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//===- SimplifyLibCalls.cpp - Optimize specific well-known library calls --===//
//
//                     The LLVM Compiler Infrastructure
//
// This file was developed by Reid Spencer and is distributed under the 
// University of Illinois Open Source License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file implements a variety of small optimizations for calls to specific
// well-known (e.g. runtime library) function calls. For example, a call to the
// function "exit(3)" that occurs within the main() function can be transformed
// into a simple "return 3" instruction. Any optimization that takes this form
// (replace call to library function with simpler code that provides same 
// result) belongs in this file. 
//
//===----------------------------------------------------------------------===//

#define DEBUG_TYPE "simplify-libcalls"
#include "llvm/Constants.h"
#include "llvm/DerivedTypes.h"
#include "llvm/Instructions.h"
#include "llvm/Module.h"
#include "llvm/Pass.h"
#include "llvm/ADT/hash_map"
#include "llvm/ADT/Statistic.h"
#include "llvm/Support/Debug.h"
#include "llvm/Target/TargetData.h"
#include "llvm/Transforms/IPO.h"
#include <iostream>
using namespace llvm;

namespace {
  Statistic<> SimplifiedLibCalls("simplified-lib-calls", 
      "Number of well-known library calls simplified");

  /// This class is the base class for a set of small but important 
  /// optimizations of calls to well-known functions, such as those in the c
  /// library. This class provides the basic infrastructure for handling 
  /// runOnModule. Subclasses register themselves and provide two methods:
  /// RecognizeCall and OptimizeCall. Whenever this class finds a function call,
  /// it asks the subclasses to recognize the call. If it is recognized, then
  /// the OptimizeCall method is called on that subclass instance. In this way
  /// the subclasses implement the calling conditions on which they trigger and
  /// the action to perform, making it easy to add new optimizations of this
  /// form.
  /// @brief A ModulePass for optimizing well-known function calls
  struct SimplifyLibCalls : public ModulePass {

    /// We need some target data for accurate signature details that are
    /// target dependent. So we require target data in our AnalysisUsage.
    virtual void getAnalysisUsage(AnalysisUsage& Info) const;

    /// For this pass, process all of the function calls in the module, calling
    /// RecognizeCall and OptimizeCall as appropriate.
    virtual bool runOnModule(Module &M);

  };

  RegisterOpt<SimplifyLibCalls> 
    X("simplify-libcalls","Simplify well-known library calls");

  struct CallOptimizer
  {
    /// @brief Constructor that registers the optimization
    CallOptimizer(const char * fname );

    virtual ~CallOptimizer();

    /// The implementation of this function in subclasses should determine if
    /// \p F is suitable for the optimization. This method is called by 
    /// runOnModule to short circuit visiting all the call sites of such a
    /// function if that function is not suitable in the first place.
    /// If the called function is suitabe, this method should return true;
    /// false, otherwise. This function should also perform any lazy 
    /// initialization that the CallOptimizer needs to do, if its to return 
    /// true. This avoids doing initialization until the optimizer is actually
    /// going to be called upon to do some optimization.
    virtual bool ValidateCalledFunction(
      const Function* F,   ///< The function that is the target of call sites
      const TargetData& TD ///< Information about the target
    ) = 0;

    /// The implementations of this function in subclasses is the heart of the 
    /// SimplifyLibCalls algorithm. Sublcasses of this class implement 
    /// OptimizeCall to determine if (a) the conditions are right for optimizing
    /// the call and (b) to perform the optimization. If an action is taken 
    /// against ci, the subclass is responsible for returning true and ensuring
    /// that ci is erased from its parent.
    /// @param ci the call instruction under consideration
    /// @param f the function that ci calls.
    /// @brief Optimize a call, if possible.
    virtual bool OptimizeCall(
      CallInst* ci,         ///< The call instruction that should be optimized.
      const TargetData& TD  ///< Information about the target
    ) = 0;

    const char * getFunctionName() const { return func_name; }

#ifndef NDEBUG
    void activate() { ++activations; }
#endif

  private:
    const char* func_name;
#ifndef NDEBUG
    std::string stat_name;
    Statistic<> activations; 
#endif
  };

  /// @brief The list of optimizations deriving from CallOptimizer

  hash_map<std::string,CallOptimizer*> optlist;

  CallOptimizer::CallOptimizer(const char* fname)
    : func_name(fname)
#ifndef NDEBUG
    , stat_name(std::string("simplify-libcalls:")+fname)
    , activations(stat_name.c_str(),"Number of calls simplified") 
#endif
  {
    // Register this call optimizer
    optlist[func_name] = this;
  }

  /// Make sure we get our virtual table in this file.
  CallOptimizer::~CallOptimizer() { }

}

ModulePass *llvm::createSimplifyLibCallsPass() 
{ 
  return new SimplifyLibCalls(); 
}

void SimplifyLibCalls::getAnalysisUsage(AnalysisUsage& Info) const
{
  // Ask that the TargetData analysis be performed before us so we can use
  // the target data.
  Info.addRequired<TargetData>();
}

bool SimplifyLibCalls::runOnModule(Module &M) 
{
  TargetData& TD = getAnalysis<TargetData>();

  bool result = false;

  // The call optimizations can be recursive. That is, the optimization might
  // generate a call to another function which can also be optimized. This way
  // we make the CallOptimizer instances very specific to the case they handle.
  // It also means we need to keep running over the function calls in the module
  // until we don't get any more optimizations possible.
  bool found_optimization = false;
  do
  {
    found_optimization = false;
    for (Module::iterator FI = M.begin(), FE = M.end(); FI != FE; ++FI)
    {
      // All the "well-known" functions are external and have external linkage
      // because they live in a runtime library somewhere and were (probably) 
      // not compiled by LLVM.  So, we only act on external functions that have 
      // external linkage and non-empty uses.
      if (!FI->isExternal() || !FI->hasExternalLinkage() || FI->use_empty())
        continue;

      // Get the optimization class that pertains to this function
      CallOptimizer* CO = optlist[FI->getName().c_str()];
      if (!CO)
        continue;

      // Make sure the called function is suitable for the optimization
      if (!CO->ValidateCalledFunction(FI,TD))
        continue;

      // Loop over each of the uses of the function
      for (Value::use_iterator UI = FI->use_begin(), UE = FI->use_end(); 
           UI != UE ; )
      {
        // If the use of the function is a call instruction
        if (CallInst* CI = dyn_cast<CallInst>(*UI++))
        {
          // Do the optimization on the CallOptimizer.
          if (CO->OptimizeCall(CI,TD))
          {
            ++SimplifiedLibCalls;
            found_optimization = result = true;
#ifndef NDEBUG
            CO->activate();
#endif
          }
        }
      }
    }
  } while (found_optimization);
  return result;
}

namespace {

  /// Provide some functions for accessing standard library prototypes and
  /// caching them so we don't have to keep recomputing them
  FunctionType* get_strlen(const Type* IntPtrTy)
  {
    static FunctionType* strlen_type = 0;
    if (!strlen_type)
    {
      std::vector<const Type*> args;
      args.push_back(PointerType::get(Type::SByteTy));
      strlen_type = FunctionType::get(IntPtrTy, args, false);
    }
    return strlen_type;
  }

  FunctionType* get_memcpy()
  {
    static FunctionType* memcpy_type = 0;
    if (!memcpy_type)
    {
      // Note: this is for llvm.memcpy intrinsic
      std::vector<const Type*> args;
      args.push_back(PointerType::get(Type::SByteTy));
      args.push_back(PointerType::get(Type::SByteTy));
      args.push_back(Type::IntTy);
      args.push_back(Type::IntTy);
      memcpy_type = FunctionType::get(Type::VoidTy, args, false);
    }
    return memcpy_type;
  }

  /// A function to compute the length of a null-terminated string of integers.
  /// This function can't rely on the size of the constant array because there 
  /// could be a null terminator in the middle of the array. We also have to 
  /// bail out if we find a non-integer constant initializer of one of the 
  /// elements or if there is no null-terminator. The logic below checks
  bool getConstantStringLength(Value* V, uint64_t& len )
  {
    assert(V != 0 && "Invalid args to getConstantStringLength");
    len = 0; // make sure we initialize this 
    User* GEP = 0;
    // If the value is not a GEP instruction nor a constant expression with a 
    // GEP instruction, then return false because ConstantArray can't occur 
    // any other way
    if (GetElementPtrInst