llvm/lib/Transforms/IPO/SimplifyLibCalls.cpp - toolchain/llvm-project - Gitiles

 //===- 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.
 //
 //===----------------------------------------------------------------------===//

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


     /// 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
     ) = 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.
     ) = 0;

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

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

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

   CallOptimizer::CallOptimizer(const char* fname)
     : func_name(fname)
   {
     // Register this call optimizer
     optlist[func_name] = this;
   }

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

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

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

 bool SimplifyLibCalls::runOnModule(Module &M)
 {
   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())
       {
         // Get the optimization class that pertains to this function
         if (CallOptimizer* CO = optlist[FI->getName().c_str()] )
         {
           // Make sure the called function is suitable for the optimization
           if (CO->ValidateCalledFunction(FI))
           {
             // 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))
                 {
                   ++SimplifiedLibCalls;
                   found_optimization = result = true;
                 }
               }
             }
           }
         }
       }
     }
   } while (found_optimization);
   return result;
 }

 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
 {
   ExitInMainOptimization() : CallOptimizer("exit") {}
   virtual ~ExitInMainOptimization() {}

   // Make sure the called function looks like exit (int argument, int return
   // type, external linkage, not varargs).
   virtual bool ValidateCalledFunction(const Function* f)
   {
     if (f->getReturnType()->getTypeID() == Type::VoidTyID && !f->isVarArg())
       if (f->arg_size() == 1)
         if (f->arg_begin()->getType()->isInteger())
           return true;
     return false;
   }

   virtual bool OptimizeCall(CallInst* ci)
   {
     // To be careful, we check that the call to exit is coming from "main", that
     // main has external linkage, and the return type of main and the argument
     // to exit have the same type.
     Function *from = ci->getParent()->getParent();
     if (from->hasExternalLinkage())
       if (from->getReturnType() == ci->getOperand(1)->getType())
         if (from->getName() == "main")
         {
           // Okay, time to actually do the optimization. First, 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);

           // Split the block at the call instruction which places it in a new
           // basic block.
           bb->splitBasicBlock(ci);

           // The block split caused a branch instruction to be inserted into
           // the end of the original block, right after the return instruction
           // that we put there. That's not a valid block, so delete the branch
           // instruction.
           bb->getInstList().pop_back();

           // Now we can finally get rid of the call instruction which now lives
           // in the new basic block.
           ci->eraseFromParent();

           // Optimization succeeded, return true.
           return true;
         }
     // We didn't pass the criteria for this optimization so return false
     return false;
   }
 } ExitInMainOptimizer;

 /// This CallOptimizer will simplify a call to the strcat library function. The
 /// simplification is possible only if the string being concatenated is a
 /// constant array or a constant expression that results in a constant array. In
 /// this case, if the array is small, we can generate a series of inline store
 /// instructions to effect the concatenation without calling strcat.
 /// @brief Simplify the strcat library function.
 struct StrCatOptimization : public CallOptimizer
 {
 private:
   Function* strlen_func;
   Function* memcpy_func;
 public:
   StrCatOptimization()
     : CallOptimizer("strcat")
     , strlen_func(0)
     , memcpy_func(0)
     {}
   virtual ~StrCatOptimization() {}

   inline Function* get_strlen_func(Module*M)
   {
     if (strlen_func)
       return strlen_func;
     return strlen_func = M->getOrInsertFunction("strlen",get_strlen());
   }

   inline Function* get_memcpy_func(Module* M)
   {
     if (memcpy_func)
       return memcpy_func;
     return memcpy_func = M->getOrInsertFunction("llvm.memcpy",get_memcpy());
   }

   /// @brief Make sure that the "strcat" function has the right prototype
   virtual bool ValidateCalledFunction(const Function* f)
   {
     if (f->getReturnType() == PointerType::get(Type::SByteTy))
       if (f->arg_size() == 2)
       {
         Function::const_arg_iterator AI = f->arg_begin();
         if (AI++->getType() == PointerType::get(Type::SByteTy))
           if (AI->getType() == PointerType::get(Type::SByteTy))
           {
             // Invalidate the pre-computed strlen_func and memcpy_func Functions
             // because, by definition, this method is only called when a new
             // Module is being traversed. Invalidation causes re-computation for
             // the new Module (if necessary).
             strlen_func = 0;
             memcpy_func = 0;

             // Indicate this is a suitable call type.
             return true;
           }
       }
     return false;
   }

   /// Perform the optimization if the length of the string concatenated
   /// is reasonably short and it is a constant array.
   virtual bool OptimizeCall(CallInst* ci)
   {
     User* GEP = 0;
     // If the thing being appended is not a GEP instruction nor a constant
     // expression with a GEP instruction, then return false because this is
     // not a situation we can optimize.
     if (GetElementPtrInst* GEPI =
         dyn_cast<GetElementPtrInst>(ci->getOperand(2)))
       GEP = GEPI;
     else if (ConstantExpr* CE = dyn_cast<ConstantExpr>(ci->getOperand(2)))
       if (CE->getOpcode() == Instruction::GetElementPtr)
         GEP = CE;
       else
         return false;
     else
       return false;

     // Check to make sure that the first operand of the GEP is an integer and
     // has value 0 so that we are sure we're indexing into the initializer.
     if (ConstantInt* op1 = dyn_cast<ConstantInt>(GEP->getOperand(1)))
       if (op1->isNullValue())
         ;
       else
         return false;
     else
       return false;

     // Ensure that the second operand is a constant int. If it isn't then this
     // GEP is wonky and we're not really sure what were referencing into and
     // better of not optimizing it.
     if (!dyn_cast<ConstantInt>(GEP->getOperand(2)))
       return false;

     // The GEP instruction, constant or instruction, must reference a global
     // variable that is a constant and is initialized. The referenced constant
     // initializer is the array that we'll use for optimization.
     GlobalVariable* GV = dyn_cast<GlobalVariable>(GEP->getOperand(0));
     if (!GV || !GV->isConstant() || !GV->hasInitializer())
       return false;

     // Get the initializer
     Constant* INTLZR = GV->getInitializer();

     // Handle the ConstantArray case.
     if (ConstantArray* A = dyn_cast<ConstantArray>(INTLZR))
     {
       // First off, we can't do this if the constant array isn't a string,
       // meaning its base type is sbyte and its constant initializers for all
       // the elements are constantInt or constantInt expressions.
       if (!A->isString())
         return false;

       // Now we need to examine the source string to find its actual length. We
       // can't rely on the size of the constant array becasue 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.
       // Also, if we never find a terminator before the end of the array.
       unsigned max_elems = A->getType()->getNumElements();
       unsigned len = 0;
       for (; len < max_elems; len++)
       {
         if (ConstantInt* CI = dyn_cast<ConstantInt>(A->getOperand(len)))
         {
           if (CI->isNullValue())
             break; // we found end of string
         }
         else
           return false; // This array isn't suitable, non-int initializer
       }
       if (len >= max_elems)
         return false; // This array isn't null terminated
       else
         len++; // increment for null terminator

       // Extract some information from the instruction
       Module* M = ci->getParent()->getParent()->getParent();

       // We need to find the end of the destination string.  That's where the
       // memory is to be moved to. We just generate a call to strlen (further
       // optimized in another pass). Note that the get_strlen_func() call
       // caches the Function* for us.
       CallInst* strlen_inst =
         new CallInst(get_strlen_func(M),ci->getOperand(1),"",ci);

       // Now that we have the destination's length, we must index into the
       // destination's pointer to get the actual memcpy destination (end of
       // the string .. we're concatenating).
       std::vector<Value*> idx;
       idx.push_back(strlen_inst);
       GetElementPtrInst* gep =
         new GetElementPtrInst(ci->getOperand(1),idx,"",ci);

       // We have enough information to now generate the memcpy call to
       // do the concatenation for us.
       std::vector<Value*> vals;
       vals.push_back(gep); // destination
       vals.push_back(ci->getOperand(2)); // source
       vals.push_back(ConstantSInt::get(Type::IntTy,len)); // length
       vals.push_back(ConstantSInt::get(Type::IntTy,1)); // alignment
       CallInst* memcpy_inst =
         new CallInst(get_memcpy_func(M), vals, "", ci);

       // Finally, substitute the first operand of the strcat call for the
       // strcat call itself since strcat returns its first operand; and,
       // kill the strcat CallInst.
       ci->replaceAllUsesWith(ci->getOperand(1));
       ci->eraseFromParent();
       return true;
     }

     // Handle the ConstantAggregateZero case
     else if (ConstantAggregateZero* CAZ =
         dyn_cast<ConstantAggregateZero>(INTLZR))
     {
       // We know this is the zero length string case so we can just avoid
       // the strcat altogether and replace the CallInst with its first operand
       // (what strcat returns).
       ci->replaceAllUsesWith(ci->getOperand(1));
       ci->eraseFromParent();
       return true;
     }

     // We didn't pass the criteria for this optimization so return false.
     return false;
   }
 } StrCatOptimizer;

 /// This CallOptimizer will simplify a call to the memcpy library function by
 /// expanding it out to a small set of stores if the copy source is a constant
 /// array.
 /// @brief Simplify the memcpy library function.
 struct MemCpyOptimization : public CallOptimizer
 {
   MemCpyOptimization() : CallOptimizer("llvm.memcpy") {}
   virtual ~MemCpyOptimization() {}

   /// @brief Make sure that the "memcpy" function has the right prototype
   virtual bool ValidateCalledFunction(const Function* f)
   {
     if (f->getReturnType() == PointerType::get(Type::VoidTy))
       if (f->arg_size() == 2)
       {
         Function::const_arg_iterator AI = f->arg_begin();
         if (AI++->getType() == PointerType::get(Type::SByteTy))
           if (AI++->getType() == PointerType::get(Type::SByteTy))
             if (AI++->getType() == Type::IntTy)
               if (AI->getType() == Type::IntTy)
             return true;
       }
     return false;
   }

   /// Perform the optimization if the length of the string concatenated
   /// is reasonably short and it is a constant array.
   virtual bool OptimizeCall(CallInst* ci)
   {
     //
     // We didn't pass the criteria for this optimization so return false.
     return false;
   }
 } MemCpyOptimizer;
 }
	//===- 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.
	//
	//===----------------------------------------------------------------------===//

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


	/// 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
	) = 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.
	) = 0;

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

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

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

	CallOptimizer::CallOptimizer(const char* fname)
	: func_name(fname)
	{
	// Register this call optimizer
	optlist[func_name] = this;
	}

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

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

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

	bool SimplifyLibCalls::runOnModule(Module &M)
	{
	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())
	{
	// Get the optimization class that pertains to this function
	if (CallOptimizer* CO = optlist[FI->getName().c_str()] )
	{
	// Make sure the called function is suitable for the optimization
	if (CO->ValidateCalledFunction(FI))
	{
	// 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))
	{
	++SimplifiedLibCalls;
	found_optimization = result = true;
	}
	}
	}
	}
	}
	}
	}
	} while (found_optimization);
	return result;
	}

	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
	{
	ExitInMainOptimization() : CallOptimizer("exit") {}
	virtual ~ExitInMainOptimization() {}

	// Make sure the called function looks like exit (int argument, int return
	// type, external linkage, not varargs).
	virtual bool ValidateCalledFunction(const Function* f)
	{
	if (f->getReturnType()->getTypeID() == Type::VoidTyID && !f->isVarArg())
	if (f->arg_size() == 1)
	if (f->arg_begin()->getType()->isInteger())
	return true;
	return false;
	}

	virtual bool OptimizeCall(CallInst* ci)
	{
	// To be careful, we check that the call to exit is coming from "main", that
	// main has external linkage, and the return type of main and the argument
	// to exit have the same type.
	Function *from = ci->getParent()->getParent();
	if (from->hasExternalLinkage())
	if (from->getReturnType() == ci->getOperand(1)->getType())
	if (from->getName() == "main")
	{
	// Okay, time to actually do the optimization. First, 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);

	// Split the block at the call instruction which places it in a new
	// basic block.
	bb->splitBasicBlock(ci);

	// The block split caused a branch instruction to be inserted into
	// the end of the original block, right after the return instruction
	// that we put there. That's not a valid block, so delete the branch
	// instruction.
	bb->getInstList().pop_back();

	// Now we can finally get rid of the call instruction which now lives
	// in the new basic block.
	ci->eraseFromParent();

	// Optimization succeeded, return true.
	return true;
	}
	// We didn't pass the criteria for this optimization so return false
	return false;
	}
	} ExitInMainOptimizer;

	/// This CallOptimizer will simplify a call to the strcat library function. The
	/// simplification is possible only if the string being concatenated is a
	/// constant array or a constant expression that results in a constant array. In
	/// this case, if the array is small, we can generate a series of inline store
	/// instructions to effect the concatenation without calling strcat.
	/// @brief Simplify the strcat library function.
	struct StrCatOptimization : public CallOptimizer
	{
	private:
	Function* strlen_func;
	Function* memcpy_func;
	public:
	StrCatOptimization()
	: CallOptimizer("strcat")
	, strlen_func(0)
	, memcpy_func(0)
	{}
	virtual ~StrCatOptimization() {}

	inline Function* get_strlen_func(Module*M)
	{
	if (strlen_func)
	return strlen_func;
	return strlen_func = M->getOrInsertFunction("strlen",get_strlen());
	}

	inline Function* get_memcpy_func(Module* M)
	{
	if (memcpy_func)
	return memcpy_func;
	return memcpy_func = M->getOrInsertFunction("llvm.memcpy",get_memcpy());
	}

	/// @brief Make sure that the "strcat" function has the right prototype
	virtual bool ValidateCalledFunction(const Function* f)
	{
	if (f->getReturnType() == PointerType::get(Type::SByteTy))
	if (f->arg_size() == 2)
	{
	Function::const_arg_iterator AI = f->arg_begin();
	if (AI++->getType() == PointerType::get(Type::SByteTy))
	if (AI->getType() == PointerType::get(Type::SByteTy))
	{
	// Invalidate the pre-computed strlen_func and memcpy_func Functions
	// because, by definition, this method is only called when a new
	// Module is being traversed. Invalidation causes re-computation for
	// the new Module (if necessary).
	strlen_func = 0;
	memcpy_func = 0;

	// Indicate this is a suitable call type.
	return true;
	}
	}
	return false;
	}

	/// Perform the optimization if the length of the string concatenated
	/// is reasonably short and it is a constant array.
	virtual bool OptimizeCall(CallInst* ci)
	{
	User* GEP = 0;
	// If the thing being appended is not a GEP instruction nor a constant
	// expression with a GEP instruction, then return false because this is
	// not a situation we can optimize.
	if (GetElementPtrInst* GEPI =
	dyn_cast<GetElementPtrInst>(ci->getOperand(2)))
	GEP = GEPI;
	else if (ConstantExpr* CE = dyn_cast<ConstantExpr>(ci->getOperand(2)))
	if (CE->getOpcode() == Instruction::GetElementPtr)
	GEP = CE;
	else
	return false;
	else
	return false;

	// Check to make sure that the first operand of the GEP is an integer and
	// has value 0 so that we are sure we're indexing into the initializer.
	if (ConstantInt* op1 = dyn_cast<ConstantInt>(GEP->getOperand(1)))
	if (op1->isNullValue())
	;
	else
	return false;
	else
	return false;

	// Ensure that the second operand is a constant int. If it isn't then this
	// GEP is wonky and we're not really sure what were referencing into and
	// better of not optimizing it.
	if (!dyn_cast<ConstantInt>(GEP->getOperand(2)))
	return false;

	// The GEP instruction, constant or instruction, must reference a global
	// variable that is a constant and is initialized. The referenced constant
	// initializer is the array that we'll use for optimization.
	GlobalVariable* GV = dyn_cast<GlobalVariable>(GEP->getOperand(0));
	if (!GV \|\| !GV->isConstant() \|\| !GV->hasInitializer())
	return false;

	// Get the initializer
	Constant* INTLZR = GV->getInitializer();

	// Handle the ConstantArray case.
	if (ConstantArray* A = dyn_cast<ConstantArray>(INTLZR))
	{
	// First off, we can't do this if the constant array isn't a string,
	// meaning its base type is sbyte and its constant initializers for all
	// the elements are constantInt or constantInt expressions.
	if (!A->isString())
	return false;

	// Now we need to examine the source string to find its actual length. We
	// can't rely on the size of the constant array becasue 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.
	// Also, if we never find a terminator before the end of the array.
	unsigned max_elems = A->getType()->getNumElements();
	unsigned len = 0;
	for (; len < max_elems; len++)
	{
	if (ConstantInt* CI = dyn_cast<ConstantInt>(A->getOperand(len)))
	{
	if (CI->isNullValue())
	break; // we found end of string
	}
	else
	return false; // This array isn't suitable, non-int initializer
	}
	if (len >= max_elems)
	return false; // This array isn't null terminated
	else
	len++; // increment for null terminator

	// Extract some information from the instruction
	Module* M = ci->getParent()->getParent()->getParent();

	// We need to find the end of the destination string. That's where the
	// memory is to be moved to. We just generate a call to strlen (further
	// optimized in another pass). Note that the get_strlen_func() call
	// caches the Function* for us.
	CallInst* strlen_inst =
	new CallInst(get_strlen_func(M),ci->getOperand(1),"",ci);

	// Now that we have the destination's length, we must index into the
	// destination's pointer to get the actual memcpy destination (end of
	// the string .. we're concatenating).
	std::vector<Value*> idx;
	idx.push_back(strlen_inst);
	GetElementPtrInst* gep =
	new GetElementPtrInst(ci->getOperand(1),idx,"",ci);

	// We have enough information to now generate the memcpy call to
	// do the concatenation for us.
	std::vector<Value*> vals;
	vals.push_back(gep); // destination
	vals.push_back(ci->getOperand(2)); // source
	vals.push_back(ConstantSInt::get(Type::IntTy,len)); // length
	vals.push_back(ConstantSInt::get(Type::IntTy,1)); // alignment
	CallInst* memcpy_inst =
	new CallInst(get_memcpy_func(M), vals, "", ci);

	// Finally, substitute the first operand of the strcat call for the
	// strcat call itself since strcat returns its first operand; and,
	// kill the strcat CallInst.
	ci->replaceAllUsesWith(ci->getOperand(1));
	ci->eraseFromParent();
	return true;
	}

	// Handle the ConstantAggregateZero case
	else if (ConstantAggregateZero* CAZ =
	dyn_cast<ConstantAggregateZero>(INTLZR))
	{
	// We know this is the zero length string case so we can just avoid
	// the strcat altogether and replace the CallInst with its first operand
	// (what strcat returns).
	ci->replaceAllUsesWith(ci->getOperand(1));
	ci->eraseFromParent();
	return true;
	}

	// We didn't pass the criteria for this optimization so return false.
	return false;
	}
	} StrCatOptimizer;

	/// This CallOptimizer will simplify a call to the memcpy library function by
	/// expanding it out to a small set of stores if the copy source is a constant
	/// array.
	/// @brief Simplify the memcpy library function.
	struct MemCpyOptimization : public CallOptimizer
	{
	MemCpyOptimization() : CallOptimizer("llvm.memcpy") {}
	virtual ~MemCpyOptimization() {}

	/// @brief Make sure that the "memcpy" function has the right prototype
	virtual bool ValidateCalledFunction(const Function* f)
	{
	if (f->getReturnType() == PointerType::get(Type::VoidTy))
	if (f->arg_size() == 2)
	{
	Function::const_arg_iterator AI = f->arg_begin();
	if (AI++->getType() == PointerType::get(Type::SByteTy))
	if (AI++->getType() == PointerType::get(Type::SByteTy))
	if (AI++->getType() == Type::IntTy)
	if (AI->getType() == Type::IntTy)
	return true;
	}
	return false;
	}

	/// Perform the optimization if the length of the string concatenated
	/// is reasonably short and it is a constant array.
	virtual bool OptimizeCall(CallInst* ci)
	{
	//
	// We didn't pass the criteria for this optimization so return false.
	return false;
	}
	} MemCpyOptimizer;
	}