mirror of https://github.com/rusefi/ChibiOS.git
82 lines
2.6 KiB
Plaintext
82 lines
2.6 KiB
Plaintext
/*
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ChibiOS/RT - Copyright (C) 2006,2007,2008,2009,2010 Giovanni Di Sirio.
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This file is part of ChibiOS/RT.
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ChibiOS/RT is free software; you can redistribute it and/or modify
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it under the terms of the GNU General Public License as published by
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the Free Software Foundation; either version 3 of the License, or
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(at your option) any later version.
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ChibiOS/RT is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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GNU General Public License for more details.
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You should have received a copy of the GNU General Public License
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along with this program. If not, see <http://www.gnu.org/licenses/>.
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*/
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/**
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* @page article_saveram Saving RAM by declaring thread functions "noreturn"
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* One of the problems, when writing embedded multi-threaded applications,
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* is that the thread functions do save the registers in the function
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* entry code even if the system does not require it, exiting such
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* a function would terminate the thread so there is no need to preserve
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* the register values. This can waste tens of bytes for each thread.<br>
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* Consider the following code:
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* @code
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#include <ch.h>
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static WORKING_AREA(waMyThread, 64);
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static t_msg MyThread(void *arg) {
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while (!chThdShoudTerminate()) {
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/* Do thread inner work */
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}
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return 1;
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}
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main() {
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chSysInit();
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...
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chThdCreateStatic(waMyThread, sizeof(waMyThread), NORMALPRIO, MyThread, NULL);
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...
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}
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* @endcode
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* The resulting ASM code for the thread function would be something like this:
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* @code
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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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* @endcode
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* Being that function a thread there is no need to save those registers, in
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* embedded applications often the RAM is a scarce resource. That space can be
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* saved by modifying the code as follow, using some advanced GCC extensions:
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* @code
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#include <ch.h>
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static WORKING_AREA(waMyThread, 64);
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__attribute__((noreturn))
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static void MyThread(void *arg) {
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while (!chThdShoudTerminate()) {
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/* Do thread inner work */
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}
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chThdExit(1);
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}
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main() {
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chSysInit();
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...
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chThdCreateStatic(waMyThread, sizeof(waMyThread), NORMALPRIO,
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(tfunc_t)MyThread, NULL);
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...
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}
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* @endcode
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* This will make GCC believe that the function cannot return and there is no
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* need to save registers. The code will be a bit less readable and less
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* portable on other compilers however.
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*/
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