单片机代码执行效率的提高

<p class="cjk" style="margin-bottom:0cm;line-height:100%;"> <span style="font-family:'Liberation Serif, Times New Roman, serif';"><span lang="en-US">1</span></span>、<span style="font-family:'Liberation Serif, Times New Roman, serif';"><span lang="en-US">for</span></span>循环采用<span style="font-family:'Liberation Serif, Times New Roman, serif';"> </span><span style="font-family:'Liberation Serif, Times New Roman, serif';"><span lang="en-US">- - </span></span>方式 </p> <p class="cjk" style="margin-bottom:0cm;line-height:100%;"> <span style="font-family:'Liberation Serif, Times New Roman, serif';"><span lang="en-US"> &nbsp;&nbsp;&nbsp;&nbsp;C</span></span>语言会生成汇编语言，而在汇编语言中，有一个不等于<span style="font-family:'Liberation Serif, Times New Roman, serif';"><span lang="en-US">0</span></span>的指令，<span style="font-family:'Liberation Serif, Times New Roman, serif';"><span lang="en-US">for</span></span>中使用<span style="font-family:'Liberation Serif, Times New Roman, serif';"> </span><span style="font-family:'Liberation Serif, Times New Roman, serif';"><span lang="en-US">- - </span></span>的方式，在判断条件是否满足时，只使用一条汇编指令，而<span style="font-family:'Liberation Serif, Times New Roman, serif';"><span lang="en-US">++</span></span>的方式，要判断与某个值的大小，这在汇编中要使用多条指令。 </p> <p class="cjk" style="margin-bottom:0cm;line-height:100%;"> <img src="//cf02.ickimg.com/bbsimages/201806/c95ac248ba398ec0e3c9fa1bbc4b7908.png" title="无标题.png" alt="无标题.png" /> </p> <p class="cjk" style="margin-bottom:0cm;line-height:100%;"> <br /> </p> <p class="cjk" style="margin-bottom:0cm;line-height:100%;"> &nbsp;&nbsp;&nbsp;&nbsp;当然，不排除有一些编译器会自动将<span style="font-family:'Liberation Serif, Times New Roman, serif';"><span lang="en-US">++</span></span>的<span style="font-family:'Liberation Serif, Times New Roman, serif';"><span lang="en-US">for</span></span>编译成<span style="font-family:'Liberation Serif, Times New Roman, serif';"> </span><span style="font-family:'Liberation Serif, Times New Roman, serif';"><span lang="en-US">- - </span></span>的<span style="font-family:'Liberation Serif, Times New Roman, serif';"><span lang="en-US">for</span></span>。 </p> <p class="cjk" style="margin-bottom:0cm;line-height:100%;"> <br /> </p> <p class="cjk" style="margin-bottom:0cm;line-height:100%;"> <span style="font-family:'Liberation Serif, Times New Roman, serif';"><span lang="en-US">2</span></span>、使用联合体进行位拆分 </p> <p class="cjk" style="margin-bottom:0cm;line-height:100%;"> <span style="font-family:'Liberation Serif, Times New Roman, serif';"><span lang="en-US"> </span></span>&nbsp;&nbsp;&nbsp;&nbsp;对于<span style="font-family:'Liberation Serif, Times New Roman, serif';"><span lang="en-US">32</span></span>位的数据，要分出其中的四个字节，常用的方式，就是与运算。 </p> <p class="cjk" style="margin-bottom:0cm;line-height:100%;"> <img src="//cf02.ickimg.com/bbsimages/201806/aa3856a3b43abd1e08c6240487224fe3.png" title="无标题.png" alt="无标题.png" /> </p> <p class="cjk" style="margin-bottom:0cm;line-height:100%;"> <br /> </p> <p class="cjk" style="margin-bottom:0cm;line-height:100%;"> <span style="font-family:'Liberation Serif, Times New Roman, serif';"><span lang="en-US"> </span></span>而使用联合体与结构体的方式： </p> <p class="cjk" style="margin-bottom:0cm;line-height:100%;"> <br /> </p> <p> <img src="//cf02.ickimg.com/bbsimages/201806/cba490b66ae7f9f6df68610ab457cf98.png" title="无标题.png" alt="无标题.png" /> </p> <p> <br /> </p> <p> <br /> </p> <p class="cjk" style="margin-bottom:0cm;line-height:100%;"> <br /> </p> <p class="cjk" style="margin-bottom:0cm;line-height:100%;"> <br /> </p> <p class="cjk" style="margin-bottom:0cm;line-height:100%;"> <span style="font-family:'Liberation Serif, Times New Roman, serif';"><span lang="en-US">3</span></span>、关键代码指定运行空间 </p> <p class="cjk" style="margin-bottom:0cm;line-height:100%;"> <span style="font-family:'Liberation Serif, Times New Roman, serif';"><span lang="en-US"> </span></span>&nbsp;&nbsp;&nbsp;&nbsp;代码的运行空间是决定程序运行效率的一个关键因素。 </p> <p class="cjk" style="margin-bottom:0cm;line-height:100%;"> <span style="font-family:'Liberation Serif, Times New Roman, serif';"><span lang="en-US"> </span></span>&nbsp;&nbsp;&nbsp;&nbsp;这里主要是指<span style="font-family:'Liberation Serif, Times New Roman, serif';"><span lang="en-US">TI</span></span>的<span style="font-family:'Liberation Serif, Times New Roman, serif';"><span lang="en-US">MCU</span></span>，其他的<span style="font-family:'Liberation Serif, Times New Roman, serif';"><span lang="en-US">DSP</span></span>也是类似的。 </p> <p class="cjk" style="margin-bottom:0cm;line-height:100%;"> <span style="font-family:'Liberation Serif, Times New Roman, serif';"><span lang="en-US"> </span></span>&nbsp;&nbsp;&nbsp;&nbsp;以其程序的运行在片内的<span style="font-family:'Liberation Serif, Times New Roman, serif';"><span lang="en-US">RAM</span></span>可以运行到其标称的<span style="font-family:'Liberation Serif, Times New Roman, serif';"><span lang="en-US">150M</span></span>或是<span style="font-family:'Liberation Serif, Times New Roman, serif';"><span lang="en-US">200M</span></span>，但是在<span style="font-family:'Liberation Serif, Times New Roman, serif';"><span lang="en-US">Flash</span></span>运行，则程序的执行速率只有<span style="font-family:'Liberation Serif, Times New Roman, serif';"><span lang="en-US">60</span></span>、<span style="font-family:'Liberation Serif, Times New Roman, serif';"><span lang="en-US">70M</span></span>的样子，当然与代码的优化等级设置与具体的代码有关，但是从总体上来说，片上的<span style="font-family:'Liberation Serif, Times New Roman, serif';"><span lang="en-US">RAM</span></span>运行速率最高，<span style="font-family:'Liberation Serif, Times New Roman, serif';"><span lang="en-US">Flash</span></span>次之，外设的<span style="font-family:'Liberation Serif, Times New Roman, serif';"><span lang="en-US">SRAM</span></span>最低。 </p> <p class="cjk" style="margin-bottom:0cm;line-height:100%;"> <span style="font-family:'Liberation Serif, Times New Roman, serif';"><span lang="en-US"> </span></span>&nbsp;&nbsp;&nbsp;&nbsp;那么可想而知，某些芯片标示的主频多高多高，而在实际使用时，往往受外设总线速度的限制，实际的运行速率是达不到的。 </p> <p class="cjk" style="margin-bottom:0cm;line-height:100%;"> <span style="font-family:'Liberation Serif, Times New Roman, serif';"><span lang="en-US"> </span></span>&nbsp;&nbsp;&nbsp;&nbsp;其他的芯片，如果支持设置，原理应该也是相通的，要将关键代码，或是实时性要求比较高的算法，放在<span style="font-family:'Liberation Serif, Times New Roman, serif';"><span lang="en-US">RAM</span></span>中运行。 </p> <p class="cjk" style="margin-bottom:0cm;line-height:100%;"> <br /> </p> <p> <img src="//cf02.ickimg.com/bbsimages/201806/f5c9bb720d3074825cbcf4a93af4ba4c.png" title="无标题.png" alt="无标题.png" /> </p> <p> <br /> </p> <p class="cjk" style="margin-bottom:0cm;line-height:100%;"> <br /> </p> <p class="cjk" style="margin-bottom:0cm;line-height:100%;"> <span style="font-family:'Liberation Serif, Times New Roman, serif';"><span lang="en-US"> </span></span>&nbsp;&nbsp;&nbsp;&nbsp;如果你的芯片支持<span style="font-family:'Liberation Serif, Times New Roman, serif';"><span lang="en-US">cache</span></span>，那么要好好<span style="font-family:'Liberation Serif, Times New Roman, serif';"> </span>利用了，效率可能会再高很多。 </p> <p class="cjk" style="margin-bottom:0cm;line-height:100%;"> <br /> <span style="font-family:'Liberation Serif, Times New Roman, serif';"><span lang="en-US"> </span></span> </p> <p class="cjk" style="margin-bottom:0cm;line-height:100%;"> <span style="font-family:&quot;font-size:14px;"><span style="font-size:14px;" lang="en-US">4</span></span><span style="font-size:14px;">、如果</span><span style="font-family:&quot;font-size:14px;"><span style="font-size:14px;" lang="en-US">Flash</span></span><span style="font-size:14px;">允许，尽量使用静态函数，</span><span style="color:#000000;font-size:14px;"><span style="font-family:&quot;font-size:14px;"><span style="font-size:14px;">避免了调用函数时压栈出栈，速度快很多。</span></span></span><span style="color:#000000;font-size:14px;"><span style="font-family:&quot;font-size:14px;"> 或者使用</span></span><span style="font-family:'Liberation Serif, Times New Roman, serif';"><span style="font-size:14px;" lang="en-US"><span style="color:#000000;font-size:14px;">inline</span></span><span style="font-family:&quot;font-size:14px;">函数，使用容量换时间。</span></span> </p> <p class="cjk" style="margin-bottom:0cm;line-height:100%;"> <br /> </p> <p class="cjk" style="margin-bottom:0cm;line-height:100%;"> <span style="font-family:'Liberation Serif, Times New Roman, serif';"><span lang="en-US">5</span></span>、使用字节对齐。 </p> <p class="cjk" style="margin-bottom:0cm;line-height:100%;"> <span style="font-family:'Liberation Serif, Times New Roman, serif';"><span lang="en-US"> </span></span>&nbsp;&nbsp;&nbsp;&nbsp;单片机内部的总线往往并不是<span style="font-family:'Liberation Serif, Times New Roman, serif';"><span lang="en-US">8</span></span>位的，而是<span style="font-family:'Liberation Serif, Times New Roman, serif';"><span lang="en-US">32</span></span>位或是更高位的，一些<span style="font-family:'Liberation Serif, Times New Roman, serif';"><span lang="en-US">DSP</span></span>内部总线也有<span style="font-family:'Liberation Serif, Times New Roman, serif';"><span lang="en-US">256</span></span>位或更高的。 </p> <p> <br /> </p> <p class="cjk" style="margin-bottom:0cm;line-height:100%;"> <img src="//cf02.ickimg.com/bbsimages/201806/56e1e6a861e24c8195e54ebede7e933e.png" title="无标题.png" alt="无标题.png" /> </p> <p class="cjk" style="margin-bottom:0cm;line-height:100%;"> <br /> </p> <p class="cjk" style="margin-bottom:0cm;line-height:100%;"> <br /> </p> <p class="cjk" style="margin-bottom:0cm;line-height:100%;"> &nbsp;&nbsp;&nbsp;&nbsp;而每次只传输<span style="font-family:'Liberation Serif, Times New Roman, serif';"><span lang="en-US">8</span></span>位数据，其他的数据位空闲浪费，这时，使用字节对齐，可以提高内部总线的利用率，提高效率。 </p> <p class="cjk" style="margin-bottom:0cm;line-height:100%;"> <br /> </p> <p class="cjk" style="margin-bottom:0cm;line-height:100%;"> <span style="font-family:'Liberation Serif, Times New Roman, serif';"><span lang="en-US">6</span></span>、使用全局变量代替函数传递的参数。 </p> <p class="cjk" style="margin-bottom:0cm;line-height:100%;"> &nbsp;&nbsp;&nbsp;&nbsp;这种方式并不提倡，要仔细对变量的值进行维护，否则会有一些逻辑上的<span style="font-family:'Liberation Serif, Times New Roman, serif';"><span lang="en-US">bug</span></span>很难发现。 </p> <p class="cjk" style="margin-bottom:0cm;line-height:100%;"> <br /> </p> <p class="cjk" style="margin-bottom:0cm;line-height:100%;"> <span style="font-family:'Liberation Serif, Times New Roman, serif';"><span lang="en-US">7</span></span>、用乘法代替除法 </p> <p class="cjk" style="margin-bottom:0cm;line-height:100%;"> <span style="font-family:'Liberation Serif, Times New Roman, serif';"><span lang="en-US"> </span></span>&nbsp;&nbsp;&nbsp;&nbsp;这种方式并没有统一性，只是有的地方可以使用，比如计算某变量的率时，分母往往都是同一个数值，这时，先计算分母的倒数，再用分子去乘，就避免了多次的除尘计算。 </p> <p class="cjk" style="margin-bottom:0cm;line-height:100%;"> <span style="font-family:'Liberation Serif, Times New Roman, serif';"><span lang="en-US"> </span></span>比如计算谐波时，要计算<span style="font-family:'Liberation Serif, Times New Roman, serif';"><span lang="en-US">2~50</span></span>次的谐波。 </p> <p> <br /> </p> <p class="cjk" style="margin-bottom:0cm;line-height:100%;"> <img src="//cf02.ickimg.com/bbsimages/201806/2c30a86005e609ee4f50facfc94c6752.png" title="无标题.png" alt="无标题.png" /> </p> <p class="cjk" style="margin-bottom:0cm;line-height:100%;"> <br /> </p> <p class="cjk" style="margin-bottom:0cm;line-height:100%;"> &nbsp;&nbsp;&nbsp;&nbsp;这样，就把近<span style="font-family:'Liberation Serif, Times New Roman, serif';"><span lang="en-US">50</span></span>次的除法运算转成一次除法与多次乘法。 </p> <p class="cjk" style="margin-bottom:0cm;line-height:100%;"> <br /> </p> <p class="cjk" style="margin-bottom:0cm;line-height:100%;"> <span style="font-family:'Liberation Serif, Times New Roman, serif';"><span lang="en-US">8</span></span>、小数表达 </p> <p class="cjk" style="margin-bottom:0cm;line-height:100%;"> <span style="font-family:'Liberation Serif, Times New Roman, serif';"><span lang="en-US"> </span></span>&nbsp;&nbsp;&nbsp;&nbsp;小数的表达，大多都是如下这样： </p> <p> <br /> </p> <p class="cjk" style="margin-bottom:0cm;line-height:100%;"> <img src="//cf02.ickimg.com/bbsimages/201806/1c57850f04710dfc690797425f6523e4.png" title="无标题.png" alt="无标题.png" /> </p> <p class="cjk" style="margin-bottom:0cm;line-height:100%;"> <br /> </p> <p class="cjk" style="margin-bottom:0cm;line-height:100%;"> <br /> </p> <p class="cjk" style="margin-bottom:0cm;line-height:100%;"> &nbsp;&nbsp;&nbsp;&nbsp;这时，有的编译器会将其默认为<span style="font-family:'Liberation Serif, Times New Roman, serif';"><span lang="en-US">double</span></span>的，这样再运算时，计算时间自然就会变长。 </p> <p class="cjk" style="margin-bottom:0cm;line-height:100%;"> <span style="font-family:'Liberation Serif, Times New Roman, serif';"><span lang="en-US"> </span></span>&nbsp;&nbsp;&nbsp;&nbsp;而使用如下表达时，而是单精度的，计算也会快一些，但是你的算法要求精度高时，那么还是使用双精度的吧。 </p> <p class="cjk" style="margin-bottom:0cm;line-height:100%;"> <br /> </p> <p class="cjk" style="margin-bottom:0cm;line-height:100%;"> <img src="//cf02.ickimg.com/bbsimages/201806/5c29026c69c5f282566ac49299dc4f49.png" title="无标题.png" alt="无标题.png" /> </p> <p class="cjk" style="margin-bottom:0cm;line-height:100%;"> <br /> </p> <p class="cjk" style="margin-bottom:0cm;line-height:100%;"> <br /> </p> <p class="cjk" style="margin-bottom:0cm;line-height:100%;"> <span style="font-family:'Liberation Serif, Times New Roman, serif';"><span lang="en-US">9</span></span>、函数调用越少越好 </p> <p class="cjk" style="margin-bottom:0cm;line-height:100%;"> <span style="font-family:'Liberation Serif, Times New Roman, serif';"><span lang="en-US"> </span></span>&nbsp;&nbsp;&nbsp;&nbsp;这也是要看具体的代码了，如果重用代码量很高时，该嵌套还是要嵌套的。 </p> <p class="cjk" style="margin-bottom:0cm;line-height:100%;"> <br /> </p> <p class="cjk" style="margin-bottom:0cm;line-height:100%;"> <br /> </p> <p class="cjk" style="margin-bottom:0cm;line-height:100%;"> <span style="font-family:'Liberation Serif, Times New Roman, serif';"><span lang="en-US">10</span></span>、习惯使用指针 </p> <p class="cjk" style="margin-bottom:0cm;line-height:100%;"> <span style="font-family:'Liberation Serif, Times New Roman, serif';"><span lang="en-US"> </span></span>&nbsp;&nbsp;&nbsp;&nbsp;指针的方式，自然不用详说了，正确的使用，会使效率有很大的提高。 </p> <p class="cjk" style="margin-bottom:0cm;line-height:100%;"> <br /> </p> <p class="cjk" style="margin-bottom:0cm;line-height:100%;"> <br /> </p> <p class="cjk" style="margin-bottom:0cm;line-height:100%;"> &nbsp;&nbsp;&nbsp;以上只是个人在实际项目中的一些愚见、体会，其依据是在个人项目中的实践结果，并没有进行完善的理论认证。所以不对之处，烦请指教。 </p> <p> <br /> </p> <p> <br /> </p>