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<title>ChibiOS/RT: Saving RAM by declaring thread functions "noreturn"</title>
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<td><big><big>ChibiOS/RT</big></big><br><br>Architecture - Reference Manual - Guides</td>
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<div class="navpath"><a class="el" href="main.html">ChibiOS/RT</a> » <a class="el" href="articles.html">Articles and Code Samples</a> » <a class="el" href="page__kb.html">Knowledge Base</a>
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<h1><a class="anchor" id="article_saveram">Saving RAM by declaring thread functions "noreturn" </a></h1><p>One of the problems, when writing embedded multi-threaded applications, is that the thread functions do save the registers in the function entry code even if the system does not require it, exiting such a function would terminate the thread so there is no need to preserve the register values. This can waste tens of bytes for each thread.<br/>
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Consider the following code: </p>
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<div class="fragment"><pre class="fragment"><span class="preprocessor">#include <<a class="code" href="ch_8h.html" title="ChibiOS/RT main include file.">ch.h</a>></span>
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<span class="keyword">static</span> <a class="code" href="group__core.html#gac8b681d521d3b6c25e7a0304674732c9" title="Static working area allocation.">WORKING_AREA</a>(waMyThread, 64);
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<span class="keyword">static</span> t_msg MyThread(<span class="keywordtype">void</span> *arg) {
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<span class="keywordflow">while</span> (!chThdShoudTerminate()) {
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<span class="comment">/* Do thread inner work */</span>
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}
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<span class="keywordflow">return</span> 1;
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}
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main() {
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<a class="code" href="group__system.html#gafe2c7de6567e98e487e009e81e3be10b" title="ChibiOS/RT initialization.">chSysInit</a>();
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...
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<a class="code" href="group__threads.html#ga048e588238dd336c6059133b3d0c3435" title="Creates a new thread into a static memory area.">chThdCreateStatic</a>(waMyThread, <span class="keyword">sizeof</span>(waMyThread), <a class="code" href="group__scheduler.html#gad6a6b28f0c1cf8e0da714ef771fb90a0" title="Normal user priority.">NORMALPRIO</a>, MyThread, NULL);
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...
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}
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</pre></div><p> The resulting ASM code for the thread function would be something like this: </p>
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<div class="fragment"><pre class="fragment">MyThread:
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stmfd sp!, {r4, r5, r6, lr}
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...
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ldmfd sp!, {r4, r5, r6, pc}
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</pre></div><p> Being that function a thread there is no need to save those registers, in embedded applications often the RAM is a scarce resource. That space can be saved by modifying the code as follow, using some advanced GCC extensions: </p>
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<div class="fragment"><pre class="fragment"><span class="preprocessor">#include <<a class="code" href="ch_8h.html" title="ChibiOS/RT main include file.">ch.h</a>></span>
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<span class="keyword">static</span> <a class="code" href="group__core.html#gac8b681d521d3b6c25e7a0304674732c9" title="Static working area allocation.">WORKING_AREA</a>(waMyThread, 64);
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<a class="code" href="group___a_r_m_c_mx___c_o_r_e.html#gadb1276c6bdf715206ee694f223112e30" title="Stack and memory alignment enforcement.">__attribute__</a>((noreturn))
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static <span class="keywordtype">void</span> MyThread(<span class="keywordtype">void</span> *arg) {
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<span class="keywordflow">while</span> (!chThdShoudTerminate()) {
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<span class="comment">/* Do thread inner work */</span>
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}
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<a class="code" href="group__threads.html#ga24ab3a3a4d70214ee360867a1c3c75ac" title="Terminates the current thread by specifying an exit status code.">chThdExit</a>(1);
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}
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main() {
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<a class="code" href="group__system.html#gafe2c7de6567e98e487e009e81e3be10b" title="ChibiOS/RT initialization.">chSysInit</a>();
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...
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<a class="code" href="group__threads.html#ga048e588238dd336c6059133b3d0c3435" title="Creates a new thread into a static memory area.">chThdCreateStatic</a>(waMyThread, <span class="keyword">sizeof</span>(waMyThread), <a class="code" href="group__scheduler.html#gad6a6b28f0c1cf8e0da714ef771fb90a0" title="Normal user priority.">NORMALPRIO</a>,
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(<a class="code" href="group__threads.html#gabfe59aa7d2ed1b8e57a2ce17bcbc1189" title="Thread function.">tfunc_t</a>)MyThread, NULL);
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...
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}
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</pre></div><p> This will make GCC believe that the function cannot return and there is no need to save registers. The code will be a bit less readable and less portable on other compilers however. </p>
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