LLVM API Documentation

InlineCost.cpp

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//===- InlineCoast.cpp - Cost analysis for inliner ------------------------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file implements inline cost analysis.
//
//===----------------------------------------------------------------------===//


#include "llvm/Transforms/Utils/InlineCost.h"
#include "llvm/Support/CallSite.h"
#include "llvm/CallingConv.h"
#include "llvm/IntrinsicInst.h"

using namespace llvm;

// CountCodeReductionForConstant - Figure out an approximation for how many
// instructions will be constant folded if the specified value is constant.
//
unsigned InlineCostAnalyzer::FunctionInfo::
         CountCodeReductionForConstant(Value *V) {
  unsigned Reduction = 0;
  for (Value::use_iterator UI = V->use_begin(), E = V->use_end(); UI != E; ++UI)
    if (isa<BranchInst>(*UI))
      Reduction += 40;          // Eliminating a conditional branch is a big win
    else if (SwitchInst *SI = dyn_cast<SwitchInst>(*UI))
      // Eliminating a switch is a big win, proportional to the number of edges
      // deleted.
      Reduction += (SI->getNumSuccessors()-1) * 40;
    else if (CallInst *CI = dyn_cast<CallInst>(*UI)) {
      // Turning an indirect call into a direct call is a BIG win
      Reduction += CI->getCalledValue() == V ? 500 : 0;
    } else if (InvokeInst *II = dyn_cast<InvokeInst>(*UI)) {
      // Turning an indirect call into a direct call is a BIG win
      Reduction += II->getCalledValue() == V ? 500 : 0;
    } else {
      // Figure out if this instruction will be removed due to simple constant
      // propagation.
      Instruction &Inst = cast<Instruction>(**UI);
      bool AllOperandsConstant = true;
      for (unsigned i = 0, e = Inst.getNumOperands(); i != e; ++i)
        if (!isa<Constant>(Inst.getOperand(i)) && Inst.getOperand(i) != V) {
          AllOperandsConstant = false;
          break;
        }

      if (AllOperandsConstant) {
        // We will get to remove this instruction...
        Reduction += 7;

        // And any other instructions that use it which become constants
        // themselves.
        Reduction += CountCodeReductionForConstant(&Inst);
      }
    }

  return Reduction;
}

// CountCodeReductionForAlloca - Figure out an approximation of how much smaller
// the function will be if it is inlined into a context where an argument
// becomes an alloca.
//
unsigned InlineCostAnalyzer::FunctionInfo::
         CountCodeReductionForAlloca(Value *V) {
  if (!isa<PointerType>(V->getType())) return 0;  // Not a pointer
  unsigned Reduction = 0;
  for (Value::use_iterator UI = V->use_begin(), E = V->use_end(); UI != E;++UI){
    Instruction *I = cast<Instruction>(*UI);
    if (isa<LoadInst>(I) || isa<StoreInst>(I))
      Reduction += 10;
    else if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(I)) {
      // If the GEP has variable indices, we won't be able to do much with it.
      for (Instruction::op_iterator I = GEP->op_begin()+1, E = GEP->op_end();
           I != E; ++I)
        if (!isa<Constant>(*I)) return 0;
      Reduction += CountCodeReductionForAlloca(GEP)+15;
    } else {
      // If there is some other strange instruction, we're not going to be able
      // to do much if we inline this.
      return 0;
    }
  }

  return Reduction;
}

/// analyzeFunction - Fill in the current structure with information gleaned
/// from the specified function.
void InlineCostAnalyzer::FunctionInfo::analyzeFunction(Function *F) {
  unsigned NumInsts = 0, NumBlocks = 0, NumVectorInsts = 0;

  // Look at the size of the callee.  Each basic block counts as 20 units, and
  // each instruction counts as 5.
  for (Function::const_iterator BB = F->begin(), E = F->end(); BB != E; ++BB) {
    for (BasicBlock::const_iterator II = BB->begin(), E = BB->end();
         II != E; ++II) {
      if (isa<PHINode>(II)) continue;           // PHI nodes don't count.

      // Special handling for calls.
      if (isa<CallInst>(II) || isa<InvokeInst>(II)) {
        if (isa<DbgInfoIntrinsic>(II))
          continue;  // Debug intrinsics don't count as size.
        
        CallSite CS = CallSite::get(const_cast<Instruction*>(&*II));
        
        // If this function contains a call to setjmp or _setjmp, never inline
        // it.  This is a hack because we depend on the user marking their local
        // variables as volatile if they are live across a setjmp call, and they
        // probably won't do this in callers.
        if (Function *F = CS.getCalledFunction())
          if (F->isDeclaration() && 
              (F->isName("setjmp") || F->isName("_setjmp"))) {
            NeverInline = true;
            return;
          }
        
        // Calls often compile into many machine instructions.  Bump up their
        // cost to reflect this.
        if (!isa<IntrinsicInst>(II))
          NumInsts += 5;
      }
      
      if (isa<ExtractElementInst>(II) || isa<VectorType>(II->getType()))
        ++NumVectorInsts; 
      
      // Noop casts, including ptr <-> int,  don't count.
      if (const CastInst *CI = dyn_cast<CastInst>(II)) {
        if (CI->isLosslessCast() || isa<IntToPtrInst>(CI) || 
            isa<PtrToIntInst>(CI))
          continue;
      } else if (const GetElementPtrInst *GEPI =
                 dyn_cast<GetElementPtrInst>(II)) {
        // If a GEP has all constant indices, it will probably be folded with
        // a load/store.
        bool AllConstant = true;
        for (unsigned i = 1, e = GEPI->getNumOperands(); i != e; ++i)
          if (!isa<ConstantInt>(GEPI->getOperand(i))) {
            AllConstant = false;
            break;
          }
        if (AllConstant) continue;
      }
      
      ++NumInsts;
    }

    ++NumBlocks;
  }

  this->NumBlocks      = NumBlocks;
  this->NumInsts       = NumInsts;
  this->NumVectorInsts = NumVectorInsts;

  // Check out all of the arguments to the function, figuring out how much
  // code can be eliminated if one of the arguments is a constant.
  for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end(); I != E; ++I)
    ArgumentWeights.push_back(ArgInfo(CountCodeReductionForConstant(I),
                                      CountCodeReductionForAlloca(I)));
}



// getInlineCost - The heuristic used to determine if we should inline the
// function call or not.
//
int InlineCostAnalyzer::getInlineCost(CallSite CS,
                               SmallPtrSet<const Function *, 16> &NeverInline) {
  Instruction *TheCall = CS.getInstruction();
  Function *Callee = CS.getCalledFunction();
  const Function *Caller = TheCall->getParent()->getParent();
  
  // Don't inline a directly recursive call.
  if (Caller == Callee ||
      // Don't inline functions which can be redefined at link-time to mean
      // something else.  link-once linkage is ok though.
      Callee->hasWeakLinkage() ||
      
      // Don't inline functions marked noinline.
      NeverInline.count(Callee))
    return 2000000000;
  
  // InlineCost - This value measures how good of an inline candidate this call
  // site is to inline.  A lower inline cost make is more likely for the call to
  // be inlined.  This value may go negative.
  //
  int InlineCost = 0;
  
  // If there is only one call of the function, and it has internal linkage,
  // make it almost guaranteed to be inlined.
  //
  if (Callee->hasInternalLinkage() && Callee->hasOneUse())
    InlineCost -= 15000;
  
  // If this function uses the coldcc calling convention, prefer not to inline
  // it.
  if (Callee->getCallingConv() == CallingConv::Cold)
    InlineCost += 2000;
  
  // If the instruction after the call, or if the normal destination of the
  // invoke is an unreachable instruction, the function is noreturn.  As such,
  // there is little point in inlining this.
  if (InvokeInst *II = dyn_cast<InvokeInst>(TheCall)) {
    if (isa<UnreachableInst>(II->getNormalDest()->begin()))
      InlineCost += 10000;
  } else if (isa<UnreachableInst>(++BasicBlock::iterator(TheCall)))
    InlineCost += 10000;
  
  // Get information about the callee...
  FunctionInfo &CalleeFI = CachedFunctionInfo[Callee];
  
  // If we haven't calculated this information yet, do so now.
  if (CalleeFI.NumBlocks == 0)
    CalleeFI.analyzeFunction(Callee);
  
  // If we should never inline this, return a huge cost.
  if (CalleeFI.NeverInline)
    return 2000000000;
    
  // Add to the inline quality for properties that make the call valuable to
  // inline.  This includes factors that indicate that the result of inlining
  // the function will be optimizable.  Currently this just looks at arguments
  // passed into the function.
  //
  unsigned ArgNo = 0;
  for (CallSite::arg_iterator I = CS.arg_begin(), E = CS.arg_end();
       I != E; ++I, ++ArgNo) {
    // Each argument passed in has a cost at both the caller and the callee
    // sides.  This favors functions that take many arguments over functions
    // that take few arguments.
    InlineCost -= 20;
    
    // If this is a function being passed in, it is very likely that we will be
    // able to turn an indirect function call into a direct function call.
    if (isa<Function>(I))
      InlineCost -= 100;
    
    // If an alloca is passed in, inlining this function is likely to allow
    // significant future optimization possibilities (like scalar promotion, and
    // scalarization), so encourage the inlining of the function.
    //
    else if (isa<AllocaInst>(I)) {
      if (ArgNo < CalleeFI.ArgumentWeights.size())
        InlineCost -= CalleeFI.ArgumentWeights[ArgNo].AllocaWeight;
      
      // If this is a constant being passed into the function, use the argument
      // weights calculated for the callee to determine how much will be folded
      // away with this information.
    } else if (isa<Constant>(I)) {
      if (ArgNo < CalleeFI.ArgumentWeights.size())
        InlineCost -= CalleeFI.ArgumentWeights[ArgNo].ConstantWeight;
    }
  }
  
  // Now that we have considered all of the factors that make the call site more
  // likely to be inlined, look at factors that make us not want to inline it.
  
  // Don't inline into something too big, which would make it bigger.
  //
  InlineCost += Caller->size()/15;
  
  // Look at the size of the callee. Each instruction counts as 5.
  InlineCost += CalleeFI.NumInsts*5;

  return InlineCost;
}

// getInlineFudgeFactor - Return a > 1.0 factor if the inliner should use a
// higher threshold to determine if the function call should be inlined.
float InlineCostAnalyzer::getInlineFudgeFactor(CallSite CS) {
  Function *Callee = CS.getCalledFunction();
  
  // Get information about the callee...
  FunctionInfo &CalleeFI = CachedFunctionInfo[Callee];
  
  // If we haven't calculated this information yet, do so now.
  if (CalleeFI.NumBlocks == 0)
    CalleeFI.analyzeFunction(Callee);

  float Factor = 1.0f;
  // Single BB functions are often written to be inlined.
  if (CalleeFI.NumBlocks == 1)
    Factor += 0.5f;

  // Be more aggressive if the function contains a good chunk (if it mades up
  // at least 10% of the instructions) of vector instructions.
  if (CalleeFI.NumVectorInsts > CalleeFI.NumInsts/2)
    Factor += 2.0f;
  else if (CalleeFI.NumVectorInsts > CalleeFI.NumInsts/10)
    Factor += 1.5f;
  return Factor;
}

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