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Hello dis
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by Pymier0
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<p>Tiếp loạt bài về Python compiler.</p>
<p><img alt="img" src="https://images.unsplash.com/photo-1563775506308-5812e69b313e?crop=entropy&cs=tinysrgb&fit=max&fm=jpg&ixid=MnwyMzI1MzN8MHwxfHJhbmRvbXx8fHx8fHx8fDE2MzAzNzU1Njg&ixlib=rb-1.2.1&q=80&w=600Wrote%20newpost.md"></p>
<p>Trong <a href="./compile.html">CPython compiler</a>, ta đã biết CPython có thực
hiện việc compile code, sinh ra các bytecode, rồi bytecode sẽ được CPython <span class="caps">VM</span>
chạy. Khi import module, CPython sinh ra các file <code>.pyc</code> sau khi compile. Nội dung
các file này ở dạng binary, người dùng sẽ không thể đọc được.</p>
<p>Python có sẵn thư viện <code>dis</code>, giúp thực hiện “disassembly”, biến code thành
dạng mã bytecode đọc được.
Bài này giới thiệu các phép toán đơn giản để làm quen:
Tạo các file code .py đơn giản rồi chạy chúng với lệnh: <code>python3 -m dis tenfile.py</code></p>
<div class="highlight"><pre><span></span><code><span class="c1"># ==> d1.py <==</span>
<span class="n">x</span> <span class="o">=</span> <span class="mi">5</span>
<span class="c1"># ==> d2.py <==</span>
<span class="n">x</span> <span class="o">=</span> <span class="mi">5</span>
<span class="n">y</span> <span class="o">=</span> <span class="mi">7</span>
<span class="c1"># ==> d3.py <==</span>
<span class="n">x</span> <span class="o">=</span> <span class="mi">5</span>
<span class="n">y</span> <span class="o">=</span> <span class="mi">7</span>
<span class="n">z</span> <span class="o">=</span> <span class="n">x</span> <span class="o">+</span> <span class="n">y</span>
<span class="c1"># ==> d4.py <==</span>
<span class="n">x</span> <span class="o">=</span> <span class="mi">5</span>
<span class="n">y</span> <span class="o">=</span> <span class="mi">7</span>
<span class="n">z</span> <span class="o">=</span> <span class="n">x</span> <span class="o">+</span> <span class="n">y</span>
<span class="n">s</span> <span class="o">=</span> <span class="n">x</span> <span class="o">-</span> <span class="n">y</span>
<span class="nb">print</span><span class="p">(</span><span class="n">z</span><span class="p">)</span>
<span class="nb">print</span><span class="p">(</span><span class="n">s</span><span class="p">)</span>
<span class="nb">print</span><span class="p">(</span><span class="nb">len</span><span class="p">(</span><span class="s2">"Python"</span><span class="p">))</span>
<span class="c1"># ==> d5.py <==</span>
<span class="n">x</span> <span class="o">=</span> <span class="mi">5</span>
<span class="n">y</span> <span class="o">=</span> <span class="o">-</span><span class="n">x</span>
</code></pre></div>
<p>File đầu tiên gán x = 5, số 1 trong cột đầu tiên thể hiện
dòng code.</p>
<div class="highlight"><pre><span></span><code>$ python3 -m dis d1.py
<span class="m">1</span> <span class="m">0</span> LOAD_CONST <span class="m">0</span> <span class="o">(</span><span class="m">5</span><span class="o">)</span>
<span class="m">2</span> STORE_NAME <span class="m">0</span> <span class="o">(</span>x<span class="o">)</span>
<span class="m">4</span> LOAD_CONST <span class="m">1</span> <span class="o">(</span>None<span class="o">)</span>
<span class="m">6</span> RETURN_VALUE
</code></pre></div>
<p>5 là một giá trị có sẵn, Python thực hiện việc lấy nó <code>LOAD_CONST</code>, sau đó gán x = 5 với <code>STORE_NAME</code>. 2 dòng sau tạm không bàn tới và xem tiếp các ví dụ còn lại:</p>
<p>chạy d2.py sẽ không khác với d1.py, chỉ thêm phần LOAD_CONST cho 7 và thưc hiện <code>STORE_NAME</code> y = 7.</p>
<p>d3.py thêm một phép tính cộng, có <span class="caps">BYTECODE</span> là <code>BINARY_ADD</code>.</p>
<div class="highlight"><pre><span></span><code>d3.py
<span class="m">1</span> <span class="m">0</span> LOAD_CONST <span class="m">0</span> <span class="o">(</span><span class="m">5</span><span class="o">)</span>
<span class="m">2</span> STORE_NAME <span class="m">0</span> <span class="o">(</span>x<span class="o">)</span>
<span class="m">2</span> <span class="m">4</span> LOAD_CONST <span class="m">1</span> <span class="o">(</span><span class="m">7</span><span class="o">)</span>
<span class="m">6</span> STORE_NAME <span class="m">1</span> <span class="o">(</span>y<span class="o">)</span>
<span class="m">3</span> <span class="m">8</span> LOAD_NAME <span class="m">0</span> <span class="o">(</span>x<span class="o">)</span>
<span class="m">10</span> LOAD_NAME <span class="m">1</span> <span class="o">(</span>y<span class="o">)</span>
<span class="m">12</span> BINARY_ADD
<span class="m">14</span> STORE_NAME <span class="m">2</span> <span class="o">(</span>z<span class="o">)</span>
<span class="m">16</span> LOAD_CONST <span class="m">2</span> <span class="o">(</span>None<span class="o">)</span>
<span class="m">18</span> RETURN_VALUE
</code></pre></div>
<p>d4.py có phép trừ, có <span class="caps">BYTECODE</span> là <code>BINARY_SUBTRACT</code>.
Chú ý chữ <code>BINARY</code> nói đây là phép toán có 2 toán tử như x + y hay x - y, một
loại phép toán khác chỉ có 1 toán tử như phép lấy số âm (-x), thì - là
<code>UNARY_NEGATIVE</code>.</p>
<div class="highlight"><pre><span></span><code>$ python3 -m dis d5.py
<span class="m">1</span> <span class="m">0</span> LOAD_CONST <span class="m">0</span> <span class="o">(</span><span class="m">5</span><span class="o">)</span>
<span class="m">2</span> STORE_NAME <span class="m">0</span> <span class="o">(</span>x<span class="o">)</span>
<span class="m">2</span> <span class="m">4</span> LOAD_NAME <span class="m">0</span> <span class="o">(</span>x<span class="o">)</span>
<span class="m">6</span> UNARY_NEGATIVE
<span class="m">8</span> STORE_NAME <span class="m">1</span> <span class="o">(</span>y<span class="o">)</span>
<span class="m">10</span> LOAD_CONST <span class="m">1</span> <span class="o">(</span>None<span class="o">)</span>
<span class="m">12</span> RETURN_VALUE
<span class="c1"># d5.py</span>
<span class="nv">x</span> <span class="o">=</span> <span class="m">5</span>
<span class="nv">y</span> <span class="o">=</span> -x
</code></pre></div>
<p>d4.py có gọi các function có sẵn, việc đầu tiên là lấy ra function với
<code>LOAD_NAME</code>, rồi gọi với <code>CALL_FUNCTION</code>. Chú ý sau mỗi lần gọi function có
<code>POP_TOP</code>, sẽ tìm hiểu ở bài sau.</p>
<div class="highlight"><pre><span></span><code><span class="n">d4</span><span class="o">.</span><span class="n">py</span>
<span class="mi">1</span> <span class="mi">0</span> <span class="n">LOAD_CONST</span> <span class="mi">0</span> <span class="p">(</span><span class="mi">5</span><span class="p">)</span>
<span class="mi">2</span> <span class="n">STORE_NAME</span> <span class="mi">0</span> <span class="p">(</span><span class="n">x</span><span class="p">)</span>
<span class="mi">2</span> <span class="mi">4</span> <span class="n">LOAD_CONST</span> <span class="mi">1</span> <span class="p">(</span><span class="mi">7</span><span class="p">)</span>
<span class="mi">6</span> <span class="n">STORE_NAME</span> <span class="mi">1</span> <span class="p">(</span><span class="n">y</span><span class="p">)</span>
<span class="mi">3</span> <span class="mi">8</span> <span class="n">LOAD_NAME</span> <span class="mi">0</span> <span class="p">(</span><span class="n">x</span><span class="p">)</span>
<span class="mi">10</span> <span class="n">LOAD_NAME</span> <span class="mi">1</span> <span class="p">(</span><span class="n">y</span><span class="p">)</span>
<span class="mi">12</span> <span class="n">BINARY_ADD</span>
<span class="mi">14</span> <span class="n">STORE_NAME</span> <span class="mi">2</span> <span class="p">(</span><span class="n">z</span><span class="p">)</span>
<span class="mi">4</span> <span class="mi">16</span> <span class="n">LOAD_NAME</span> <span class="mi">0</span> <span class="p">(</span><span class="n">x</span><span class="p">)</span>
<span class="mi">18</span> <span class="n">LOAD_NAME</span> <span class="mi">1</span> <span class="p">(</span><span class="n">y</span><span class="p">)</span>
<span class="mi">20</span> <span class="n">BINARY_SUBTRACT</span>
<span class="mi">22</span> <span class="n">STORE_NAME</span> <span class="mi">3</span> <span class="p">(</span><span class="n">s</span><span class="p">)</span>
<span class="mi">5</span> <span class="mi">24</span> <span class="n">LOAD_NAME</span> <span class="mi">4</span> <span class="p">(</span><span class="nb">print</span><span class="p">)</span>
<span class="mi">26</span> <span class="n">LOAD_NAME</span> <span class="mi">2</span> <span class="p">(</span><span class="n">z</span><span class="p">)</span>
<span class="mi">28</span> <span class="n">CALL_FUNCTION</span> <span class="mi">1</span>
<span class="mi">30</span> <span class="n">POP_TOP</span>
<span class="mi">6</span> <span class="mi">32</span> <span class="n">LOAD_NAME</span> <span class="mi">4</span> <span class="p">(</span><span class="nb">print</span><span class="p">)</span>
<span class="mi">34</span> <span class="n">LOAD_NAME</span> <span class="mi">3</span> <span class="p">(</span><span class="n">s</span><span class="p">)</span>
<span class="mi">36</span> <span class="n">CALL_FUNCTION</span> <span class="mi">1</span>
<span class="mi">38</span> <span class="n">POP_TOP</span>
<span class="mi">7</span> <span class="mi">40</span> <span class="n">LOAD_NAME</span> <span class="mi">4</span> <span class="p">(</span><span class="nb">print</span><span class="p">)</span>
<span class="mi">42</span> <span class="n">LOAD_NAME</span> <span class="mi">5</span> <span class="p">(</span><span class="nb">len</span><span class="p">)</span>
<span class="mi">44</span> <span class="n">LOAD_CONST</span> <span class="mi">2</span> <span class="p">(</span><span class="s1">'Python'</span><span class="p">)</span>
<span class="mi">46</span> <span class="n">CALL_FUNCTION</span> <span class="mi">1</span>
<span class="mi">48</span> <span class="n">CALL_FUNCTION</span> <span class="mi">1</span>
<span class="mi">50</span> <span class="n">POP_TOP</span>
<span class="mi">52</span> <span class="n">LOAD_CONST</span> <span class="mi">3</span> <span class="p">(</span><span class="kc">None</span><span class="p">)</span>
<span class="mi">54</span> <span class="n">RETURN_VALUE</span>
</code></pre></div>
<p><span class="caps">BYTECODE</span> trông lạ nhưng không hề khó.</p>
<p>https://docs.python.org/3/library/dis.html
Đăng ký ngay tại <a href="https://pymi.vn">PyMI.vn</a> để học Python tại Hà Nội <span class="caps">TP</span> <span class="caps">HCM</span> (Sài Gòn),
trở thành lập trình viên #python chuyên nghiệp ngay sau khóa học.</p>
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