source/Expression/IRForTarget.cpp

//===-- IRForTarget.cpp -------------------------------------------*- C++ -*-===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//

#include "lldb/Expression/IRForTarget.h"

#include "llvm/Support/raw_ostream.h"
#include "llvm/InstrTypes.h"
#include "llvm/Instructions.h"
#include "llvm/Module.h"
#include "llvm/Target/TargetData.h"
#include "llvm/ValueSymbolTable.h"

#include "clang/AST/ASTContext.h"

#include "lldb/Core/dwarf.h"
#include "lldb/Core/Log.h"
#include "lldb/Core/Scalar.h"
#include "lldb/Core/StreamString.h"
#include "lldb/Expression/ClangExpressionDeclMap.h"

#include <map>

using namespace llvm;

static char ID;

IRForTarget::IRForTarget(lldb_private::ClangExpressionDeclMap *decl_map,
                         const TargetData *target_data,
                         bool resolve_vars,
                         const char *func_name) :
    ModulePass(&ID),
    m_decl_map(decl_map),
    m_target_data(target_data),
    m_sel_registerName(NULL),
    m_func_name(func_name),
    m_resolve_vars(resolve_vars)
{
}

/* A handy utility function used at several places in the code */

static std::string 
PrintValue(Value *V, bool truncate = false)
{
    std::string s;
    raw_string_ostream rso(s);
    V->print(rso);
    rso.flush();
    if (truncate)
        s.resize(s.length() - 1);
    return s;
}

IRForTarget::~IRForTarget()
{
}

bool 
IRForTarget::createResultVariable(llvm::Module &M,
                                  llvm::Function &F)
{
    lldb_private::Log *log = lldb_private::GetLogIfAllCategoriesSet (LIBLLDB_LOG_EXPRESSIONS);
    
    if (!m_resolve_vars)
        return true;
    
    // Find the result variable.  If it doesn't exist, we can give up right here.
            
    Value *result_value = M.getNamedValue("___clang_expr_result");
    
    if (!result_value)
    {
        if (log)
            log->PutCString("Couldn't find result variable");
                
        return true;
    }
        
    if (log)
        log->Printf("Found result in the IR: %s", PrintValue(result_value, false).c_str());
    
    GlobalVariable *result_global = dyn_cast<GlobalVariable>(result_value);
    
    if (!result_global)
    {
        if (log)
            log->PutCString("Result variable isn't a GlobalVariable");
        return false;
    }
    
    // Find the metadata and follow it to the VarDecl
    
    NamedMDNode *named_metadata = M.getNamedMetadata("clang.global.decl.ptrs");
    
    if (!named_metadata)
    {
        if (log)
            log->PutCString("No global metadata");
        
        return false;
    }
        
    unsigned num_nodes = named_metadata->getNumOperands();
    unsigned node_index;
    
    MDNode *metadata_node = NULL;
    
    for (node_index = 0;
         node_index < num_nodes;
         ++node_index)
    {
        metadata_node = named_metadata->getOperand(node_index);
        
        if (metadata_node->getNumOperands() != 2)
            continue;
        
        if (metadata_node->getOperand(0) == result_global)
            break;
    }
    
    if (!metadata_node)
    {
        if (log)
            log->PutCString("Couldn't find result metadata");
        return false;
    }
        
    ConstantInt *constant_int = dyn_cast<ConstantInt>(metadata_node->getOperand(1));
        
    uint64_t result_decl_intptr = constant_int->getZExtValue();
    
    clang::VarDecl *result_decl = reinterpret_cast<clang::VarDecl *>(result_decl_intptr);
        
    // Get the next available result name from m_decl_map and create the persistent
    // variable for it
    
    lldb_private::TypeFromParser result_decl_type (result_decl->getType().getAsOpaquePtr(),
                                                   &result_decl->getASTContext());
    std::string new_result_name;
    
    m_decl_map->GetPersistentResultName(new_result_name);
    m_decl_map->AddPersistentVariable(result_decl, new_result_name.c_str(), result_decl_type);
    
    if (log)
        log->Printf("Creating a new result global: %s", new_result_name.c_str());
        
    // Construct a new result global and set up its metadata
    
    GlobalVariable *new_result_global = new GlobalVariable(M, 
                                                           result_global->getType()->getElementType(),
                                                           false, /* not constant */
                                                           GlobalValue::ExternalLinkage,
                                                           NULL, /* no initializer */
                                                           new_result_name.c_str());
    
    // It's too late in compilation to create a new VarDecl for this, but we don't
    // need to.  We point the metadata at the old VarDecl.  This creates an odd
    // anomaly: a variable with a Value whose name is something like $0 and a
    // Decl whose name is ___clang_expr_result.  This condition is handled in
    // ClangExpressionDeclMap::DoMaterialize, and the name of the variable is
    // fixed up.
    
    ConstantInt *new_constant_int = ConstantInt::get(constant_int->getType(), 
                                                     result_decl_intptr,
                                                     false);
    
    llvm::Value* values[2];
    values[0] = new_result_global;
    values[1] = new_constant_int;
    
    MDNode *persistent_global_md = MDNode::get(M.getContext(), values, 2);
    named_metadata->addOperand(persistent_global_md);
    
    if (log)
        log->Printf("Replacing %s with %s",
                    PrintValue(result_global).c_str(),
                    PrintValue(new_result_global).c_str());
    
    if (result_global->hasNUses(0))
    {
        // We need to synthesize a store for this variable, because otherwise
        // there's nothing to put into its equivalent persistent variable.
        
        BasicBlock &entry_block(F.getEntryBlock());
        Instruction *first_entry_instruction(entry_block.getFirstNonPHIOrDbg());
        
        if (!first_entry_instruction)
            return false;
        
        if (!result_global->hasInitializer())
        {
            if (log)
                log->Printf("Couldn't find initializer for unused variable");
            return false;
        }
        
        Constant *initializer = result_global->getInitializer();
                
        StoreInst *synthesized_store = new StoreInst::StoreInst(initializer,
                                                                new_result_global,
                                                                first_entry_instruction);
        
        if (log)
            log->Printf("Synthesized result store %s\n", PrintValue(synthesized_store).c_str());
    }
    else
    {
        result_global->replaceAllUsesWith(new_result_global);
    }
        
    result_global->eraseFromParent();
    
    return true;
}

static bool isObjCSelectorRef(Value *V)
{
    GlobalVariable *GV = dyn_cast<GlobalVariable>(V);
    
    if (!GV || !GV->hasName() || !GV->getName().startswith("\01L_OBJC_SELECTOR_REFERENCES_"))
        return false;
    
    return true;
}

bool 
IRForTarget::RewriteObjCSelector(Instruction* selector_load,
                                 Module &M)
{
    lldb_private::Log *log = lldb_private::GetLogIfAllCategoriesSet (LIBLLDB_LOG_EXPRESSIONS);

    LoadInst *load = dyn_cast<LoadInst>(selector_load);
    
    if (!load)
        return false;
    
    // Unpack the message name from the selector.  In LLVM IR, an objc_msgSend gets represented as
    //
    // %tmp     = load i8** @"\01L_OBJC_SELECTOR_REFERENCES_" ; <i8*>
    // %call    = call i8* (i8*, i8*, ...)* @objc_msgSend(i8* %obj, i8* %tmp, ...) ; <i8*>
    //
    // where %obj is the object pointer and %tmp is the selector.
    // 
    // @"\01L_OBJC_SELECTOR_REFERENCES_" is a pointer to a character array called @"\01L_OBJC_METH_VAR_NAME_".
    // @"\01L_OBJC_METH_VAR_NAME_" contains the string.
    
    // Find the pointer's initializer (a ConstantExpr with opcode GetElementPtr) and get the string from its target
    
    GlobalVariable *_objc_selector_references_ = dyn_cast<GlobalVariable>(load->getPointerOperand());
    
    if (!_objc_selector_references_ || !_objc_selector_references_->hasInitializer())
        return false;
    
    Constant *osr_initializer = _objc_selector_references_->getInitializer();
    
    ConstantExpr *osr_initializer_expr = dyn_cast<ConstantExpr>(osr_initializer);
    
    if (!osr_initializer_expr || osr_initializer_expr->getOpcode() != Instruction::GetElementPtr)
        return false;
    
    Value *osr_initializer_base = osr_initializer_expr->getOperand(0);

    if (!osr_initializer_base)
        return false;
    
    // Find the string's initializer (a ConstantArray) and get the string from it
    
    GlobalVariable *_objc_meth_var_name_ = dyn_cast<GlobalVariable>(osr_initializer_base);
    
    if (!_objc_meth_var_name_ || !_objc_meth_var_name_->hasInitializer())
        return false;
    
    Constant *omvn_initializer = _objc_meth_var_name_->getInitializer();

    ConstantArray *omvn_initializer_array = dyn_cast<ConstantArray>(omvn_initializer);
    
    if (!omvn_initializer_array->isString())
        return false;
    
    std::string omvn_initializer_string = omvn_initializer_array->getAsString();
    
    if (log)
        log->Printf("Found Objective-C selector reference %s", omvn_initializer_string.c_str());
    
    // Construct a call to sel_registerName
    
    if (!m_sel_registerName)
    {
        uint64_t srN_addr;
        
        if (!m_decl_map->GetFunctionAddress("sel_registerName", srN_addr))
            return false;
        
        // Build the function type: struct objc_selector *sel_registerName(uint8_t*)
        
        // The below code would be "more correct," but in actuality what's required is uint8_t*
        //Type *sel_type = StructType::get(M.getContext());
        //Type *sel_ptr_type = PointerType::getUnqual(sel_type);
        const Type *sel_ptr_type = Type::getInt8PtrTy(M.getContext());
        
        std::vector <const Type *> srN_arg_types;
        srN_arg_types.push_back(Type::getInt8PtrTy(M.getContext()));
        llvm::Type *srN_type = FunctionType::get(sel_ptr_type, srN_arg_types, false);
        
        // Build the constant containing the pointer to the function
        const IntegerType *intptr_ty = Type::getIntNTy(M.getContext(),
                                                       (M.getPointerSize() == Module::Pointer64) ? 64 : 32);
        PointerType *srN_ptr_ty = PointerType::getUnqual(srN_type);
        Constant *srN_addr_int = ConstantInt::get(intptr_ty, srN_addr, false);
        m_sel_registerName = ConstantExpr::getIntToPtr(srN_addr_int, srN_ptr_ty);
    }
    
    SmallVector <Value*, 1> srN_arguments;
    
    Constant *omvn_pointer = ConstantExpr::getBitCast(_objc_meth_var_name_, Type::getInt8PtrTy(M.getContext()));
    
    srN_arguments.push_back(omvn_pointer);
    
    CallInst *srN_call = CallInst::Create(m_sel_registerName, 
                                          srN_arguments.begin(),
                                          srN_arguments.end(),
                                          "srN",
                                          selector_load);
    
    // Replace the load with the call in all users
    
    selector_load->replaceAllUsesWith(srN_call);
    
    selector_load->eraseFromParent();
    
    return true;
}

bool
IRForTarget::rewriteObjCSelectors(Module &M, 
                                  BasicBlock &BB)
{
    lldb_private::Log *log = lldb_private::GetLogIfAllCategoriesSet (LIBLLDB_LOG_EXPRESSIONS);

    BasicBlock::iterator ii;
    
    typedef SmallVector <Instruction*, 2> InstrList;
    typedef InstrList::iterator InstrIterator;
    
    InstrList selector_loads;
    
    for (ii = BB.begin();
         ii != BB.end();
         ++ii)
    {
        Instruction &inst = *ii;
        
        if (LoadInst *load = dyn_cast<LoadInst>(&inst))
            if (isObjCSelectorRef(load->getPointerOperand()))
                selector_loads.push_back(&inst);
    }
    
    InstrIterator iter;
    
    for (iter = selector_loads.begin();
         iter != selector_loads.end();
         ++iter)
    {
        if (!RewriteObjCSelector(*iter, M))
        {
            if(log)
                log->PutCString("Couldn't rewrite a reference to an Objective-C selector");
            return false;
        }
    }
        
    return true;
}

bool 
IRForTarget::RewritePersistentAlloc(llvm::Instruction *persistent_alloc,
                                    llvm::Module &M)
{
    AllocaInst *alloc = dyn_cast<AllocaInst>(persistent_alloc);
    
    MDNode *alloc_md = alloc->getMetadata("clang.decl.ptr");

    if (!alloc_md || !alloc_md->getNumOperands())
        return false;
    
    ConstantInt *constant_int = dyn_cast<ConstantInt>(alloc_md->getOperand(0));
    
    if (!constant_int)
        return false;
    
    // We attempt to register this as a new persistent variable with the DeclMap.
    
    uintptr_t ptr = constant_int->getZExtValue();
    
    clang::VarDecl *decl = reinterpret_cast<clang::VarDecl *>(ptr);
    
    lldb_private::TypeFromParser result_decl_type (decl->getType().getAsOpaquePtr(),
                                                   &decl->getASTContext());
    
    if (!m_decl_map->AddPersistentVariable(decl, decl->getName().str().c_str(), result_decl_type))
        return false;
    
    GlobalVariable *persistent_global = new GlobalVariable(M, 
                                                           alloc->getType()->getElementType(),
                                                           false, /* not constant */
                                                           GlobalValue::ExternalLinkage,
                                                           NULL, /* no initializer */
                                                           alloc->getName().str().c_str());
    
    // What we're going to do here is make believe this was a regular old external
    // variable.  That means we need to make the metadata valid.
    
    NamedMDNode *named_metadata = M.getNamedMetadata("clang.global.decl.ptrs");
    
    llvm::Value* values[2];
    values[0] = persistent_global;
    values[1] = constant_int;

    MDNode *persistent_global_md = MDNode::get(M.getContext(), values, 2);
    named_metadata->addOperand(persistent_global_md);
    
    alloc->replaceAllUsesWith(persistent_global);
    alloc->eraseFromParent();
    
    return true;
}

bool 
IRForTarget::rewritePersistentAllocs(llvm::Module &M,
                                     llvm::BasicBlock &BB)
{
    if (!m_resolve_vars)
        return true;
    
    lldb_private::Log *log = lldb_private::GetLogIfAllCategoriesSet (LIBLLDB_LOG_EXPRESSIONS);
    
    BasicBlock::iterator ii;
    
    typedef SmallVector <Instruction*, 2> InstrList;
    typedef InstrList::iterator InstrIterator;
    
    InstrList pvar_allocs;
    
    for (ii = BB.begin();
         ii != BB.end();
         ++ii)
    {
        Instruction &inst = *ii;
        
        if (AllocaInst *alloc = dyn_cast<AllocaInst>(&inst))
            if (alloc->getName().startswith("$"))
                pvar_allocs.push_back(alloc);
    }
    
    InstrIterator iter;
    
    for (iter = pvar_allocs.begin();
         iter != pvar_allocs.end();
         ++iter)
    {
        if (!RewritePersistentAlloc(*iter, M))
        {
            if(log)
                log->PutCString("Couldn't rewrite the creation of a persistent variable");
            return false;
        }
    }
    
    return true;
}

static clang::NamedDecl *
DeclForGlobalValue(Module &module,
                   GlobalValue *global_value)
{
    NamedMDNode *named_metadata = module.getNamedMetadata("clang.global.decl.ptrs");
    
    if (!named_metadata)
        return NULL;
    
    unsigned num_nodes = named_metadata->getNumOperands();
    unsigned node_index;
    
    for (node_index = 0;
         node_index < num_nodes;
         ++node_index)
    {
        MDNode *metadata_node = named_metadata->getOperand(node_index);
        
        if (!metadata_node)
            return NULL;
        
        if (metadata_node->getNumOperands() != 2)
            continue;
        
        if (metadata_node->getOperand(0) != global_value)
            continue;
        
        ConstantInt *constant_int = dyn_cast<ConstantInt>(metadata_node->getOperand(1));
        
        if (!constant_int)
            return NULL;
        
        uintptr_t ptr = constant_int->getZExtValue();
        
        return reinterpret_cast<clang::NamedDecl *>(ptr);
    }
    
    return NULL;
}

bool 
IRForTarget::MaybeHandleVariable(Module &M, 
                                 Value *V,
                                 bool Store)
{
    lldb_private::Log *log = lldb_private::GetLogIfAllCategoriesSet (LIBLLDB_LOG_EXPRESSIONS);

    if (ConstantExpr *constant_expr = dyn_cast<ConstantExpr>(V))
    {
        switch (constant_expr->getOpcode())
        {
        default:
            break;
        case Instruction::GetElementPtr:
        case Instruction::BitCast:
            Value *s = constant_expr->getOperand(0);
            MaybeHandleVariable(M, s, Store);
        }
    }
    if (GlobalVariable *global_variable = dyn_cast<GlobalVariable>(V))
    {
        clang::NamedDecl *named_decl = DeclForGlobalValue(M, global_variable);
        
        if (!named_decl)
        {
            if (isObjCSelectorRef(V))
                return true;
            
            if (log)
                log->Printf("Found global variable %s without metadata", global_variable->getName().str().c_str());
            return false;
        }
        
        std::string name = named_decl->getName().str();
        
        void *qual_type = NULL;
        clang::ASTContext *ast_context = NULL;
        
        if (clang::ValueDecl *value_decl = dyn_cast<clang::ValueDecl>(named_decl))
        {
            qual_type = value_decl->getType().getAsOpaquePtr();
            ast_context = &value_decl->getASTContext();
        }
        else
        {
            return false;
        }
            
        const Type *value_type = global_variable->getType();
        
        size_t value_size = m_target_data->getTypeStoreSize(value_type);
        off_t value_alignment = m_target_data->getPrefTypeAlignment(value_type);
        
        if (named_decl && !m_decl_map->AddValueToStruct(named_decl,
                                                        name.c_str(),
                                                        V,
                                                        value_size, 
                                                        value_alignment))
            return false;
    }
    
    return true;
}

bool
IRForTarget::MaybeHandleCall(Module &M,
                             CallInst *C)
{
    lldb_private::Log *log = lldb_private::GetLogIfAllCategoriesSet (LIBLLDB_LOG_EXPRESSIONS);

    Function *fun = C->getCalledFunction();
    
    if (fun == NULL)
    {
        Value *val = C->getCalledValue();
        
        ConstantExpr *const_expr = dyn_cast<ConstantExpr>(val);
        
        if (const_expr && const_expr->getOpcode() == Instruction::BitCast)
        {
            fun = dyn_cast<Function>(const_expr->getOperand(0));
            
            if (!fun)
                return true;
        }
        else
        {
            return true;
        }
    }
    
    clang::NamedDecl *fun_decl = DeclForGlobalValue(M, fun);
    uint64_t fun_addr;
    Value **fun_value_ptr = NULL;
    
    if (fun_decl)
    {
        if (!m_decl_map->GetFunctionInfo(fun_decl, fun_value_ptr, fun_addr)) 
        {
            fun_value_ptr = NULL;
            
            if (!m_decl_map->GetFunctionAddress(fun->getName().str().c_str(), fun_addr))
            {
                if (log)
                    log->Printf("Function %s had no address", fun->getName().str().c_str());
                
                return false;
            }
        }
    }
    else 
    {
        if (!m_decl_map->GetFunctionAddress(fun->getName().str().c_str(), fun_addr))
        {
            if (log)
                log->Printf("Metadataless function %s had no address", fun->getName().str().c_str());
            return false;
        }
    }
        
    if (log)
        log->Printf("Found %s at %llx", fun->getName().str().c_str(), fun_addr);
    
    Value *fun_addr_ptr;
            
    if (!fun_value_ptr || !*fun_value_ptr)
    {
        const IntegerType *intptr_ty = Type::getIntNTy(M.getContext(),
                                                       (M.getPointerSize() == Module::Pointer64) ? 64 : 32);
        const FunctionType *fun_ty = fun->getFunctionType();
        PointerType *fun_ptr_ty = PointerType::getUnqual(fun_ty);
        Constant *fun_addr_int = ConstantInt::get(intptr_ty, fun_addr, false);
        fun_addr_ptr = ConstantExpr::getIntToPtr(fun_addr_int, fun_ptr_ty);
            
        if (fun_value_ptr)
            *fun_value_ptr = fun_addr_ptr;
    }
            
    if (fun_value_ptr)
        fun_addr_ptr = *fun_value_ptr;
    
    C->setCalledFunction(fun_addr_ptr);
    
    ConstantArray *func_name = (ConstantArray*)ConstantArray::get(M.getContext(), fun->getName());
    
    Value *values[1];
    values[0] = func_name;
    MDNode *func_metadata = MDNode::get(M.getContext(), values, 1);
    
    C->setMetadata("lldb.call.realName", func_metadata);
    
    if (log)
        log->Printf("Set metadata for %p [%d, %s]", C, func_name->isString(), func_name->getAsString().c_str());
    
    return true;
}

bool
IRForTarget::resolveExternals(Module &M, BasicBlock &BB)
{        
    /////////////////////////////////////////////////////////////////////////
    // Prepare the current basic block for execution in the remote process
    //
    
    BasicBlock::iterator ii;

    for (ii = BB.begin();
         ii != BB.end();
         ++ii)
    {
        Instruction &inst = *ii;
        
        if (m_resolve_vars)
        {
            if (LoadInst *load = dyn_cast<LoadInst>(&inst))
                if (!MaybeHandleVariable(M, load->getPointerOperand(), false))
                    return false;
            
            if (StoreInst *store = dyn_cast<StoreInst>(&inst))
                if (!MaybeHandleVariable(M, store->getPointerOperand(), true))
                    return false;
        }
        
        if (CallInst *call = dyn_cast<CallInst>(&inst))
            if (!MaybeHandleCall(M, call))
                return false;
    }
    
    return true;
}

static bool isGuardVariableRef(Value *V)
{
    ConstantExpr *C = dyn_cast<ConstantExpr>(V);
    
    if (!C || C->getOpcode() != Instruction::BitCast)
        return false;
    
    GlobalVariable *GV = dyn_cast<GlobalVariable>(C->getOperand(0));
    
    if (!GV || !GV->hasName() || !GV->getName().startswith("_ZGV"))
        return false;
    
    return true;
}

static void TurnGuardLoadIntoZero(Instruction* guard_load, Module &M)
{
    Constant* zero(ConstantInt::get(Type::getInt8Ty(M.getContext()), 0, true));

    Value::use_iterator ui;
    
    for (ui = guard_load->use_begin();
         ui != guard_load->use_end();
         ++ui)
    {
        if (isa<Constant>(*ui))
        {
            // do nothing for the moment
        }
        else
        {
            ui->replaceUsesOfWith(guard_load, zero);
        }
    }
    
    guard_load->eraseFromParent();
}

static void ExciseGuardStore(Instruction* guard_store)
{
    guard_store->eraseFromParent();
}

bool
IRForTarget::removeGuards(Module &M, BasicBlock &BB)
{        
    ///////////////////////////////////////////////////////
    // Eliminate any reference to guard variables found.
    //
    
    BasicBlock::iterator ii;
    
    typedef SmallVector <Instruction*, 2> InstrList;
    typedef InstrList::iterator InstrIterator;
    
    InstrList guard_loads;
    InstrList guard_stores;
    
    for (ii = BB.begin();
         ii != BB.end();
         ++ii)
    {
        Instruction &inst = *ii;
        
        if (LoadInst *load = dyn_cast<LoadInst>(&inst))
            if (isGuardVariableRef(load->getPointerOperand()))
                guard_loads.push_back(&inst);                
        
        if (StoreInst *store = dyn_cast<StoreInst>(&inst))            
            if (isGuardVariableRef(store->getPointerOperand()))
                guard_stores.push_back(&inst);
    }
    
    InstrIterator iter;
    
    for (iter = guard_loads.begin();
         iter != guard_loads.end();
         ++iter)
        TurnGuardLoadIntoZero(*iter, M);
    
    for (iter = guard_stores.begin();
         iter != guard_stores.end();
         ++iter)
        ExciseGuardStore(*iter);
    
    return true;
}

// UnfoldConstant operates on a constant [C] which has just been replaced with a value
// [new_value].  We assume that new_value has been properly placed early in the function,
// most likely somewhere in front of the first instruction in the entry basic block 
// [first_entry_instruction].  
//
// UnfoldConstant reads through the uses of C and replaces C in those uses with new_value.
// Where those uses are constants, the function generates new instructions to compute the
// result of the new, non-constant expression and places them before first_entry_instruction.  
// These instructions replace the constant uses, so UnfoldConstant calls itself recursively
// for those.

static bool
UnfoldConstant(Constant *C, Value *new_value, Instruction *first_entry_instruction)
{
    lldb_private::Log *log = lldb_private::GetLogIfAllCategoriesSet (LIBLLDB_LOG_EXPRESSIONS);

    Value::use_iterator ui;
    
    SmallVector<User*, 16> users;
    
    // We do this because the use list might change, invalidating our iterator.
    // Much better to keep a work list ourselves.
    for (ui = C->use_begin();
         ui != C->use_end();
         ++ui)
        users.push_back(*ui);
        
    for (int i = 0;
         i < users.size();
         ++i)
    {
        User *user = users[i];
                
        if (Constant *constant = dyn_cast<Constant>(user))
        {
            // synthesize a new non-constant equivalent of the constant
            
            if (ConstantExpr *constant_expr = dyn_cast<ConstantExpr>(constant))
            {
                switch (constant_expr->getOpcode())
                {
                default:
                    if (log)
                        log->Printf("Unhandled constant expression type: %s", PrintValue(constant_expr).c_str());
                    return false;
                case Instruction::BitCast:
                    {
                        // UnaryExpr
                        //   OperandList[0] is value
                        
                        Value *s = constant_expr->getOperand(0);
                        
                        if (s == C)
                            s = new_value;
                        
                        BitCastInst *bit_cast(new BitCastInst(s, C->getType(), "", first_entry_instruction));
                        
                        UnfoldConstant(constant_expr, bit_cast, first_entry_instruction);
                    }
                    break;
                case Instruction::GetElementPtr:
                    {
                        // GetElementPtrConstantExpr
                        //   OperandList[0] is base
                        //   OperandList[1]... are indices
                        
                        Value *ptr = constant_expr->getOperand(0);
                        
                        if (ptr == C)
                            ptr = new_value;
                        
                        SmallVector<Value*, 16> indices;
                        
                        unsigned operand_index;
                        unsigned num_operands = constant_expr->getNumOperands();
                        
                        for (operand_index = 1;
                             operand_index < num_operands;
                             ++operand_index)
                        {
                            Value *operand = constant_expr->getOperand(operand_index);
                            
                            if (operand == C)
                                operand = new_value;
                            
                            indices.push_back(operand);
                        }
                        
                        GetElementPtrInst *get_element_ptr(GetElementPtrInst::Create(ptr, indices.begin(), indices.end(), "", first_entry_instruction));
                        
                        UnfoldConstant(constant_expr, get_element_ptr, first_entry_instruction);
                    }
                    break;
                }
            }
            else
            {
                if (log)
                    log->Printf("Unhandled constant type: %s", PrintValue(constant).c_str());
                return false;
            }
        }
        else
        {
            // simple fall-through case for non-constants
            user->replaceUsesOfWith(C, new_value);
        }
    }
    
    return true;
}

bool 
IRForTarget::replaceVariables(Module &M, Function &F)
{
    if (!m_resolve_vars)
        return true;
    
    lldb_private::Log *log = lldb_private::GetLogIfAllCategoriesSet (LIBLLDB_LOG_EXPRESSIONS);

    m_decl_map->DoStructLayout();
    
    if (log)
        log->Printf("Element arrangement:");
    
    uint32_t num_elements;
    uint32_t element_index;
    
    size_t size;
    off_t alignment;
    
    if (!m_decl_map->GetStructInfo (num_elements, size, alignment))
        return false;
    
    Function::arg_iterator iter(F.getArgumentList().begin());
    
    if (iter == F.getArgumentList().end())
        return false;
    
    Argument *argument = iter;
    
    if (!argument->getName().equals("___clang_arg"))
        return false;
    
    if (log)
        log->Printf("Arg: %s", PrintValue(argument).c_str());
    
    BasicBlock &entry_block(F.getEntryBlock());
    Instruction *first_entry_instruction(entry_block.getFirstNonPHIOrDbg());
    
    if (!first_entry_instruction)
        return false;
    
    LLVMContext &context(M.getContext());
    const IntegerType *offset_type(Type::getInt32Ty(context));
    
    if (!offset_type)
        return false;
        
    for (element_index = 0; element_index < num_elements; ++element_index)
    {
        const clang::NamedDecl *decl;
        Value *value;
        off_t offset;
        const char *name;
        
        if (!m_decl_map->GetStructElement (decl, value, offset, name, element_index))
            return false;
        
        if (log)
            log->Printf("  %s [%s] (%s) placed at %d",
                        value->getName().str().c_str(),
                        name,
                        PrintValue(value, true).c_str(),
                        offset);
        
        ConstantInt *offset_int(ConstantInt::getSigned(offset_type, offset));
        GetElementPtrInst *get_element_ptr = GetElementPtrInst::Create(argument, offset_int, "", first_entry_instruction);
        BitCastInst *bit_cast = new BitCastInst(get_element_ptr, value->getType(), "", first_entry_instruction);
        
        if (Constant *constant = dyn_cast<Constant>(value))
            UnfoldConstant(constant, bit_cast, first_entry_instruction);
        else
            value->replaceAllUsesWith(bit_cast);
    }
    
    if (log)
        log->Printf("Total structure [align %d, size %d]", alignment, size);
    
    return true;
}

bool
IRForTarget::runOnModule(Module &M)
{
    lldb_private::Log *log = lldb_private::GetLogIfAllCategoriesSet (LIBLLDB_LOG_EXPRESSIONS);
    
    Function* function = M.getFunction(StringRef(m_func_name.c_str()));
    
    if (!function)
    {
        if (log)
            log->Printf("Couldn't find %s() in the module", m_func_name.c_str());
        
        return false;
    }
        
    Function::iterator bbi;
    
    ////////////////////////////////////////////////////////////
    // Replace __clang_expr_result with a persistent variable
    //
    
    if (!createResultVariable(M, *function))
        return false;
    
    //////////////////////////////////
    // Run basic-block level passes
    //
    
    for (bbi = function->begin();
         bbi != function->end();
         ++bbi)
    {
        if (!removeGuards(M, *bbi))
            return false;
        
        if (!rewritePersistentAllocs(M, *bbi))
            return false;
        
        if (!rewriteObjCSelectors(M, *bbi))
            return false;

        if (!resolveExternals(M, *bbi))
            return false;
    }
    
    if (log)
    {
        std::string s;
        raw_string_ostream oss(s);
        
        M.print(oss, NULL);
        
        oss.flush();
        
        log->Printf("Module after preparing for execution: \n%s", s.c_str());
    }
    
    ///////////////////////////////
    // Run function-level passes
    //
    
    if (!replaceVariables(M, *function))
        return false;
    
    return true;    
}

void
IRForTarget::assignPassManager(PMStack &PMS,
                               PassManagerType T)
{
}

PassManagerType
IRForTarget::getPotentialPassManagerType() const
{
    return PMT_ModulePassManager;
}