llvm/lib/Transforms/IPO/ArgumentPromotion.cpp

//===-- ArgumentPromotion.cpp - Promote by-reference arguments ------------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This pass promotes "by reference" arguments to be "by value" arguments.  In
// practice, this means looking for internal functions that have pointer
// arguments.  If it can prove, through the use of alias analysis, that an
// argument is *only* loaded, then it can pass the value into the function
// instead of the address of the value.  This can cause recursive simplification
// of code and lead to the elimination of allocas (especially in C++ template
// code like the STL).
//
// This pass also handles aggregate arguments that are passed into a function,
// scalarizing them if the elements of the aggregate are only loaded.  Note that
// it refuses to scalarize aggregates which would require passing in more than
// three operands to the function, because passing thousands of operands for a
// large array or structure is unprofitable!
//
// Note that this transformation could also be done for arguments that are only
// stored to (returning the value instead), but does not currently.  This case
// would be best handled when and if LLVM begins supporting multiple return
// values from functions.
//
//===----------------------------------------------------------------------===//

#define DEBUG_TYPE "argpromotion"
#include "llvm/Transforms/IPO.h"
#include "llvm/Constants.h"
#include "llvm/DerivedTypes.h"
#include "llvm/Module.h"
#include "llvm/CallGraphSCCPass.h"
#include "llvm/Instructions.h"
#include "llvm/ParameterAttributes.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Analysis/CallGraph.h"
#include "llvm/Target/TargetData.h"
#include "llvm/Support/CallSite.h"
#include "llvm/Support/CFG.h"
#include "llvm/Support/Debug.h"
#include "llvm/ADT/DepthFirstIterator.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/ADT/StringExtras.h"
#include "llvm/Support/Compiler.h"
#include <set>
using namespace llvm;

STATISTIC(NumArgumentsPromoted , "Number of pointer arguments promoted");
STATISTIC(NumAggregatesPromoted, "Number of aggregate arguments promoted");
STATISTIC(NumArgumentsDead     , "Number of dead pointer args eliminated");

namespace {
  /// ArgPromotion - The 'by reference' to 'by value' argument promotion pass.
  ///
  struct VISIBILITY_HIDDEN ArgPromotion : public CallGraphSCCPass {
    virtual void getAnalysisUsage(AnalysisUsage &AU) const {
      AU.addRequired<AliasAnalysis>();
      AU.addRequired<TargetData>();
      CallGraphSCCPass::getAnalysisUsage(AU);
    }

    virtual bool runOnSCC(const std::vector<CallGraphNode *> &SCC);
    static char ID; // Pass identification, replacement for typeid
    ArgPromotion() : CallGraphSCCPass((intptr_t)&ID) {}

  private:
    bool PromoteArguments(CallGraphNode *CGN);
    bool isSafeToPromoteArgument(Argument *Arg, bool isByVal) const;
    Function *DoPromotion(Function *F, 
                          SmallPtrSet<Argument*, 8> &ArgsToPromote);
  };

  char ArgPromotion::ID = 0;
  RegisterPass<ArgPromotion> X("argpromotion",
                               "Promote 'by reference' arguments to scalars");
}

Pass *llvm::createArgumentPromotionPass() {
  return new ArgPromotion();
}

bool ArgPromotion::runOnSCC(const std::vector<CallGraphNode *> &SCC) {
  bool Changed = false, LocalChange;

  do {  // Iterate until we stop promoting from this SCC.
    LocalChange = false;
    // Attempt to promote arguments from all functions in this SCC.
    for (unsigned i = 0, e = SCC.size(); i != e; ++i)
      LocalChange |= PromoteArguments(SCC[i]);
    Changed |= LocalChange;               // Remember that we changed something.
  } while (LocalChange);

  return Changed;
}

/// PromoteArguments - This method checks the specified function to see if there
/// are any promotable arguments and if it is safe to promote the function (for
/// example, all callers are direct).  If safe to promote some arguments, it
/// calls the DoPromotion method.
///
bool ArgPromotion::PromoteArguments(CallGraphNode *CGN) {
  Function *F = CGN->getFunction();

  // Make sure that it is local to this module.
  if (!F || !F->hasInternalLinkage()) return false;

  // First check: see if there are any pointer arguments!  If not, quick exit.
  SmallVector<std::pair<Argument*, unsigned>, 16> PointerArgs;
  unsigned ArgNo = 0;
  for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end();
       I != E; ++I, ++ArgNo)
    if (isa<PointerType>(I->getType()))
      PointerArgs.push_back(std::pair<Argument*, unsigned>(I, ArgNo));
  if (PointerArgs.empty()) return false;

  // Second check: make sure that all callers are direct callers.  We can't
  // transform functions that have indirect callers.
  for (Value::use_iterator UI = F->use_begin(), E = F->use_end();
       UI != E; ++UI) {
    CallSite CS = CallSite::get(*UI);
    if (!CS.getInstruction())       // "Taking the address" of the function
      return false;

    // Ensure that this call site is CALLING the function, not passing it as
    // an argument.
    if (UI.getOperandNo() != 0) 
      return false;
  }

  // Check to see which arguments are promotable.  If an argument is promotable,
  // add it to ArgsToPromote.
  SmallPtrSet<Argument*, 8> ArgsToPromote;
  for (unsigned i = 0; i != PointerArgs.size(); ++i) {
    bool isByVal = F->paramHasAttr(PointerArgs[i].second, ParamAttr::ByVal);
    if (isSafeToPromoteArgument(PointerArgs[i].first, isByVal))
      ArgsToPromote.insert(PointerArgs[i].first);
  }
  
  // No promotable pointer arguments.
  if (ArgsToPromote.empty()) return false;

  Function *NewF = DoPromotion(F, ArgsToPromote);

  // Update the call graph to know that the function has been transformed.
  getAnalysis<CallGraph>().changeFunction(F, NewF);
  return true;
}

/// IsAlwaysValidPointer - Return true if the specified pointer is always legal
/// to load.
static bool IsAlwaysValidPointer(Value *V) {
  if (isa<AllocaInst>(V) || isa<GlobalVariable>(V)) return true;
  if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(V))
    return IsAlwaysValidPointer(GEP->getOperand(0));
  if (ConstantExpr *CE = dyn_cast<ConstantExpr>(V))
    if (CE->getOpcode() == Instruction::GetElementPtr)
      return IsAlwaysValidPointer(CE->getOperand(0));

  return false;
}

/// AllCalleesPassInValidPointerForArgument - Return true if we can prove that
/// all callees pass in a valid pointer for the specified function argument.
static bool AllCalleesPassInValidPointerForArgument(Argument *Arg) {
  Function *Callee = Arg->getParent();

  unsigned ArgNo = std::distance(Callee->arg_begin(),
                                 Function::arg_iterator(Arg));

  // Look at all call sites of the function.  At this pointer we know we only
  // have direct callees.
  for (Value::use_iterator UI = Callee->use_begin(), E = Callee->use_end();
       UI != E; ++UI) {
    CallSite CS = CallSite::get(*UI);
    assert(CS.getInstruction() && "Should only have direct calls!");

    if (!IsAlwaysValidPointer(CS.getArgument(ArgNo)))
      return false;
  }
  return true;
}


/// isSafeToPromoteArgument - As you might guess from the name of this method,
/// it checks to see if it is both safe and useful to promote the argument.
/// This method limits promotion of aggregates to only promote up to three
/// elements of the aggregate in order to avoid exploding the number of
/// arguments passed in.
bool ArgPromotion::isSafeToPromoteArgument(Argument *Arg, bool isByVal) const {
  // We can only promote this argument if all of the uses are loads, or are GEP
  // instructions (with constant indices) that are subsequently loaded.

  // We can also only promote the load if we can guarantee that it will happen.
  // Promoting a load causes the load to be unconditionally executed in the
  // caller, so we can't turn a conditional load into an unconditional load in
  // general.
  bool SafeToUnconditionallyLoad = false;
  if (isByVal)   // ByVal arguments are always safe to load from.
    SafeToUnconditionallyLoad = true;
  
  BasicBlock *EntryBlock = Arg->getParent()->begin();
  SmallVector<LoadInst*, 16> Loads;
  std::vector<SmallVector<ConstantInt*, 8> > GEPIndices;
  for (Value::use_iterator UI = Arg->use_begin(), E = Arg->use_end();
       UI != E; ++UI)
    if (LoadInst *LI = dyn_cast<LoadInst>(*UI)) {
      if (LI->isVolatile()) return false;  // Don't hack volatile loads
      Loads.push_back(LI);
      
      // If this load occurs in the entry block, then the pointer is 
      // unconditionally loaded.
      SafeToUnconditionallyLoad |= LI->getParent() == EntryBlock;
    } else if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(*UI)) {
      if (GEP->use_empty()) {
        // Dead GEP's cause trouble later.  Just remove them if we run into
        // them.
        getAnalysis<AliasAnalysis>().deleteValue(GEP);
        GEP->eraseFromParent();
        return isSafeToPromoteArgument(Arg, isByVal);
      }
      // Ensure that all of the indices are constants.
      SmallVector<ConstantInt*, 8> Operands;
      for (unsigned i = 1, e = GEP->getNumOperands(); i != e; ++i)
        if (ConstantInt *C = dyn_cast<ConstantInt>(GEP->getOperand(i)))
          Operands.push_back(C);
        else
          return false;  // Not a constant operand GEP!

      // Ensure that the only users of the GEP are load instructions.
      for (Value::use_iterator UI = GEP->use_begin(), E = GEP->use_end();
           UI != E; ++UI)
        if (LoadInst *LI = dyn_cast<LoadInst>(*UI)) {
          if (LI->isVolatile()) return false;  // Don't hack volatile loads
          Loads.push_back(LI);
          
          // If this load occurs in the entry block, then the pointer is 
          // unconditionally loaded.
          SafeToUnconditionallyLoad |= LI->getParent() == EntryBlock;
        } else {
          return false;
        }

      // See if there is already a GEP with these indices.  If not, check to
      // make sure that we aren't promoting too many elements.  If so, nothing
      // to do.
      if (std::find(GEPIndices.begin(), GEPIndices.end(), Operands) ==
          GEPIndices.end()) {
        if (GEPIndices.size() == 3) {
          DOUT << "argpromotion disable promoting argument '"
               << Arg->getName() << "' because it would require adding more "
               << "than 3 arguments to the function.\n";
          // We limit aggregate promotion to only promoting up to three elements
          // of the aggregate.
          return false;
        }
        GEPIndices.push_back(Operands);
      }
    } else {
      return false;  // Not a load or a GEP.
    }

  if (Loads.empty()) return true;  // No users, this is a dead argument.

  // If we decide that we want to promote this argument, the value is going to
  // be unconditionally loaded in all callees.  This is only safe to do if the
  // pointer was going to be unconditionally loaded anyway (i.e. there is a load
  // of the pointer in the entry block of the function) or if we can prove that
  // all pointers passed in are always to legal locations (for example, no null
  // pointers are passed in, no pointers to free'd memory, etc).
  if (!SafeToUnconditionallyLoad &&
      !AllCalleesPassInValidPointerForArgument(Arg))
    return false;   // Cannot prove that this is safe!!

  // Okay, now we know that the argument is only used by load instructions and
  // it is safe to unconditionally load the pointer.  Use alias analysis to
  // check to see if the pointer is guaranteed to not be modified from entry of
  // the function to each of the load instructions.

  // Because there could be several/many load instructions, remember which
  // blocks we know to be transparent to the load.
  SmallPtrSet<BasicBlock*, 16> TranspBlocks;

  AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
  TargetData &TD = getAnalysis<TargetData>();

  for (unsigned i = 0, e = Loads.size(); i != e; ++i) {
    // Check to see if the load is invalidated from the start of the block to
    // the load itself.
    LoadInst *Load = Loads[i];
    BasicBlock *BB = Load->getParent();

    const PointerType *LoadTy =
      cast<PointerType>(Load->getOperand(0)->getType());
    unsigned LoadSize = (unsigned)TD.getTypeStoreSize(LoadTy->getElementType());

    if (AA.canInstructionRangeModify(BB->front(), *Load, Arg, LoadSize))
      return false;  // Pointer is invalidated!

    // Now check every path from the entry block to the load for transparency.
    // To do this, we perform a depth first search on the inverse CFG from the
    // loading block.
    for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI)
      for (idf_ext_iterator<BasicBlock*, SmallPtrSet<BasicBlock*, 16> >
             I = idf_ext_begin(*PI, TranspBlocks),
             E = idf_ext_end(*PI, TranspBlocks); I != E; ++I)
        if (AA.canBasicBlockModify(**I, Arg, LoadSize))
          return false;
  }

  // If the path from the entry of the function to each load is free of
  // instructions that potentially invalidate the load, we can make the
  // transformation!
  return true;
}

namespace {
  /// GEPIdxComparator - Provide a strong ordering for GEP indices.  All Value*
  /// elements are instances of ConstantInt.
  ///
  struct GEPIdxComparator {
    bool operator()(const std::vector<Value*> &LHS,
                    const std::vector<Value*> &RHS) const {
      unsigned idx = 0;
      for (; idx < LHS.size() && idx < RHS.size(); ++idx) {
        if (LHS[idx] != RHS[idx]) {
          return cast<ConstantInt>(LHS[idx])->getZExtValue() <
                 cast<ConstantInt>(RHS[idx])->getZExtValue();
        }
      }

      // Return less than if we ran out of stuff in LHS and we didn't run out of
      // stuff in RHS.
      return idx == LHS.size() && idx != RHS.size();
    }
  };
}


/// DoPromotion - This method actually performs the promotion of the specified
/// arguments, and returns the new function.  At this point, we know that it's
/// safe to do so.
Function *ArgPromotion::DoPromotion(Function *F,
                                    SmallPtrSet<Argument*, 8> &ArgsToPromote) {

  // Start by computing a new prototype for the function, which is the same as
  // the old function, but has modified arguments.
  const FunctionType *FTy = F->getFunctionType();
  std::vector<const Type*> Params;

  typedef std::set<std::vector<Value*>, GEPIdxComparator> ScalarizeTable;

  // ScalarizedElements - If we are promoting a pointer that has elements
  // accessed out of it, keep track of which elements are accessed so that we
  // can add one argument for each.
  //
  // Arguments that are directly loaded will have a zero element value here, to
  // handle cases where there are both a direct load and GEP accesses.
  //
  std::map<Argument*, ScalarizeTable> ScalarizedElements;

  // OriginalLoads - Keep track of a representative load instruction from the
  // original function so that we can tell the alias analysis implementation
  // what the new GEP/Load instructions we are inserting look like.
  std::map<std::vector<Value*>, LoadInst*> OriginalLoads;

  // ParamAttrs - Keep track of the parameter attributes for the arguments
  // that we are *not* promoting. For the ones that we do promote, the parameter
  // attributes are lost
  ParamAttrsVector ParamAttrsVec;
  const ParamAttrsList *PAL = F->getParamAttrs();

  unsigned index = 1;
  for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end(); I != E;
       ++I, ++index)
    if (!ArgsToPromote.count(I)) {
      Params.push_back(I->getType());
      if (PAL) {
        unsigned attrs = PAL->getParamAttrs(index);
        if (attrs)
          ParamAttrsVec.push_back(ParamAttrsWithIndex::get(Params.size(),
                                  attrs));
      }
    } else if (I->use_empty()) {
      ++NumArgumentsDead;
    } else {
      // Okay, this is being promoted.  Check to see if there are any GEP uses
      // of the argument.
      ScalarizeTable &ArgIndices = ScalarizedElements[I];
      for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); UI != E;
           ++UI) {
        Instruction *User = cast<Instruction>(*UI);
        assert(isa<LoadInst>(User) || isa<GetElementPtrInst>(User));
        std::vector<Value*> Indices(User->op_begin()+1, User->op_end());
        ArgIndices.insert(Indices);
        LoadInst *OrigLoad;
        if (LoadInst *L = dyn_cast<LoadInst>(User))
          OrigLoad = L;
        else
          OrigLoad = cast<LoadInst>(User->use_back());
        OriginalLoads[Indices] = OrigLoad;
      }

      // Add a parameter to the function for each element passed in.
      for (ScalarizeTable::iterator SI = ArgIndices.begin(),
             E = ArgIndices.end(); SI != E; ++SI)
        Params.push_back(GetElementPtrInst::getIndexedType(I->getType(),
                                                           SI->begin(),
                                                           SI->end()));

      if (ArgIndices.size() == 1 && ArgIndices.begin()->empty())
        ++NumArgumentsPromoted;
      else
        ++NumAggregatesPromoted;
    }

  const Type *RetTy = FTy->getReturnType();

  // Recompute the parameter attributes list based on the new arguments for
  // the function.
  if (ParamAttrsVec.empty())
    PAL = 0;
  else
    PAL = ParamAttrsList::get(ParamAttrsVec);

  // Work around LLVM bug PR56: the CWriter cannot emit varargs functions which
  // have zero fixed arguments.
  bool ExtraArgHack = false;
  if (Params.empty() && FTy->isVarArg()) {
    ExtraArgHack = true;
    Params.push_back(Type::Int32Ty);
  }

  // Construct the new function type using the new arguments.
  FunctionType *NFTy = FunctionType::get(RetTy, Params, FTy->isVarArg());

  // Create the new function body and insert it into the module...
  Function *NF = new Function(NFTy, F->getLinkage(), F->getName());
  NF->setCallingConv(F->getCallingConv());
  NF->setParamAttrs(PAL);
  if (F->hasCollector())
    NF->setCollector(F->getCollector());
  F->getParent()->getFunctionList().insert(F, NF);

  // Get the alias analysis information that we need to update to reflect our
  // changes.
  AliasAnalysis &AA = getAnalysis<AliasAnalysis>();

  // Loop over all of the callers of the function, transforming the call sites
  // to pass in the loaded pointers.
  //
  std::vector<Value*> Args;
  while (!F->use_empty()) {
    CallSite CS = CallSite::get(F->use_back());
    Instruction *Call = CS.getInstruction();

    // Loop over the operands, inserting GEP and loads in the caller as
    // appropriate.
    CallSite::arg_iterator AI = CS.arg_begin();
    for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end();
         I != E; ++I, ++AI)
      if (!ArgsToPromote.count(I))
        Args.push_back(*AI);          // Unmodified argument
      else if (!I->use_empty()) {
        // Non-dead argument: insert GEPs and loads as appropriate.
        ScalarizeTable &ArgIndices = ScalarizedElements[I];
        for (ScalarizeTable::iterator SI = ArgIndices.begin(),
               E = ArgIndices.end(); SI != E; ++SI) {
          Value *V = *AI;
          LoadInst *OrigLoad = OriginalLoads[*SI];
          if (!SI->empty()) {
            V = new GetElementPtrInst(V, SI->begin(), SI->end(),
                                      V->getName()+".idx", Call);
            AA.copyValue(OrigLoad->getOperand(0), V);
          }
          Args.push_back(new LoadInst(V, V->getName()+".val", Call));
          AA.copyValue(OrigLoad, Args.back());
        }
      }

    if (ExtraArgHack)
      Args.push_back(Constant::getNullValue(Type::Int32Ty));

    // Push any varargs arguments on the list
    for (; AI != CS.arg_end(); ++AI)
      Args.push_back(*AI);

    Instruction *New;
    if (InvokeInst *II = dyn_cast<InvokeInst>(Call)) {
      New = new InvokeInst(NF, II->getNormalDest(), II->getUnwindDest(),
                           Args.begin(), Args.end(), "", Call);
      cast<InvokeInst>(New)->setCallingConv(CS.getCallingConv());
      cast<InvokeInst>(New)->setParamAttrs(PAL);
    } else {
      New = new CallInst(NF, Args.begin(), Args.end(), "", Call);
      cast<CallInst>(New)->setCallingConv(CS.getCallingConv());
      cast<CallInst>(New)->setParamAttrs(PAL);
      if (cast<CallInst>(Call)->isTailCall())
        cast<CallInst>(New)->setTailCall();
    }
    Args.clear();

    // Update the alias analysis implementation to know that we are replacing
    // the old call with a new one.
    AA.replaceWithNewValue(Call, New);

    if (!Call->use_empty()) {
      Call->replaceAllUsesWith(New);
      New->takeName(Call);
    }

    // Finally, remove the old call from the program, reducing the use-count of
    // F.
    Call->eraseFromParent();
  }

  // Since we have now created the new function, splice the body of the old
  // function right into the new function, leaving the old rotting hulk of the
  // function empty.
  NF->getBasicBlockList().splice(NF->begin(), F->getBasicBlockList());

  // Loop over the argument list, transfering uses of the old arguments over to
  // the new arguments, also transfering over the names as well.
  //
  for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end(),
       I2 = NF->arg_begin(); I != E; ++I)
    if (!ArgsToPromote.count(I)) {
      // If this is an unmodified argument, move the name and users over to the
      // new version.
      I->replaceAllUsesWith(I2);
      I2->takeName(I);
      AA.replaceWithNewValue(I, I2);
      ++I2;
    } else if (I->use_empty()) {
      AA.deleteValue(I);
    } else {
      // Otherwise, if we promoted this argument, then all users are load
      // instructions, and all loads should be using the new argument that we
      // added.
      ScalarizeTable &ArgIndices = ScalarizedElements[I];

      while (!I->use_empty()) {
        if (LoadInst *LI = dyn_cast<LoadInst>(I->use_back())) {
          assert(ArgIndices.begin()->empty() &&
                 "Load element should sort to front!");
          I2->setName(I->getName()+".val");
          LI->replaceAllUsesWith(I2);
          AA.replaceWithNewValue(LI, I2);
          LI->eraseFromParent();
          DOUT << "*** Promoted load of argument '" << I->getName()
               << "' in function '" << F->getName() << "'\n";
        } else {
          GetElementPtrInst *GEP = cast<GetElementPtrInst>(I->use_back());
          std::vector<Value*> Operands(GEP->op_begin()+1, GEP->op_end());

          Function::arg_iterator TheArg = I2;
          for (ScalarizeTable::iterator It = ArgIndices.begin();
               *It != Operands; ++It, ++TheArg) {
            assert(It != ArgIndices.end() && "GEP not handled??");
          }

          std::string NewName = I->getName();
          for (unsigned i = 0, e = Operands.size(); i != e; ++i)
            if (ConstantInt *CI = dyn_cast<ConstantInt>(Operands[i]))
              NewName += "." + CI->getValue().toStringUnsigned(10);
            else
              NewName += ".x";
          TheArg->setName(NewName+".val");

          DOUT << "*** Promoted agg argument '" << TheArg->getName()
               << "' of function '" << F->getName() << "'\n";

          // All of the uses must be load instructions.  Replace them all with
          // the argument specified by ArgNo.
          while (!GEP->use_empty()) {
            LoadInst *L = cast<LoadInst>(GEP->use_back());
            L->replaceAllUsesWith(TheArg);
            AA.replaceWithNewValue(L, TheArg);
            L->eraseFromParent();
          }
          AA.deleteValue(GEP);
          GEP->eraseFromParent();
        }
      }

      // Increment I2 past all of the arguments added for this promoted pointer.
      for (unsigned i = 0, e = ArgIndices.size(); i != e; ++i)
        ++I2;
    }

  // Notify the alias analysis implementation that we inserted a new argument.
  if (ExtraArgHack)
    AA.copyValue(Constant::getNullValue(Type::Int32Ty), NF->arg_begin());


  // Tell the alias analysis that the old function is about to disappear.
  AA.replaceWithNewValue(F, NF);

  // Now that the old function is dead, delete it.
  F->eraseFromParent();
  return NF;
}