LLVM API Documentation
00001 //===-- IPConstantPropagation.cpp - Propagate constants through calls -----===// 00002 // 00003 // The LLVM Compiler Infrastructure 00004 // 00005 // This file was developed by the LLVM research group and is distributed under 00006 // the University of Illinois Open Source License. See LICENSE.TXT for details. 00007 // 00008 //===----------------------------------------------------------------------===// 00009 // 00010 // This pass implements an _extremely_ simple interprocedural constant 00011 // propagation pass. It could certainly be improved in many different ways, 00012 // like using a worklist. This pass makes arguments dead, but does not remove 00013 // them. The existing dead argument elimination pass should be run after this 00014 // to clean up the mess. 00015 // 00016 //===----------------------------------------------------------------------===// 00017 00018 #include "llvm/Transforms/IPO.h" 00019 #include "llvm/Constants.h" 00020 #include "llvm/Instructions.h" 00021 #include "llvm/Module.h" 00022 #include "llvm/Pass.h" 00023 #include "llvm/Support/CallSite.h" 00024 #include "llvm/ADT/Statistic.h" 00025 using namespace llvm; 00026 00027 namespace { 00028 Statistic<> NumArgumentsProped("ipconstprop", 00029 "Number of args turned into constants"); 00030 Statistic<> NumReturnValProped("ipconstprop", 00031 "Number of return values turned into constants"); 00032 00033 /// IPCP - The interprocedural constant propagation pass 00034 /// 00035 struct IPCP : public ModulePass { 00036 bool runOnModule(Module &M); 00037 private: 00038 bool PropagateConstantsIntoArguments(Function &F); 00039 bool PropagateConstantReturn(Function &F); 00040 }; 00041 RegisterOpt<IPCP> X("ipconstprop", "Interprocedural constant propagation"); 00042 } 00043 00044 ModulePass *llvm::createIPConstantPropagationPass() { return new IPCP(); } 00045 00046 bool IPCP::runOnModule(Module &M) { 00047 bool Changed = false; 00048 bool LocalChange = true; 00049 00050 // FIXME: instead of using smart algorithms, we just iterate until we stop 00051 // making changes. 00052 while (LocalChange) { 00053 LocalChange = false; 00054 for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I) 00055 if (!I->isExternal()) { 00056 // Delete any klingons. 00057 I->removeDeadConstantUsers(); 00058 if (I->hasInternalLinkage()) 00059 LocalChange |= PropagateConstantsIntoArguments(*I); 00060 Changed |= PropagateConstantReturn(*I); 00061 } 00062 Changed |= LocalChange; 00063 } 00064 return Changed; 00065 } 00066 00067 /// PropagateConstantsIntoArguments - Look at all uses of the specified 00068 /// function. If all uses are direct call sites, and all pass a particular 00069 /// constant in for an argument, propagate that constant in as the argument. 00070 /// 00071 bool IPCP::PropagateConstantsIntoArguments(Function &F) { 00072 if (F.aempty() || F.use_empty()) return false; // No arguments? Early exit. 00073 00074 std::vector<std::pair<Constant*, bool> > ArgumentConstants; 00075 ArgumentConstants.resize(F.asize()); 00076 00077 unsigned NumNonconstant = 0; 00078 00079 for (Value::use_iterator I = F.use_begin(), E = F.use_end(); I != E; ++I) 00080 if (!isa<Instruction>(*I)) 00081 return false; // Used by a non-instruction, do not transform 00082 else { 00083 CallSite CS = CallSite::get(cast<Instruction>(*I)); 00084 if (CS.getInstruction() == 0 || 00085 CS.getCalledFunction() != &F) 00086 return false; // Not a direct call site? 00087 00088 // Check out all of the potentially constant arguments 00089 CallSite::arg_iterator AI = CS.arg_begin(); 00090 Function::aiterator Arg = F.abegin(); 00091 for (unsigned i = 0, e = ArgumentConstants.size(); i != e; 00092 ++i, ++AI, ++Arg) { 00093 if (*AI == &F) return false; // Passes the function into itself 00094 00095 if (!ArgumentConstants[i].second) { 00096 if (Constant *C = dyn_cast<Constant>(*AI)) { 00097 if (!ArgumentConstants[i].first) 00098 ArgumentConstants[i].first = C; 00099 else if (ArgumentConstants[i].first != C) { 00100 // Became non-constant 00101 ArgumentConstants[i].second = true; 00102 ++NumNonconstant; 00103 if (NumNonconstant == ArgumentConstants.size()) return false; 00104 } 00105 } else if (*AI != &*Arg) { // Ignore recursive calls with same arg 00106 // This is not a constant argument. Mark the argument as 00107 // non-constant. 00108 ArgumentConstants[i].second = true; 00109 ++NumNonconstant; 00110 if (NumNonconstant == ArgumentConstants.size()) return false; 00111 } 00112 } 00113 } 00114 } 00115 00116 // If we got to this point, there is a constant argument! 00117 assert(NumNonconstant != ArgumentConstants.size()); 00118 Function::aiterator AI = F.abegin(); 00119 bool MadeChange = false; 00120 for (unsigned i = 0, e = ArgumentConstants.size(); i != e; ++i, ++AI) 00121 // Do we have a constant argument!? 00122 if (!ArgumentConstants[i].second && !AI->use_empty()) { 00123 Value *V = ArgumentConstants[i].first; 00124 if (V == 0) V = UndefValue::get(AI->getType()); 00125 AI->replaceAllUsesWith(V); 00126 ++NumArgumentsProped; 00127 MadeChange = true; 00128 } 00129 return MadeChange; 00130 } 00131 00132 00133 // Check to see if this function returns a constant. If so, replace all callers 00134 // that user the return value with the returned valued. If we can replace ALL 00135 // callers, 00136 bool IPCP::PropagateConstantReturn(Function &F) { 00137 if (F.getReturnType() == Type::VoidTy) 00138 return false; // No return value. 00139 00140 // Check to see if this function returns a constant. 00141 Value *RetVal = 0; 00142 for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB) 00143 if (ReturnInst *RI = dyn_cast<ReturnInst>(BB->getTerminator())) 00144 if (isa<UndefValue>(RI->getOperand(0))) { 00145 // Ignore. 00146 } else if (Constant *C = dyn_cast<Constant>(RI->getOperand(0))) { 00147 if (RetVal == 0) 00148 RetVal = C; 00149 else if (RetVal != C) 00150 return false; // Does not return the same constant. 00151 } else { 00152 return false; // Does not return a constant. 00153 } 00154 00155 if (RetVal == 0) RetVal = UndefValue::get(F.getReturnType()); 00156 00157 // If we got here, the function returns a constant value. Loop over all 00158 // users, replacing any uses of the return value with the returned constant. 00159 bool ReplacedAllUsers = true; 00160 bool MadeChange = false; 00161 for (Value::use_iterator I = F.use_begin(), E = F.use_end(); I != E; ++I) 00162 if (!isa<Instruction>(*I)) 00163 ReplacedAllUsers = false; 00164 else { 00165 CallSite CS = CallSite::get(cast<Instruction>(*I)); 00166 if (CS.getInstruction() == 0 || 00167 CS.getCalledFunction() != &F) { 00168 ReplacedAllUsers = false; 00169 } else { 00170 if (!CS.getInstruction()->use_empty()) { 00171 CS.getInstruction()->replaceAllUsesWith(RetVal); 00172 MadeChange = true; 00173 } 00174 } 00175 } 00176 00177 // If we replace all users with the returned constant, and there can be no 00178 // other callers of the function, replace the constant being returned in the 00179 // function with an undef value. 00180 if (ReplacedAllUsers && F.hasInternalLinkage() && !isa<UndefValue>(RetVal)) { 00181 Value *RV = UndefValue::get(RetVal->getType()); 00182 for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB) 00183 if (ReturnInst *RI = dyn_cast<ReturnInst>(BB->getTerminator())) 00184 RI->setOperand(0, RV); 00185 MadeChange = true; 00186 } 00187 00188 if (MadeChange) ++NumReturnValProped; 00189 return MadeChange; 00190 }