Function emit_shift_handler

src/codec.cpp:1735–1795

    }

    static void emit_shift_handler(X64Emitter& e, const VMConfig& vm, void (X64Emitter::*op_cl)(std::uint8_t, bool)) {
        const auto& r = vm.dispatcher_regs();
        emit_fetch_byte_dec(e, vm, r.scratch_a); // dst slot
        emit_fetch_byte_dec(e, vm, r.scratch_b); // dst width
        e.push_reg(r.scratch_a); e.push_reg(r.scratch_b);
        e.mov_reg_mem_sib(
            rx::rax,
            r.state_ptr,
            r.scratch_a,
            3,
            static_cast<std::int32_t>(vm.state_layout().regs_base), 
            true
        );
        emit_fetch_byte_dec(e, vm, r.scratch_a);
        auto lbl_reg = e.new_label(), lbl_done = e.new_label();
        e.cmp_reg_imm32(r.scratch_a, 0);
        e.jcc_label(cc::z, lbl_reg);
        emit_fetch_byte_dec(e, vm, rx::rcx);
        e.jmp_label(lbl_done);
        e.bind(lbl_reg);
        emit_fetch_byte_dec(e, vm, r.scratch_b);
        emit_fetch_byte_dec(e, vm, r.scratch_a);
        e.mov_reg_mem_sib(
            rx::rcx, 
            r.state_ptr, 
            r.scratch_b, 
            3,
            static_cast<std::int32_t>(vm.state_layout().regs_base), 
            true
        );
        e.bind(lbl_done);
        e.and_reg_imm32(rx::rcx, 0x3F);

        // width-aware shift
        auto lbl_w8_shift = e.new_label(), lbl_shift_done = e.new_label();
        e.mov_reg_mem(
            rx::rdx,
            rx::rsp,
            0,
            true
        ); // rdx = saved width byte

        e.and_reg_imm32(rx::rdx, 0x7F);
        e.cmp_reg_imm32(rx::rdx, 8);
        e.jcc_label(cc::z, lbl_w8_shift);
        (e.*op_cl)(rx::rax, false); // 32-bit shift zero-extends
        e.jmp_label(lbl_shift_done);
        e.bind(lbl_w8_shift);
        (e.*op_cl)(rx::rax, true); // 64-bit shift
        e.bind(lbl_shift_done);
        e.pop_reg(r.scratch_b);
        e.pop_reg(r.scratch_a);
        emit_store_slot_value(
            e,
            vm,
            r.scratch_a,
            rx::rax,
            r.scratch_b