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611 lines
22 KiB
C++
611 lines
22 KiB
C++
//===- SimplifyLibCalls.cpp - Optimize specific well-known library calls --===//
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//
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// The LLVM Compiler Infrastructure
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//
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// This file was developed by Reid Spencer and is distributed under the
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// University of Illinois Open Source License. See LICENSE.TXT for details.
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//
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//===----------------------------------------------------------------------===//
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//
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// This file implements a variety of small optimizations for calls to specific
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// well-known (e.g. runtime library) function calls. For example, a call to the
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// function "exit(3)" that occurs within the main() function can be transformed
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// into a simple "return 3" instruction. Any optimization that takes this form
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// (replace call to library function with simpler code that provides same
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// result) belongs in this file.
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//
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//===----------------------------------------------------------------------===//
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#define DEBUG_TYPE "simplify-libcalls"
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#include "llvm/Constants.h"
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#include "llvm/DerivedTypes.h"
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#include "llvm/Instructions.h"
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#include "llvm/Module.h"
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#include "llvm/Pass.h"
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#include "llvm/ADT/hash_map"
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#include "llvm/ADT/Statistic.h"
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#include "llvm/Support/Debug.h"
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#include "llvm/Target/TargetData.h"
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#include "llvm/Transforms/IPO.h"
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#include <iostream>
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using namespace llvm;
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namespace {
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Statistic<> SimplifiedLibCalls("simplified-lib-calls",
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"Number of well-known library calls simplified");
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/// This class is the base class for a set of small but important
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/// optimizations of calls to well-known functions, such as those in the c
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/// library. This class provides the basic infrastructure for handling
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/// runOnModule. Subclasses register themselves and provide two methods:
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/// RecognizeCall and OptimizeCall. Whenever this class finds a function call,
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/// it asks the subclasses to recognize the call. If it is recognized, then
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/// the OptimizeCall method is called on that subclass instance. In this way
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/// the subclasses implement the calling conditions on which they trigger and
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/// the action to perform, making it easy to add new optimizations of this
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/// form.
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/// @brief A ModulePass for optimizing well-known function calls
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struct SimplifyLibCalls : public ModulePass {
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/// We need some target data for accurate signature details that are
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/// target dependent. So we require target data in our AnalysisUsage.
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virtual void getAnalysisUsage(AnalysisUsage& Info) const;
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/// For this pass, process all of the function calls in the module, calling
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/// RecognizeCall and OptimizeCall as appropriate.
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virtual bool runOnModule(Module &M);
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};
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RegisterOpt<SimplifyLibCalls>
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X("simplify-libcalls","Simplify well-known library calls");
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struct CallOptimizer
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{
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/// @brief Constructor that registers the optimization
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CallOptimizer(const char * fname );
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virtual ~CallOptimizer();
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/// The implementation of this function in subclasses should determine if
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/// \p F is suitable for the optimization. This method is called by
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/// runOnModule to short circuit visiting all the call sites of such a
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/// function if that function is not suitable in the first place.
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/// If the called function is suitabe, this method should return true;
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/// false, otherwise. This function should also perform any lazy
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/// initialization that the CallOptimizer needs to do, if its to return
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/// true. This avoids doing initialization until the optimizer is actually
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/// going to be called upon to do some optimization.
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virtual bool ValidateCalledFunction(
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const Function* F, ///< The function that is the target of call sites
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const TargetData& TD ///< Information about the target
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) = 0;
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/// The implementations of this function in subclasses is the heart of the
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/// SimplifyLibCalls algorithm. Sublcasses of this class implement
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/// OptimizeCall to determine if (a) the conditions are right for optimizing
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/// the call and (b) to perform the optimization. If an action is taken
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/// against ci, the subclass is responsible for returning true and ensuring
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/// that ci is erased from its parent.
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/// @param ci the call instruction under consideration
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/// @param f the function that ci calls.
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/// @brief Optimize a call, if possible.
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virtual bool OptimizeCall(
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CallInst* ci, ///< The call instruction that should be optimized.
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const TargetData& TD ///< Information about the target
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) = 0;
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const char * getFunctionName() const { return func_name; }
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private:
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const char* func_name;
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};
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/// @brief The list of optimizations deriving from CallOptimizer
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hash_map<std::string,CallOptimizer*> optlist;
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CallOptimizer::CallOptimizer(const char* fname)
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: func_name(fname)
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{
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// Register this call optimizer
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optlist[func_name] = this;
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}
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/// Make sure we get our virtual table in this file.
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CallOptimizer::~CallOptimizer() { }
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}
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ModulePass *llvm::createSimplifyLibCallsPass()
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{
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return new SimplifyLibCalls();
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}
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void SimplifyLibCalls::getAnalysisUsage(AnalysisUsage& Info) const
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{
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// Ask that the TargetData analysis be performed before us so we can use
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// the target data.
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Info.addRequired<TargetData>();
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}
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bool SimplifyLibCalls::runOnModule(Module &M)
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{
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TargetData& TD = getAnalysis<TargetData>();
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bool result = false;
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// The call optimizations can be recursive. That is, the optimization might
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// generate a call to another function which can also be optimized. This way
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// we make the CallOptimizer instances very specific to the case they handle.
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// It also means we need to keep running over the function calls in the module
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// until we don't get any more optimizations possible.
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bool found_optimization = false;
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do
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{
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found_optimization = false;
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for (Module::iterator FI = M.begin(), FE = M.end(); FI != FE; ++FI)
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{
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// All the "well-known" functions are external and have external linkage
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// because they live in a runtime library somewhere and were (probably)
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// not compiled by LLVM. So, we only act on external functions that have
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// external linkage and non-empty uses.
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if (FI->isExternal() && FI->hasExternalLinkage() && !FI->use_empty())
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{
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// Get the optimization class that pertains to this function
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if (CallOptimizer* CO = optlist[FI->getName().c_str()] )
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{
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// Make sure the called function is suitable for the optimization
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if (CO->ValidateCalledFunction(FI,TD))
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{
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// Loop over each of the uses of the function
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for (Value::use_iterator UI = FI->use_begin(), UE = FI->use_end();
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UI != UE ; )
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{
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// If the use of the function is a call instruction
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if (CallInst* CI = dyn_cast<CallInst>(*UI++))
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{
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// Do the optimization on the CallOptimizer.
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if (CO->OptimizeCall(CI,TD))
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{
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++SimplifiedLibCalls;
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found_optimization = result = true;
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DEBUG(std::cerr << "simplify-libcall: " << CO->getFunctionName() << "\n");
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}
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}
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}
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}
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}
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}
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}
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} while (found_optimization);
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return result;
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}
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namespace {
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/// Provide some functions for accessing standard library prototypes and
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/// caching them so we don't have to keep recomputing them
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FunctionType* get_strlen(const Type* IntPtrTy)
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{
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static FunctionType* strlen_type = 0;
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if (!strlen_type)
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{
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std::vector<const Type*> args;
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args.push_back(PointerType::get(Type::SByteTy));
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strlen_type = FunctionType::get(IntPtrTy, args, false);
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}
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return strlen_type;
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}
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FunctionType* get_memcpy()
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{
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static FunctionType* memcpy_type = 0;
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if (!memcpy_type)
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{
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// Note: this is for llvm.memcpy intrinsic
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std::vector<const Type*> args;
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args.push_back(PointerType::get(Type::SByteTy));
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args.push_back(PointerType::get(Type::SByteTy));
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args.push_back(Type::IntTy);
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args.push_back(Type::IntTy);
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memcpy_type = FunctionType::get(Type::VoidTy, args, false);
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}
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return memcpy_type;
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}
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/// A function to compute the length of a null-terminated string of integers.
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/// This function can't rely on the size of the constant array because there
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/// could be a null terminator in the middle of the array. We also have to
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/// bail out if we find a non-integer constant initializer of one of the
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/// elements or if there is no null-terminator. The logic below checks
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bool getConstantStringLength(Value* V, uint64_t& len )
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{
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assert(V != 0 && "Invalid args to getConstantStringLength");
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len = 0; // make sure we initialize this
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User* GEP = 0;
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// If the value is not a GEP instruction nor a constant expression with a
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// GEP instruction, then return false because ConstantArray can't occur
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// any other way
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if (GetElementPtrInst* GEPI = dyn_cast<GetElementPtrInst>(V))
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GEP = GEPI;
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else if (ConstantExpr* CE = dyn_cast<ConstantExpr>(V))
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if (CE->getOpcode() == Instruction::GetElementPtr)
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GEP = CE;
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else
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return false;
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else
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return false;
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// Make sure the GEP has exactly three arguments.
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if (GEP->getNumOperands() != 3)
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return false;
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// Check to make sure that the first operand of the GEP is an integer and
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// has value 0 so that we are sure we're indexing into the initializer.
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if (ConstantInt* op1 = dyn_cast<ConstantInt>(GEP->getOperand(1)))
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{
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if (!op1->isNullValue())
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return false;
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}
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else
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return false;
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// Ensure that the second operand is a ConstantInt. If it isn't then this
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// GEP is wonky and we're not really sure what were referencing into and
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// better of not optimizing it. While we're at it, get the second index
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// value. We'll need this later for indexing the ConstantArray.
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uint64_t start_idx = 0;
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if (ConstantInt* CI = dyn_cast<ConstantInt>(GEP->getOperand(2)))
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start_idx = CI->getRawValue();
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else
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return false;
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// The GEP instruction, constant or instruction, must reference a global
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// variable that is a constant and is initialized. The referenced constant
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// initializer is the array that we'll use for optimization.
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GlobalVariable* GV = dyn_cast<GlobalVariable>(GEP->getOperand(0));
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if (!GV || !GV->isConstant() || !GV->hasInitializer())
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return false;
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// Get the initializer.
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Constant* INTLZR = GV->getInitializer();
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// Handle the ConstantAggregateZero case
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if (ConstantAggregateZero* CAZ = dyn_cast<ConstantAggregateZero>(INTLZR))
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{
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// This is a degenerate case. The initializer is constant zero so the
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// length of the string must be zero.
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len = 0;
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return true;
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}
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// Must be a Constant Array
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ConstantArray* A = dyn_cast<ConstantArray>(INTLZR);
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if (!A)
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return false;
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// Get the number of elements in the array
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uint64_t max_elems = A->getType()->getNumElements();
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// Traverse the constant array from start_idx (derived above) which is
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// the place the GEP refers to in the array.
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for ( len = start_idx; len < max_elems; len++)
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{
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if (ConstantInt* CI = dyn_cast<ConstantInt>(A->getOperand(len)))
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{
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// Check for the null terminator
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if (CI->isNullValue())
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break; // we found end of string
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}
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else
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return false; // This array isn't suitable, non-int initializer
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}
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if (len >= max_elems)
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return false; // This array isn't null terminated
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// Subtract out the initial value from the length
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len -= start_idx;
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return true; // success!
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}
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/// This CallOptimizer will find instances of a call to "exit" that occurs
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/// within the "main" function and change it to a simple "ret" instruction with
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/// the same value as passed to the exit function. It assumes that the
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/// instructions after the call to exit(3) can be deleted since they are
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/// unreachable anyway.
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/// @brief Replace calls to exit in main with a simple return
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struct ExitInMainOptimization : public CallOptimizer
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{
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ExitInMainOptimization() : CallOptimizer("exit") {}
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virtual ~ExitInMainOptimization() {}
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// Make sure the called function looks like exit (int argument, int return
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// type, external linkage, not varargs).
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virtual bool ValidateCalledFunction(const Function* f, const TargetData& TD)
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{
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if (f->arg_size() >= 1)
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if (f->arg_begin()->getType()->isInteger())
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return true;
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return false;
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}
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virtual bool OptimizeCall(CallInst* ci, const TargetData& TD)
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{
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// To be careful, we check that the call to exit is coming from "main", that
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// main has external linkage, and the return type of main and the argument
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// to exit have the same type.
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Function *from = ci->getParent()->getParent();
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if (from->hasExternalLinkage())
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if (from->getReturnType() == ci->getOperand(1)->getType())
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if (from->getName() == "main")
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{
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// Okay, time to actually do the optimization. First, get the basic
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// block of the call instruction
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BasicBlock* bb = ci->getParent();
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// Create a return instruction that we'll replace the call with.
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// Note that the argument of the return is the argument of the call
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// instruction.
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ReturnInst* ri = new ReturnInst(ci->getOperand(1), ci);
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// Split the block at the call instruction which places it in a new
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// basic block.
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bb->splitBasicBlock(ci);
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// The block split caused a branch instruction to be inserted into
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// the end of the original block, right after the return instruction
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// that we put there. That's not a valid block, so delete the branch
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// instruction.
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bb->getInstList().pop_back();
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// Now we can finally get rid of the call instruction which now lives
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// in the new basic block.
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ci->eraseFromParent();
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// Optimization succeeded, return true.
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return true;
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}
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// We didn't pass the criteria for this optimization so return false
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return false;
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}
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} ExitInMainOptimizer;
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/// This CallOptimizer will simplify a call to the strcat library function. The
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/// simplification is possible only if the string being concatenated is a
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/// constant array or a constant expression that results in a constant array. In
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/// this case, if the array is small, we can generate a series of inline store
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/// instructions to effect the concatenation without calling strcat.
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/// @brief Simplify the strcat library function.
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struct StrCatOptimization : public CallOptimizer
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{
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private:
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Function* strlen_func;
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Function* memcpy_func;
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public:
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StrCatOptimization()
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: CallOptimizer("strcat")
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, strlen_func(0)
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, memcpy_func(0)
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{}
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virtual ~StrCatOptimization() {}
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inline Function* get_strlen_func(Module*M,const Type* IntPtrTy)
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{
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if (strlen_func)
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return strlen_func;
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return strlen_func = M->getOrInsertFunction("strlen",get_strlen(IntPtrTy));
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}
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inline Function* get_memcpy_func(Module* M)
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{
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if (memcpy_func)
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return memcpy_func;
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return memcpy_func = M->getOrInsertFunction("llvm.memcpy",get_memcpy());
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}
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/// @brief Make sure that the "strcat" function has the right prototype
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virtual bool ValidateCalledFunction(const Function* f, const TargetData& TD)
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{
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if (f->getReturnType() == PointerType::get(Type::SByteTy))
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if (f->arg_size() == 2)
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{
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Function::const_arg_iterator AI = f->arg_begin();
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if (AI++->getType() == PointerType::get(Type::SByteTy))
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if (AI->getType() == PointerType::get(Type::SByteTy))
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{
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// Invalidate the pre-computed strlen_func and memcpy_func Functions
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// because, by definition, this method is only called when a new
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// Module is being traversed. Invalidation causes re-computation for
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// the new Module (if necessary).
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strlen_func = 0;
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memcpy_func = 0;
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// Indicate this is a suitable call type.
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return true;
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}
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}
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return false;
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}
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/// Perform the optimization if the length of the string concatenated
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/// is reasonably short and it is a constant array.
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virtual bool OptimizeCall(CallInst* ci, const TargetData& TD)
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{
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// Extract the initializer (while making numerous checks) from the
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// source operand of the call to strcat. If we get null back, one of
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// a variety of checks in get_GVInitializer failed
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uint64_t len = 0;
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if (!getConstantStringLength(ci->getOperand(2),len))
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return false;
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// Handle the simple, do-nothing case
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if (len == 0)
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{
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ci->replaceAllUsesWith(ci->getOperand(1));
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ci->eraseFromParent();
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return true;
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}
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// Increment the length because we actually want to memcpy the null
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// terminator as well.
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len++;
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// Extract some information from the instruction
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Module* M = ci->getParent()->getParent()->getParent();
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// We need to find the end of the destination string. That's where the
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// memory is to be moved to. We just generate a call to strlen (further
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// optimized in another pass). Note that the get_strlen_func() call
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// caches the Function* for us.
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CallInst* strlen_inst =
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new CallInst(get_strlen_func(M,TD.getIntPtrType()),
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ci->getOperand(1),"",ci);
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// Now that we have the destination's length, we must index into the
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// destination's pointer to get the actual memcpy destination (end of
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// the string .. we're concatenating).
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std::vector<Value*> idx;
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idx.push_back(strlen_inst);
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GetElementPtrInst* gep =
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new GetElementPtrInst(ci->getOperand(1),idx,"",ci);
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// We have enough information to now generate the memcpy call to
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// do the concatenation for us.
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std::vector<Value*> vals;
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vals.push_back(gep); // destination
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vals.push_back(ci->getOperand(2)); // source
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vals.push_back(ConstantSInt::get(Type::IntTy,len)); // length
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vals.push_back(ConstantSInt::get(Type::IntTy,1)); // alignment
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CallInst* memcpy_inst = new CallInst(get_memcpy_func(M), vals, "", ci);
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// Finally, substitute the first operand of the strcat call for the
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// strcat call itself since strcat returns its first operand; and,
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// kill the strcat CallInst.
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ci->replaceAllUsesWith(ci->getOperand(1));
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ci->eraseFromParent();
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return true;
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}
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} StrCatOptimizer;
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/// This CallOptimizer will simplify a call to the strlen library function by
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/// replacing it with a constant value if the string provided to it is a
|
|
/// constant array.
|
|
/// @brief Simplify the strlen library function.
|
|
struct StrLenOptimization : public CallOptimizer
|
|
{
|
|
StrLenOptimization() : CallOptimizer("strlen") {}
|
|
virtual ~StrLenOptimization() {}
|
|
|
|
/// @brief Make sure that the "strlen" function has the right prototype
|
|
virtual bool ValidateCalledFunction(const Function* f, const TargetData& TD)
|
|
{
|
|
if (f->getReturnType() == TD.getIntPtrType())
|
|
if (f->arg_size() == 1)
|
|
if (Function::const_arg_iterator AI = f->arg_begin())
|
|
if (AI->getType() == PointerType::get(Type::SByteTy))
|
|
return true;
|
|
return false;
|
|
}
|
|
|
|
/// @brief Perform the strlen optimization
|
|
virtual bool OptimizeCall(CallInst* ci, const TargetData& TD)
|
|
{
|
|
// Get the length of the string
|
|
uint64_t len = 0;
|
|
if (!getConstantStringLength(ci->getOperand(1),len))
|
|
return false;
|
|
|
|
ci->replaceAllUsesWith(ConstantInt::get(TD.getIntPtrType(),len));
|
|
ci->eraseFromParent();
|
|
return true;
|
|
}
|
|
} StrLenOptimizer;
|
|
|
|
/// 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") {}
|
|
protected:
|
|
MemCpyOptimization(const char* fname) : CallOptimizer(fname) {}
|
|
public:
|
|
virtual ~MemCpyOptimization() {}
|
|
|
|
/// @brief Make sure that the "memcpy" function has the right prototype
|
|
virtual bool ValidateCalledFunction(const Function* f, const TargetData& TD)
|
|
{
|
|
// Just make sure this has 4 arguments per LLVM spec.
|
|
return (f->arg_size() == 4);
|
|
}
|
|
|
|
/// Because of alignment and instruction information that we don't have, we
|
|
/// leave the bulk of this to the code generators. The optimization here just
|
|
/// deals with a few degenerate cases where the length of the string and the
|
|
/// alignment match the sizes of our intrinsic types so we can do a load and
|
|
/// store instead of the memcpy call.
|
|
/// @brief Perform the memcpy optimization.
|
|
virtual bool OptimizeCall(CallInst* ci, const TargetData& TD)
|
|
{
|
|
// Make sure we have constant int values to work with
|
|
ConstantInt* LEN = dyn_cast<ConstantInt>(ci->getOperand(3));
|
|
if (!LEN)
|
|
return false;
|
|
ConstantInt* ALIGN = dyn_cast<ConstantInt>(ci->getOperand(4));
|
|
if (!ALIGN)
|
|
return false;
|
|
|
|
// If the length is larger than the alignment, we can't optimize
|
|
uint64_t len = LEN->getRawValue();
|
|
uint64_t alignment = ALIGN->getRawValue();
|
|
if (len > alignment)
|
|
return false;
|
|
|
|
Value* dest = ci->getOperand(1);
|
|
Value* src = ci->getOperand(2);
|
|
CastInst* SrcCast = 0;
|
|
CastInst* DestCast = 0;
|
|
switch (len)
|
|
{
|
|
case 0:
|
|
// The memcpy is a no-op so just dump its call.
|
|
ci->eraseFromParent();
|
|
return true;
|
|
case 1:
|
|
SrcCast = new CastInst(src,PointerType::get(Type::SByteTy),"",ci);
|
|
DestCast = new CastInst(dest,PointerType::get(Type::SByteTy),"",ci);
|
|
break;
|
|
case 2:
|
|
SrcCast = new CastInst(src,PointerType::get(Type::ShortTy),"",ci);
|
|
DestCast = new CastInst(dest,PointerType::get(Type::ShortTy),"",ci);
|
|
break;
|
|
case 4:
|
|
SrcCast = new CastInst(src,PointerType::get(Type::IntTy),"",ci);
|
|
DestCast = new CastInst(dest,PointerType::get(Type::IntTy),"",ci);
|
|
break;
|
|
case 8:
|
|
SrcCast = new CastInst(src,PointerType::get(Type::LongTy),"",ci);
|
|
DestCast = new CastInst(dest,PointerType::get(Type::LongTy),"",ci);
|
|
break;
|
|
default:
|
|
return false;
|
|
}
|
|
LoadInst* LI = new LoadInst(SrcCast,"",ci);
|
|
StoreInst* SI = new StoreInst(LI, DestCast, ci);
|
|
ci->eraseFromParent();
|
|
return true;
|
|
}
|
|
} MemCpyOptimizer;
|
|
|
|
/// This CallOptimizer will simplify a call to the memmove library function. It
|
|
/// is identical to MemCopyOptimization except for the name of the intrinsic.
|
|
/// @brief Simplify the memmove library function.
|
|
struct MemMoveOptimization : public MemCpyOptimization
|
|
{
|
|
MemMoveOptimization() : MemCpyOptimization("llvm.memmove") {}
|
|
|
|
} MemMoveOptimizer;
|
|
|
|
}
|