llvm/lib/Transforms/IPO/SimplifyLibCalls.cpp

//===- SimplifyLibCalls.cpp - Optimize specific well-known librayr calls --===//
//
//                     The LLVM Compiler Infrastructure
//
// This file was developed by Reid Spencer group 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. Many of the ideas for these 
// optimizations were taken from GCC's "builtins.c" file but their 
// implementation here is completely knew and LLVM-centric
//
//===----------------------------------------------------------------------===//

#include "llvm/Transforms/IPO.h"
#include "llvm/Module.h"
#include "llvm/Pass.h"
#include "llvm/Instructions.h"
#include "llvm/ADT/Statistic.h"
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 {


    /// 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();

    virtual ~CallOptimizer();

    /// 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, const Function* f) const = 0;
  };

  /// @brief The list of optimizations deriving from CallOptimizer
  std::vector<struct CallOptimizer*> optlist;

  CallOptimizer::CallOptimizer()
  {
    // Register this call optimizer
    optlist.push_back(this);
  }

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

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

bool SimplifyLibCalls::runOnModule(Module &M) 
{
  for (Module::iterator FI = M.begin(), FE = M.end(); FI != FE; ++FI)
  {
    // All the "well-known" functions are external because they live in a 
    // runtime library somewhere and were (probably) not compiled by LLVM.
    // So, we only act on external functions that have non-empty uses.
    if (FI->isExternal() && !FI->use_empty())
    {
      // Loop over each of the uses of the function
      for (Value::use_iterator UI = FI->use_begin(), UE = FI->use_end(); 
           UI != UE ; )
      {
        CallInst* CI = dyn_cast<CallInst>(*UI);
        ++UI;

        // If the use of the function is a call instruction
        if (CI)
        {
          // Loop over each of the registered optimizations and find the one 
          // that can optimize this call.
          std::vector<CallOptimizer*>::iterator OI = optlist.begin();
          std::vector<CallOptimizer*>::iterator OE = optlist.end();
          for ( ; OI != OE ; ++OI)
          {
            if ((*OI)->OptimizeCall(CI,&(*FI)))
            {
              ++SimplifiedLibCalls;
              break;
            }
          }
        }
      }
    }
  }
  return true;
}

namespace {

/// This CallOptimizer will find instances of a call to "exit" that occurs
/// within the "main" function and change it to a simple "ret" instruction with
/// the same value as passed to the exit function. It assumes that the 
/// instructions after the call to exit(3) can be deleted since they are 
/// unreachable anyway.
/// @brief Replace calls to exit in main with a simple return
struct ExitInMainOptimization : public CallOptimizer
{
virtual ~ExitInMainOptimization() {}
bool OptimizeCall(CallInst* ci, const Function* func) const
{
  // If this isn't the exit function then we don't act
  if (func->getName() != "exit")
    return false;

  // If the call isn't coming from main then we don't act
  if (const Function* f = ci->getParent()->getParent())
    if (f->getName() != "main")
      return false;

  // Okay, time to replace it. Get the basic block of the call instruction
  BasicBlock* bb = ci->getParent();

  // Create a return instruction that we'll replace the call with. Note that
  // the argument of the return is the argument of the call instruction.
  ReturnInst* ri = new ReturnInst(ci->getOperand(1), ci);

  // Erase everything from the call instruction to the end of the block. There
  // really shouldn't be anything other than the call instruction, but just in
  // case there is we delete it all because its now dead.
  bb->getInstList().erase(ci, bb->end());

  return true;
}

} ExitInMainOptimizer;

}