LLVM API Documentation

InlineSimple.cpp

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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/CallingConv.h"
00016 #include "llvm/Instructions.h"
00017 #include "llvm/IntrinsicInst.h"
00018 #include "llvm/Function.h"
00019 #include "llvm/Type.h"
00020 #include "llvm/Support/CallSite.h"
00021 #include "llvm/Transforms/IPO.h"
00022 using namespace llvm;
00023 
00024 namespace {
00025   struct ArgInfo {
00026     unsigned ConstantWeight;
00027     unsigned AllocaWeight;
00028 
00029     ArgInfo(unsigned CWeight, unsigned AWeight)
00030       : ConstantWeight(CWeight), AllocaWeight(AWeight) {}
00031   };
00032 
00033   // FunctionInfo - For each function, calculate the size of it in blocks and
00034   // instructions.
00035   struct FunctionInfo {
00036     // NumInsts, NumBlocks - Keep track of how large each function is, which is
00037     // used to estimate the code size cost of inlining it.
00038     unsigned NumInsts, NumBlocks;
00039 
00040     // ArgumentWeights - Each formal argument of the function is inspected to
00041     // see if it is used in any contexts where making it a constant or alloca
00042     // would reduce the code size.  If so, we add some value to the argument
00043     // entry here.
00044     std::vector<ArgInfo> ArgumentWeights;
00045 
00046     FunctionInfo() : NumInsts(0), NumBlocks(0) {}
00047 
00048     /// analyzeFunction - Fill in the current structure with information gleaned
00049     /// from the specified function.
00050     void analyzeFunction(Function *F);
00051   };
00052 
00053   class SimpleInliner : public Inliner {
00054     std::map<const Function*, FunctionInfo> CachedFunctionInfo;
00055   public:
00056     int getInlineCost(CallSite CS);
00057   };
00058   RegisterOpt<SimpleInliner> X("inline", "Function Integration/Inlining");
00059 }
00060 
00061 ModulePass *llvm::createFunctionInliningPass() { return new SimpleInliner(); }
00062 
00063 // CountCodeReductionForConstant - Figure out an approximation for how many
00064 // instructions will be constant folded if the specified value is constant.
00065 //
00066 static unsigned CountCodeReductionForConstant(Value *V) {
00067   unsigned Reduction = 0;
00068   for (Value::use_iterator UI = V->use_begin(), E = V->use_end(); UI != E; ++UI)
00069     if (isa<BranchInst>(*UI))
00070       Reduction += 40;          // Eliminating a conditional branch is a big win
00071     else if (SwitchInst *SI = dyn_cast<SwitchInst>(*UI))
00072       // Eliminating a switch is a big win, proportional to the number of edges
00073       // deleted.
00074       Reduction += (SI->getNumSuccessors()-1) * 40;
00075     else if (CallInst *CI = dyn_cast<CallInst>(*UI)) {
00076       // Turning an indirect call into a direct call is a BIG win
00077       Reduction += CI->getCalledValue() == V ? 500 : 0;
00078     } else if (InvokeInst *II = dyn_cast<InvokeInst>(*UI)) {
00079       // Turning an indirect call into a direct call is a BIG win
00080       Reduction += II->getCalledValue() == V ? 500 : 0;
00081     } else {
00082       // Figure out if this instruction will be removed due to simple constant
00083       // propagation.
00084       Instruction &Inst = cast<Instruction>(**UI);
00085       bool AllOperandsConstant = true;
00086       for (unsigned i = 0, e = Inst.getNumOperands(); i != e; ++i)
00087         if (!isa<Constant>(Inst.getOperand(i)) && Inst.getOperand(i) != V) {
00088           AllOperandsConstant = false;
00089           break;
00090         }
00091 
00092       if (AllOperandsConstant) {
00093         // We will get to remove this instruction...
00094         Reduction += 7;
00095 
00096         // And any other instructions that use it which become constants
00097         // themselves.
00098         Reduction += CountCodeReductionForConstant(&Inst);
00099       }
00100     }
00101 
00102   return Reduction;
00103 }
00104 
00105 // CountCodeReductionForAlloca - Figure out an approximation of how much smaller
00106 // the function will be if it is inlined into a context where an argument
00107 // becomes an alloca.
00108 //
00109 static unsigned CountCodeReductionForAlloca(Value *V) {
00110   if (!isa<PointerType>(V->getType())) return 0;  // Not a pointer
00111   unsigned Reduction = 0;
00112   for (Value::use_iterator UI = V->use_begin(), E = V->use_end(); UI != E;++UI){
00113     Instruction *I = cast<Instruction>(*UI);
00114     if (isa<LoadInst>(I) || isa<StoreInst>(I))
00115       Reduction += 10;
00116     else if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(I)) {
00117       // If the GEP has variable indices, we won't be able to do much with it.
00118       for (Instruction::op_iterator I = GEP->op_begin()+1, E = GEP->op_end();
00119            I != E; ++I)
00120         if (!isa<Constant>(*I)) return 0;
00121       Reduction += CountCodeReductionForAlloca(GEP)+15;
00122     } else {
00123       // If there is some other strange instruction, we're not going to be able
00124       // to do much if we inline this.
00125       return 0;
00126     }
00127   }
00128 
00129   return Reduction;
00130 }
00131 
00132 /// analyzeFunction - Fill in the current structure with information gleaned
00133 /// from the specified function.
00134 void FunctionInfo::analyzeFunction(Function *F) {
00135   unsigned NumInsts = 0, NumBlocks = 0;
00136 
00137   // Look at the size of the callee.  Each basic block counts as 20 units, and
00138   // each instruction counts as 10.
00139   for (Function::const_iterator BB = F->begin(), E = F->end(); BB != E; ++BB) {
00140     for (BasicBlock::const_iterator II = BB->begin(), E = BB->end();
00141          II != E; ++II)
00142       if (!isa<DbgInfoIntrinsic>(II)) 
00143         ++NumInsts;
00144 
00145     ++NumBlocks;
00146   }
00147 
00148   this->NumBlocks = NumBlocks;
00149   this->NumInsts  = NumInsts;
00150 
00151   // Check out all of the arguments to the function, figuring out how much
00152   // code can be eliminated if one of the arguments is a constant.
00153   for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end(); I != E; ++I)
00154     ArgumentWeights.push_back(ArgInfo(CountCodeReductionForConstant(I),
00155                                       CountCodeReductionForAlloca(I)));
00156 }
00157 
00158 
00159 // getInlineCost - The heuristic used to determine if we should inline the
00160 // function call or not.
00161 //
00162 int SimpleInliner::getInlineCost(CallSite CS) {
00163   Instruction *TheCall = CS.getInstruction();
00164   Function *Callee = CS.getCalledFunction();
00165   const Function *Caller = TheCall->getParent()->getParent();
00166 
00167   // Don't inline a directly recursive call.
00168   if (Caller == Callee) return 2000000000;
00169 
00170   // InlineCost - This value measures how good of an inline candidate this call
00171   // site is to inline.  A lower inline cost make is more likely for the call to
00172   // be inlined.  This value may go negative.
00173   //
00174   int InlineCost = 0;
00175 
00176   // If there is only one call of the function, and it has internal linkage,
00177   // make it almost guaranteed to be inlined.
00178   //
00179   if (Callee->hasInternalLinkage() && Callee->hasOneUse())
00180     InlineCost -= 30000;
00181 
00182   // If this function uses the coldcc calling convention, prefer not to inline
00183   // it.
00184   if (Callee->getCallingConv() == CallingConv::Cold)
00185     InlineCost += 2000;
00186 
00187   // If the instruction after the call, or if the normal destination of the
00188   // invoke is an unreachable instruction, the function is noreturn.  As such,
00189   // there is little point in inlining this.
00190   if (InvokeInst *II = dyn_cast<InvokeInst>(TheCall)) {
00191     if (isa<UnreachableInst>(II->getNormalDest()->begin()))
00192       InlineCost += 10000;
00193   } else if (isa<UnreachableInst>(++BasicBlock::iterator(TheCall)))
00194     InlineCost += 10000;
00195 
00196   // Get information about the callee...
00197   FunctionInfo &CalleeFI = CachedFunctionInfo[Callee];
00198 
00199   // If we haven't calculated this information yet, do so now.
00200   if (CalleeFI.NumBlocks == 0)
00201     CalleeFI.analyzeFunction(Callee);
00202 
00203   // Add to the inline quality for properties that make the call valuable to
00204   // inline.  This includes factors that indicate that the result of inlining
00205   // the function will be optimizable.  Currently this just looks at arguments
00206   // passed into the function.
00207   //
00208   unsigned ArgNo = 0;
00209   for (CallSite::arg_iterator I = CS.arg_begin(), E = CS.arg_end();
00210        I != E; ++I, ++ArgNo) {
00211     // Each argument passed in has a cost at both the caller and the callee
00212     // sides.  This favors functions that take many arguments over functions
00213     // that take few arguments.
00214     InlineCost -= 20;
00215 
00216     // If this is a function being passed in, it is very likely that we will be
00217     // able to turn an indirect function call into a direct function call.
00218     if (isa<Function>(I))
00219       InlineCost -= 100;
00220 
00221     // If an alloca is passed in, inlining this function is likely to allow
00222     // significant future optimization possibilities (like scalar promotion, and
00223     // scalarization), so encourage the inlining of the function.
00224     //
00225     else if (AllocaInst *AI = dyn_cast<AllocaInst>(I)) {
00226       if (ArgNo < CalleeFI.ArgumentWeights.size())
00227         InlineCost -= CalleeFI.ArgumentWeights[ArgNo].AllocaWeight;
00228 
00229     // If this is a constant being passed into the function, use the argument
00230     // weights calculated for the callee to determine how much will be folded
00231     // away with this information.
00232     } else if (isa<Constant>(I)) {
00233       if (ArgNo < CalleeFI.ArgumentWeights.size())
00234         InlineCost -= CalleeFI.ArgumentWeights[ArgNo].ConstantWeight;
00235     }
00236   }
00237 
00238   // Now that we have considered all of the factors that make the call site more
00239   // likely to be inlined, look at factors that make us not want to inline it.
00240 
00241   // Don't inline into something too big, which would make it bigger.  Here, we
00242   // count each basic block as a single unit.
00243   //
00244   InlineCost += Caller->size()/20;
00245 
00246 
00247   // Look at the size of the callee.  Each basic block counts as 20 units, and
00248   // each instruction counts as 5.
00249   InlineCost += CalleeFI.NumInsts*5 + CalleeFI.NumBlocks*20;
00250   return InlineCost;
00251 }
00252