[optimze]添加基础的除法指令优化，目前只对除以2的幂数生效
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329
src/backend/RISCv64/Optimize/DivStrengthReduction.cpp
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329
src/backend/RISCv64/Optimize/DivStrengthReduction.cpp
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#include "DivStrengthReduction.h"
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namespace sysy {
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char DivStrengthReduction::ID = 0;
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bool DivStrengthReduction::runOnFunction(Function *F, AnalysisManager& AM) {
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// This pass works on MachineFunction level, not IR level
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return false;
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}
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void DivStrengthReduction::runOnMachineFunction(MachineFunction *mfunc) {
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if (!mfunc)
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return;
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bool debug = false; // Set to true for debugging
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if (debug)
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std::cout << "Running DivStrengthReduction optimization..." << std::endl;
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// 虚拟寄存器分配器
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int next_temp_reg = 1000;
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auto createTempReg = [&]() -> int {
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return next_temp_reg++;
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};
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// Magic number 信息结构
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struct MagicInfo {
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int64_t magic;
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int shift;
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bool add_indicator; // 是否需要额外的加法修正
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};
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// 针对缺少MULH指令的简化magic number计算
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auto computeMagicNumber = [](int64_t divisor, bool is_32bit) -> MagicInfo {
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if (divisor == 0) return {0, 0, false};
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if (divisor == 1) return {1, 0, false};
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if (divisor == -1) return {-1, 0, false};
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// 对于没有MULH的情况，我们使用更简单但有效的算法
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// 基于 2^n / divisor 的近似
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bool neg = divisor < 0;
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int64_t d = neg ? -divisor : divisor;
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int word_size = is_32bit ? 32 : 64;
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// 计算合适的移位量
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int shift = word_size;
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int64_t magic = ((1LL << shift) + d - 1) / d;
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// 调整magic number以适应MUL指令
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if (is_32bit) {
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// 32位情况：调整magic使其适合符号扩展后的乘法
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shift = 32;
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magic = ((1LL << shift) + d - 1) / d;
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} else {
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// 64位情况：使用更保守的算法
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shift = 32; // 使用32位作为基础移位
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magic = ((1LL << shift) + d - 1) / d;
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}
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bool add_indicator = false;
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// 检查是否需要加法修正
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if (magic >= (1LL << (word_size - 1))) {
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add_indicator = true;
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magic -= (1LL << word_size);
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}
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if (neg) {
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magic = -magic;
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}
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return {magic, shift, add_indicator};
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};
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// 检查是否为2的幂次
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auto isPowerOfTwo = [](int64_t n) -> bool {
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return n > 0 && (n & (n - 1)) == 0;
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};
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// 获取2的幂次的指数
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auto getPowerOfTwoExponent = [](int64_t n) -> int {
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if (n <= 0 || (n & (n - 1)) != 0) return -1;
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int shift = 0;
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while (n > 1) {
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n >>= 1;
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shift++;
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}
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return shift;
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};
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// 收集需要替换的指令
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struct InstructionReplacement {
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size_t index;
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std::vector<std::unique_ptr<MachineInstr>> newInstrs;
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};
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for (auto &mbb_uptr : mfunc->getBlocks()) {
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auto &mbb = *mbb_uptr;
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auto &instrs = mbb.getInstructions();
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std::vector<InstructionReplacement> replacements;
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for (size_t i = 0; i < instrs.size(); ++i) {
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auto *instr = instrs[i].get();
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bool is_32bit = (instr->getOpcode() == RVOpcodes::DIVW);
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// 只处理 DIV 和 DIVW 指令
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if (instr->getOpcode() != RVOpcodes::DIV && !is_32bit) {
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continue;
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}
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if (instr->getOperands().size() != 3) {
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continue;
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}
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auto *dst_op = instr->getOperands()[0].get();
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auto *src1_op = instr->getOperands()[1].get();
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auto *src2_op = instr->getOperands()[2].get();
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// 检查操作数类型
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if (dst_op->getKind() != MachineOperand::KIND_REG ||
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src1_op->getKind() != MachineOperand::KIND_REG ||
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src2_op->getKind() != MachineOperand::KIND_IMM) {
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continue;
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}
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auto *dst_reg = static_cast<RegOperand *>(dst_op);
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auto *src1_reg = static_cast<RegOperand *>(src1_op);
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auto *src2_imm = static_cast<ImmOperand *>(src2_op);
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int64_t divisor = src2_imm->getValue();
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// 跳过除数为0的情况
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if (divisor == 0) continue;
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std::vector<std::unique_ptr<MachineInstr>> newInstrs;
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// 情况1: 除数为1
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if (divisor == 1) {
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// dst = src1 (直接复制)
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auto moveInstr = std::make_unique<MachineInstr>(is_32bit ? RVOpcodes::ADDW : RVOpcodes::ADD);
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moveInstr->addOperand(std::make_unique<RegOperand>(*dst_reg));
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moveInstr->addOperand(std::make_unique<RegOperand>(*src1_reg));
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moveInstr->addOperand(std::make_unique<RegOperand>(PhysicalReg::ZERO));
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newInstrs.push_back(std::move(moveInstr));
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}
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// 情况2: 除数为-1
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else if (divisor == -1) {
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// dst = -src1
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auto negInstr = std::make_unique<MachineInstr>(is_32bit ? RVOpcodes::SUBW : RVOpcodes::SUB);
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negInstr->addOperand(std::make_unique<RegOperand>(*dst_reg));
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negInstr->addOperand(std::make_unique<RegOperand>(PhysicalReg::ZERO));
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negInstr->addOperand(std::make_unique<RegOperand>(*src1_reg));
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newInstrs.push_back(std::move(negInstr));
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}
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// 情况3: 正的2的幂次除法
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else if (isPowerOfTwo(divisor)) {
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int shift = getPowerOfTwoExponent(divisor);
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int temp_reg = createTempReg();
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// 对于有符号除法，需要处理负数的舍入
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// if (src1 < 0) src1 += (divisor - 1)
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// 获取符号位：temp = src1 >> (word_size - 1)
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auto sraSignInstr = std::make_unique<MachineInstr>(is_32bit ? RVOpcodes::SRAIW : RVOpcodes::SRAI);
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sraSignInstr->addOperand(std::make_unique<RegOperand>(temp_reg));
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sraSignInstr->addOperand(std::make_unique<RegOperand>(*src1_reg));
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sraSignInstr->addOperand(std::make_unique<ImmOperand>(is_32bit ? 31 : 63));
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newInstrs.push_back(std::move(sraSignInstr));
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// 计算偏移：temp = temp >> (word_size - shift)
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if (shift < (is_32bit ? 32 : 64)) {
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auto srlInstr = std::make_unique<MachineInstr>(is_32bit ? RVOpcodes::SRLIW : RVOpcodes::SRLI);
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srlInstr->addOperand(std::make_unique<RegOperand>(temp_reg));
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srlInstr->addOperand(std::make_unique<RegOperand>(temp_reg));
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srlInstr->addOperand(std::make_unique<ImmOperand>((is_32bit ? 32 : 64) - shift));
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newInstrs.push_back(std::move(srlInstr));
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}
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// 加上偏移：temp = src1 + temp
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auto addInstr = std::make_unique<MachineInstr>(is_32bit ? RVOpcodes::ADDW : RVOpcodes::ADD);
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addInstr->addOperand(std::make_unique<RegOperand>(temp_reg));
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addInstr->addOperand(std::make_unique<RegOperand>(*src1_reg));
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addInstr->addOperand(std::make_unique<RegOperand>(temp_reg));
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newInstrs.push_back(std::move(addInstr));
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// 最终右移：dst = temp >> shift
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auto sraInstr = std::make_unique<MachineInstr>(is_32bit ? RVOpcodes::SRAIW : RVOpcodes::SRAI);
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sraInstr->addOperand(std::make_unique<RegOperand>(*dst_reg));
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sraInstr->addOperand(std::make_unique<RegOperand>(temp_reg));
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sraInstr->addOperand(std::make_unique<ImmOperand>(shift));
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newInstrs.push_back(std::move(sraInstr));
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}
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// 情况4: 负的2的幂次除法
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else if (divisor < 0 && isPowerOfTwo(-divisor)) {
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int shift = getPowerOfTwoExponent(-divisor);
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int temp_reg = createTempReg();
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// 先按正数处理
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auto sraSignInstr = std::make_unique<MachineInstr>(is_32bit ? RVOpcodes::SRAIW : RVOpcodes::SRAI);
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sraSignInstr->addOperand(std::make_unique<RegOperand>(temp_reg));
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sraSignInstr->addOperand(std::make_unique<RegOperand>(*src1_reg));
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sraSignInstr->addOperand(std::make_unique<ImmOperand>(is_32bit ? 31 : 63));
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newInstrs.push_back(std::move(sraSignInstr));
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if (shift < (is_32bit ? 32 : 64)) {
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auto srlInstr = std::make_unique<MachineInstr>(is_32bit ? RVOpcodes::SRLIW : RVOpcodes::SRLI);
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srlInstr->addOperand(std::make_unique<RegOperand>(temp_reg));
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srlInstr->addOperand(std::make_unique<RegOperand>(temp_reg));
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srlInstr->addOperand(std::make_unique<ImmOperand>((is_32bit ? 32 : 64) - shift));
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newInstrs.push_back(std::move(srlInstr));
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}
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auto addInstr = std::make_unique<MachineInstr>(is_32bit ? RVOpcodes::ADDW : RVOpcodes::ADD);
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addInstr->addOperand(std::make_unique<RegOperand>(temp_reg));
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addInstr->addOperand(std::make_unique<RegOperand>(*src1_reg));
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addInstr->addOperand(std::make_unique<RegOperand>(temp_reg));
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newInstrs.push_back(std::move(addInstr));
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auto sraInstr = std::make_unique<MachineInstr>(is_32bit ? RVOpcodes::SRAIW : RVOpcodes::SRAI);
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sraInstr->addOperand(std::make_unique<RegOperand>(temp_reg));
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sraInstr->addOperand(std::make_unique<RegOperand>(temp_reg));
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sraInstr->addOperand(std::make_unique<ImmOperand>(shift));
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newInstrs.push_back(std::move(sraInstr));
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// 然后取反
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auto negInstr = std::make_unique<MachineInstr>(is_32bit ? RVOpcodes::SUBW : RVOpcodes::SUB);
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negInstr->addOperand(std::make_unique<RegOperand>(*dst_reg));
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negInstr->addOperand(std::make_unique<RegOperand>(PhysicalReg::ZERO));
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negInstr->addOperand(std::make_unique<RegOperand>(temp_reg));
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newInstrs.push_back(std::move(negInstr));
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}
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// 情况5: 通用magic number算法（针对没有MULH的情况进行了简化）
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else {
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// 对于一般除法，在没有MULH的情况下，我们采用更保守的策略
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// 只处理一些简单的常数除法，复杂的情况保持原始除法指令
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// 检查是否为小的常数（可以用简单乘法处理）
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if (std::abs(divisor) <= 1024) { // 限制在较小的除数范围内
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auto magic_info = computeMagicNumber(divisor, is_32bit);
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if (magic_info.magic == 0) continue;
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int magic_reg = createTempReg();
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int temp_reg = createTempReg();
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// 加载magic number到寄存器
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auto loadInstr = std::make_unique<MachineInstr>(RVOpcodes::LI);
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loadInstr->addOperand(std::make_unique<RegOperand>(magic_reg));
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loadInstr->addOperand(std::make_unique<ImmOperand>(magic_info.magic));
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newInstrs.push_back(std::move(loadInstr));
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// 使用普通乘法模拟高位乘法
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if (is_32bit) {
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// 32位：使用MULW
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auto mulInstr = std::make_unique<MachineInstr>(RVOpcodes::MULW);
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mulInstr->addOperand(std::make_unique<RegOperand>(temp_reg));
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mulInstr->addOperand(std::make_unique<RegOperand>(*src1_reg));
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mulInstr->addOperand(std::make_unique<RegOperand>(magic_reg));
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newInstrs.push_back(std::move(mulInstr));
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// 右移得到近似结果
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auto sraInstr = std::make_unique<MachineInstr>(RVOpcodes::SRAIW);
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sraInstr->addOperand(std::make_unique<RegOperand>(temp_reg));
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sraInstr->addOperand(std::make_unique<RegOperand>(temp_reg));
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sraInstr->addOperand(std::make_unique<ImmOperand>(magic_info.shift));
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newInstrs.push_back(std::move(sraInstr));
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} else {
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// 64位：使用MUL
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auto mulInstr = std::make_unique<MachineInstr>(RVOpcodes::MUL);
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mulInstr->addOperand(std::make_unique<RegOperand>(temp_reg));
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mulInstr->addOperand(std::make_unique<RegOperand>(*src1_reg));
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mulInstr->addOperand(std::make_unique<RegOperand>(magic_reg));
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newInstrs.push_back(std::move(mulInstr));
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// 右移得到近似结果
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auto sraInstr = std::make_unique<MachineInstr>(RVOpcodes::SRAI);
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sraInstr->addOperand(std::make_unique<RegOperand>(temp_reg));
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sraInstr->addOperand(std::make_unique<RegOperand>(temp_reg));
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sraInstr->addOperand(std::make_unique<ImmOperand>(magic_info.shift));
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newInstrs.push_back(std::move(sraInstr));
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}
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// 符号修正：处理负数被除数
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int sign_reg = createTempReg();
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// 获取被除数的符号位
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auto sraSignInstr = std::make_unique<MachineInstr>(is_32bit ? RVOpcodes::SRAIW : RVOpcodes::SRAI);
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sraSignInstr->addOperand(std::make_unique<RegOperand>(sign_reg));
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sraSignInstr->addOperand(std::make_unique<RegOperand>(*src1_reg));
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sraSignInstr->addOperand(std::make_unique<ImmOperand>(is_32bit ? 31 : 63));
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newInstrs.push_back(std::move(sraSignInstr));
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// 最终结果：dst = temp - sign（对于正除数）或 dst = temp + sign（对于负除数）
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if (divisor > 0) {
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auto finalSubInstr = std::make_unique<MachineInstr>(is_32bit ? RVOpcodes::SUBW : RVOpcodes::SUB);
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finalSubInstr->addOperand(std::make_unique<RegOperand>(*dst_reg));
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finalSubInstr->addOperand(std::make_unique<RegOperand>(temp_reg));
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finalSubInstr->addOperand(std::make_unique<RegOperand>(sign_reg));
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newInstrs.push_back(std::move(finalSubInstr));
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} else {
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auto finalAddInstr = std::make_unique<MachineInstr>(is_32bit ? RVOpcodes::ADDW : RVOpcodes::ADD);
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finalAddInstr->addOperand(std::make_unique<RegOperand>(*dst_reg));
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finalAddInstr->addOperand(std::make_unique<RegOperand>(temp_reg));
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finalAddInstr->addOperand(std::make_unique<RegOperand>(sign_reg));
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newInstrs.push_back(std::move(finalAddInstr));
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}
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}
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// 对于大的除数或复杂情况，保持原始除法指令不变
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}
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if (!newInstrs.empty()) {
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replacements.push_back({i, std::move(newInstrs)});
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}
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}
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// 批量应用替换（从后往前处理避免索引问题）
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for (auto it = replacements.rbegin(); it != replacements.rend(); ++it) {
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instrs.erase(instrs.begin() + it->index);
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instrs.insert(instrs.begin() + it->index,
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std::make_move_iterator(it->newInstrs.begin()),
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std::make_move_iterator(it->newInstrs.end()));
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}
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}
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}
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} // namespace sysy
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