276 lines
9.9 KiB
C++
276 lines
9.9 KiB
C++
#include "DeadCodeElimination.h"
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#include <iostream>
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extern int DEBUG;
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namespace sysy {
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void DeadCodeElimination::runDCEPipeline() {
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const auto& functions = pModule->getFunctions();
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for (const auto& function : functions) {
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const auto& func = function.second;
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bool changed = true;
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while (changed) {
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changed = false;
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eliminateDeadStores(func.get(), changed);
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eliminateDeadLoads(func.get(), changed);
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eliminateDeadAllocas(func.get(), changed);
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eliminateDeadRedundantLoadStore(func.get(), changed);
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eliminateDeadGlobals(changed);
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}
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}
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}
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// 消除无用存储 消除条件：
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// 存储的目标指针（pointer）不是全局变量（!isGlobal(pointer)）。
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// 存储的目标指针不是数组参数（!isArr(pointer) 或不在函数参数列表里）。
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// 该指针的所有使用者（uses）仅限 alloca 或 store（即没有 load 或其他指令使用它）。
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void DeadCodeElimination::eliminateDeadStores(Function* func, bool& changed) {
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for (const auto& block : func->getBasicBlocks()) {
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auto& instrs = block->getInstructions();
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for (auto iter = instrs.begin(); iter != instrs.end();) {
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auto inst = iter->get();
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if (!inst->isStore()) {
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++iter;
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continue;
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}
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auto storeInst = dynamic_cast<StoreInst*>(inst);
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auto pointer = storeInst->getPointer();
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// 如果是全局变量或者是函数的数组参数
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if (isGlobal(pointer) || (isArr(pointer) &&
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std::find(func->getEntryBlock()->getArguments().begin(),
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func->getEntryBlock()->getArguments().end(),
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pointer) != func->getEntryBlock()->getArguments().end())) {
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++iter;
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continue;
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}
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bool changetag = true;
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for (auto& use : pointer->getUses()) {
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// 依次判断store的指针是否被其他指令使用
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auto user = use->getUser();
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auto userInst = dynamic_cast<Instruction*>(user);
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// 如果使用store的指针的指令不是Alloca或Store，则不删除
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if (userInst != nullptr && !userInst->isAlloca() && !userInst->isStore()) {
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changetag = false;
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break;
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}
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}
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if (changetag) {
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changed = true;
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if(DEBUG){
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std::cout << "=== Dead Store Found ===\n";
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SysYPrinter::printInst(storeInst);
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}
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usedelete(storeInst);
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iter = instrs.erase(iter);
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} else {
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++iter;
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}
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}
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}
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}
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// 消除无用加载 消除条件：
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// 该指令的结果未被使用（inst->getUses().empty()）。
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void DeadCodeElimination::eliminateDeadLoads(Function* func, bool& changed) {
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for (const auto& block : func->getBasicBlocks()) {
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auto& instrs = block->getInstructions();
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for (auto iter = instrs.begin(); iter != instrs.end();) {
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auto inst = iter->get();
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if (inst->isBinary() || inst->isUnary() || inst->isLoad()) {
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if (inst->getUses().empty()) {
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changed = true;
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if(DEBUG){
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std::cout << "=== Dead Load Binary Unary Found ===\n";
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SysYPrinter::printInst(inst);
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}
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usedelete(inst);
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iter = instrs.erase(iter);
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continue;
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}
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}
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++iter;
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}
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}
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}
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// 消除无用加载 消除条件：
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// 该 alloca 未被任何指令使用（allocaInst->getUses().empty()）。
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// 该 alloca 不是函数的参数（不在 entry 块的参数列表里）。
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void DeadCodeElimination::eliminateDeadAllocas(Function* func, bool& changed) {
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for (const auto& block : func->getBasicBlocks()) {
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auto& instrs = block->getInstructions();
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for (auto iter = instrs.begin(); iter != instrs.end();) {
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auto inst = iter->get();
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if (inst->isAlloca()) {
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auto allocaInst = dynamic_cast<AllocaInst*>(inst);
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if (allocaInst->getUses().empty() &&
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std::find(func->getEntryBlock()->getArguments().begin(),
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func->getEntryBlock()->getArguments().end(),
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allocaInst) == func->getEntryBlock()->getArguments().end()) {
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changed = true;
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if(DEBUG){
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std::cout << "=== Dead Alloca Found ===\n";
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SysYPrinter::printInst(inst);
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}
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usedelete(inst);
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iter = instrs.erase(iter);
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continue;
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}
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}
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++iter;
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}
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}
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}
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void DeadCodeElimination::eliminateDeadIndirectiveAllocas(Function* func, bool& changed) {
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// 删除mem2reg时引入的且现在已经没有value使用了的隐式alloca
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FunctionAnalysisInfo* funcInfo = pCFA->getFunctionAnalysisInfo(func);
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for (auto it = funcInfo->getIndirectAllocas().begin(); it != funcInfo->getIndirectAllocas().end();) {
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auto &allocaInst = *it;
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if (allocaInst->getUses().empty()) {
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changed = true;
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if(DEBUG){
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std::cout << "=== Dead Indirect Alloca Found ===\n";
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SysYPrinter::printInst(allocaInst.get());
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}
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it = funcInfo->getIndirectAllocas().erase(it);
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} else {
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++it;
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}
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}
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}
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// 该全局变量未被任何指令使用（global->getUses().empty()）。
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void DeadCodeElimination::eliminateDeadGlobals(bool& changed) {
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auto& globals = pModule->getGlobals();
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for (auto it = globals.begin(); it != globals.end();) {
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auto& global = *it;
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if (global->getUses().empty()) {
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changed = true;
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if(DEBUG){
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std::cout << "=== Dead Global Found ===\n";
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SysYPrinter::printValue(global.get());
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}
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it = globals.erase(it);
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} else {
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++it;
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}
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}
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}
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// 消除冗余加载和存储 消除条件：
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// phi 指令的目标指针仅被该 phi 使用（无其他 store/load 使用）。
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// memset 指令的目标指针未被使用（pointer->getUses().empty()）
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// store -> load -> store 模式
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void DeadCodeElimination::eliminateDeadRedundantLoadStore(Function* func, bool& changed) {
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for (const auto& block : func->getBasicBlocks()) {
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auto& instrs = block->getInstructions();
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for (auto iter = instrs.begin(); iter != instrs.end();) {
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auto inst = iter->get();
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if (inst->isPhi()) {
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auto phiInst = dynamic_cast<PhiInst*>(inst);
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auto pointer = phiInst->getPointer();
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bool tag = true;
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for (const auto& use : pointer->getUses()) {
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auto user = use->getUser();
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if (user != inst) {
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tag = false;
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break;
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}
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}
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/// 如果 pointer 仅被该 phi 使用，可以删除 ph
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if (tag) {
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changed = true;
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usedelete(inst);
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iter = instrs.erase(iter);
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continue;
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}
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// 数组指令还不完善，不保证memset优化效果
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} else if (inst->isMemset()) {
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auto memsetInst = dynamic_cast<MemsetInst*>(inst);
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auto pointer = memsetInst->getPointer();
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if (pointer->getUses().empty()) {
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changed = true;
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usedelete(inst);
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iter = instrs.erase(iter);
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continue;
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}
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}else if(inst->isLoad()) {
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if (iter != instrs.begin()) {
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auto loadInst = dynamic_cast<LoadInst*>(inst);
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auto loadPointer = loadInst->getPointer();
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// TODO:store -> load -> store 模式
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auto prevIter = std::prev(iter);
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auto prevInst = prevIter->get();
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if (prevInst->isStore()) {
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auto prevStore = dynamic_cast<StoreInst*>(prevInst);
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auto prevStorePointer = prevStore->getPointer();
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auto prevStoreValue = prevStore->getOperand(0);
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// 确保前一个 store 不是数组操作
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if (prevStore->getIndices().empty()) {
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// 检查后一条指令是否是 store 同一个值
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auto nextIter = std::next(iter);
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if (nextIter != instrs.end()) {
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auto nextInst = nextIter->get();
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if (nextInst->isStore()) {
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auto nextStore = dynamic_cast<StoreInst*>(nextInst);
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auto nextStorePointer = nextStore->getPointer();
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auto nextStoreValue = nextStore->getOperand(0);
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// 确保后一个 store 不是数组操作
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if (nextStore->getIndices().empty()) {
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// 判断优化条件：
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// 1. prevStore 的指针操作数 == load 的指针操作数
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// 2. nextStore 的值操作数 == load 指令本身
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if (prevStorePointer == loadPointer &&
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nextStoreValue == loadInst) {
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// 可以优化直接把prevStorePointer的值存到nextStorePointer
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changed = true;
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nextStore->setOperand(0, prevStoreValue);
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if(DEBUG){
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std::cout << "=== Dead Store Load Store Found(now only del Load) ===\n";
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SysYPrinter::printInst(prevStore);
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SysYPrinter::printInst(loadInst);
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SysYPrinter::printInst(nextStore);
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}
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usedelete(loadInst);
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iter = instrs.erase(iter);
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// 删除 prevStore 这里是不是可以留给删除无用store处理？
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// if (prevStore->getUses().empty()) {
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// usedelete(prevStore);
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// instrs.erase(prevIter); // 删除 prevStore
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// }
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continue; // 跳过 ++iter，因为已经移动迭代器
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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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}
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++iter;
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}
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}
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}
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bool DeadCodeElimination::isGlobal(Value *val){
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auto gval = dynamic_cast<GlobalValue *>(val);
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return gval != nullptr;
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}
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bool DeadCodeElimination::isArr(Value *val){
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auto aval = dynamic_cast<AllocaInst *>(val);
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return aval != nullptr && aval->getNumDims() != 0;
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}
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void DeadCodeElimination::usedelete(Instruction *instr){
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for (auto &use1 : instr->getOperands()) {
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auto val1 = use1->getValue();
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val1->removeUse(use1);
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}
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}
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} // namespace sysy
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