LLVM API Documentation
00001 //===- InlineSimple.cpp - Code to perform simple function inlining --------===// 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 file implements bottom-up inlining of functions into callees. 00011 // 00012 //===----------------------------------------------------------------------===// 00013 00014 #include "Inliner.h" 00015 #include "llvm/Instructions.h" 00016 #include "llvm/IntrinsicInst.h" 00017 #include "llvm/Function.h" 00018 #include "llvm/Type.h" 00019 #include "llvm/Support/CallSite.h" 00020 #include "llvm/Transforms/IPO.h" 00021 using namespace llvm; 00022 00023 namespace { 00024 struct ArgInfo { 00025 unsigned ConstantWeight; 00026 unsigned AllocaWeight; 00027 00028 ArgInfo(unsigned CWeight, unsigned AWeight) 00029 : ConstantWeight(CWeight), AllocaWeight(AWeight) {} 00030 }; 00031 00032 // FunctionInfo - For each function, calculate the size of it in blocks and 00033 // instructions. 00034 struct FunctionInfo { 00035 // HasAllocas - Keep track of whether or not a function contains an alloca 00036 // instruction that is not in the entry block of the function. Inlining 00037 // this call could cause us to blow out the stack, because the stack memory 00038 // would never be released. 00039 // 00040 // FIXME: LLVM needs a way of dealloca'ing memory, which would make this 00041 // irrelevant! 00042 // 00043 bool HasAllocas; 00044 00045 // NumInsts, NumBlocks - Keep track of how large each function is, which is 00046 // used to estimate the code size cost of inlining it. 00047 unsigned NumInsts, NumBlocks; 00048 00049 // ArgumentWeights - Each formal argument of the function is inspected to 00050 // see if it is used in any contexts where making it a constant or alloca 00051 // would reduce the code size. If so, we add some value to the argument 00052 // entry here. 00053 std::vector<ArgInfo> ArgumentWeights; 00054 00055 FunctionInfo() : HasAllocas(false), NumInsts(0), NumBlocks(0) {} 00056 00057 /// analyzeFunction - Fill in the current structure with information gleaned 00058 /// from the specified function. 00059 void analyzeFunction(Function *F); 00060 }; 00061 00062 class SimpleInliner : public Inliner { 00063 std::map<const Function*, FunctionInfo> CachedFunctionInfo; 00064 public: 00065 int getInlineCost(CallSite CS); 00066 }; 00067 RegisterOpt<SimpleInliner> X("inline", "Function Integration/Inlining"); 00068 } 00069 00070 ModulePass *llvm::createFunctionInliningPass() { return new SimpleInliner(); } 00071 00072 // CountCodeReductionForConstant - Figure out an approximation for how many 00073 // instructions will be constant folded if the specified value is constant. 00074 // 00075 static unsigned CountCodeReductionForConstant(Value *V) { 00076 unsigned Reduction = 0; 00077 for (Value::use_iterator UI = V->use_begin(), E = V->use_end(); UI != E; ++UI) 00078 if (isa<BranchInst>(*UI)) 00079 Reduction += 40; // Eliminating a conditional branch is a big win 00080 else if (SwitchInst *SI = dyn_cast<SwitchInst>(*UI)) 00081 // Eliminating a switch is a big win, proportional to the number of edges 00082 // deleted. 00083 Reduction += (SI->getNumSuccessors()-1) * 40; 00084 else if (CallInst *CI = dyn_cast<CallInst>(*UI)) { 00085 // Turning an indirect call into a direct call is a BIG win 00086 Reduction += CI->getCalledValue() == V ? 500 : 0; 00087 } else if (InvokeInst *II = dyn_cast<InvokeInst>(*UI)) { 00088 // Turning an indirect call into a direct call is a BIG win 00089 Reduction += II->getCalledValue() == V ? 500 : 0; 00090 } else { 00091 // Figure out if this instruction will be removed due to simple constant 00092 // propagation. 00093 Instruction &Inst = cast<Instruction>(**UI); 00094 bool AllOperandsConstant = true; 00095 for (unsigned i = 0, e = Inst.getNumOperands(); i != e; ++i) 00096 if (!isa<Constant>(Inst.getOperand(i)) && Inst.getOperand(i) != V) { 00097 AllOperandsConstant = false; 00098 break; 00099 } 00100 00101 if (AllOperandsConstant) { 00102 // We will get to remove this instruction... 00103 Reduction += 7; 00104 00105 // And any other instructions that use it which become constants 00106 // themselves. 00107 Reduction += CountCodeReductionForConstant(&Inst); 00108 } 00109 } 00110 00111 return Reduction; 00112 } 00113 00114 // CountCodeReductionForAlloca - Figure out an approximation of how much smaller 00115 // the function will be if it is inlined into a context where an argument 00116 // becomes an alloca. 00117 // 00118 static unsigned CountCodeReductionForAlloca(Value *V) { 00119 if (!isa<PointerType>(V->getType())) return 0; // Not a pointer 00120 unsigned Reduction = 0; 00121 for (Value::use_iterator UI = V->use_begin(), E = V->use_end(); UI != E;++UI){ 00122 Instruction *I = cast<Instruction>(*UI); 00123 if (isa<LoadInst>(I) || isa<StoreInst>(I)) 00124 Reduction += 10; 00125 else if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(I)) { 00126 // If the GEP has variable indices, we won't be able to do much with it. 00127 for (Instruction::op_iterator I = GEP->op_begin()+1, E = GEP->op_end(); 00128 I != E; ++I) 00129 if (!isa<Constant>(*I)) return 0; 00130 Reduction += CountCodeReductionForAlloca(GEP)+15; 00131 } else { 00132 // If there is some other strange instruction, we're not going to be able 00133 // to do much if we inline this. 00134 return 0; 00135 } 00136 } 00137 00138 return Reduction; 00139 } 00140 00141 /// analyzeFunction - Fill in the current structure with information gleaned 00142 /// from the specified function. 00143 void FunctionInfo::analyzeFunction(Function *F) { 00144 unsigned NumInsts = 0, NumBlocks = 0; 00145 00146 // Look at the size of the callee. Each basic block counts as 20 units, and 00147 // each instruction counts as 10. 00148 for (Function::const_iterator BB = F->begin(), E = F->end(); BB != E; ++BB) { 00149 for (BasicBlock::const_iterator II = BB->begin(), E = BB->end(); 00150 II != E; ++II) { 00151 if (!isa<DbgInfoIntrinsic>(II)) ++NumInsts; 00152 00153 // If there is an alloca in the body of the function, we cannot currently 00154 // inline the function without the risk of exploding the stack. 00155 if (isa<AllocaInst>(II) && BB != F->begin()) { 00156 HasAllocas = true; 00157 this->NumBlocks = this->NumInsts = 1; 00158 return; 00159 } 00160 } 00161 00162 ++NumBlocks; 00163 } 00164 00165 this->NumBlocks = NumBlocks; 00166 this->NumInsts = NumInsts; 00167 00168 // Check out all of the arguments to the function, figuring out how much 00169 // code can be eliminated if one of the arguments is a constant. 00170 for (Function::aiterator I = F->abegin(), E = F->aend(); I != E; ++I) 00171 ArgumentWeights.push_back(ArgInfo(CountCodeReductionForConstant(I), 00172 CountCodeReductionForAlloca(I))); 00173 } 00174 00175 00176 // getInlineCost - The heuristic used to determine if we should inline the 00177 // function call or not. 00178 // 00179 int SimpleInliner::getInlineCost(CallSite CS) { 00180 Instruction *TheCall = CS.getInstruction(); 00181 Function *Callee = CS.getCalledFunction(); 00182 const Function *Caller = TheCall->getParent()->getParent(); 00183 00184 // Don't inline a directly recursive call. 00185 if (Caller == Callee) return 2000000000; 00186 00187 // InlineCost - This value measures how good of an inline candidate this call 00188 // site is to inline. A lower inline cost make is more likely for the call to 00189 // be inlined. This value may go negative. 00190 // 00191 int InlineCost = 0; 00192 00193 // If there is only one call of the function, and it has internal linkage, 00194 // make it almost guaranteed to be inlined. 00195 // 00196 if (Callee->hasInternalLinkage() && Callee->hasOneUse()) 00197 InlineCost -= 30000; 00198 00199 // Get information about the callee... 00200 FunctionInfo &CalleeFI = CachedFunctionInfo[Callee]; 00201 00202 // If we haven't calculated this information yet, do so now. 00203 if (CalleeFI.NumBlocks == 0) 00204 CalleeFI.analyzeFunction(Callee); 00205 00206 // Don't inline calls to functions with allocas that are not in the entry 00207 // block of the function. 00208 if (CalleeFI.HasAllocas) 00209 return 2000000000; 00210 00211 // Add to the inline quality for properties that make the call valuable to 00212 // inline. This includes factors that indicate that the result of inlining 00213 // the function will be optimizable. Currently this just looks at arguments 00214 // passed into the function. 00215 // 00216 unsigned ArgNo = 0; 00217 for (CallSite::arg_iterator I = CS.arg_begin(), E = CS.arg_end(); 00218 I != E; ++I, ++ArgNo) { 00219 // Each argument passed in has a cost at both the caller and the callee 00220 // sides. This favors functions that take many arguments over functions 00221 // that take few arguments. 00222 InlineCost -= 20; 00223 00224 // If this is a function being passed in, it is very likely that we will be 00225 // able to turn an indirect function call into a direct function call. 00226 if (isa<Function>(I)) 00227 InlineCost -= 100; 00228 00229 // If an alloca is passed in, inlining this function is likely to allow 00230 // significant future optimization possibilities (like scalar promotion, and 00231 // scalarization), so encourage the inlining of the function. 00232 // 00233 else if (AllocaInst *AI = dyn_cast<AllocaInst>(I)) { 00234 if (ArgNo < CalleeFI.ArgumentWeights.size()) 00235 InlineCost -= CalleeFI.ArgumentWeights[ArgNo].AllocaWeight; 00236 00237 // If this is a constant being passed into the function, use the argument 00238 // weights calculated for the callee to determine how much will be folded 00239 // away with this information. 00240 } else if (isa<Constant>(I)) { 00241 if (ArgNo < CalleeFI.ArgumentWeights.size()) 00242 InlineCost -= CalleeFI.ArgumentWeights[ArgNo].ConstantWeight; 00243 } 00244 } 00245 00246 // Now that we have considered all of the factors that make the call site more 00247 // likely to be inlined, look at factors that make us not want to inline it. 00248 00249 // Don't inline into something too big, which would make it bigger. Here, we 00250 // count each basic block as a single unit. 00251 // 00252 InlineCost += Caller->size()/20; 00253 00254 00255 // Look at the size of the callee. Each basic block counts as 20 units, and 00256 // each instruction counts as 5. 00257 InlineCost += CalleeFI.NumInsts*5 + CalleeFI.NumBlocks*20; 00258 return InlineCost; 00259 } 00260