260 // replace it. 261 if (index != -1) { 262 Value *New = RetVals[index]; 263 if (New) { 264 if (Argument *A = dyn_cast<Argument>(New)) 265 // Was an argument returned? Then find the corresponding argument in 266 // the call instruction and use that. 267 New = CS.getArgument(A->getArgNo()); 268 Ins->replaceAllUsesWith(New); 269 Ins->eraseFromParent(); 270 } 271 } 272 } 273 } 274 275 if (MadeChange) ++NumReturnValProped; 276 return MadeChange; 277} 278