查找最高优先级就绪任务
FreeRTOS\Source\tasks.c
#if ( configUSE_PORT_OPTIMISED_TASK_SELECTION == 0 )
/* If configUSE_PORT_OPTIMISED_TASK_SELECTION is 0 then task selection is
* performed in a generic way that is not optimised to any particular
* microcontroller architecture. */
/* uxTopReadyPriority holds the priority of the highest priority ready
* state task. */
#define taskRECORD_READY_PRIORITY( uxPriority ) \
{ \
if( ( uxPriority ) > uxTopReadyPriority ) \
{ \
uxTopReadyPriority = ( uxPriority ); \
} \
} /* taskRECORD_READY_PRIORITY */
/*-----------------------------------------------------------*/
#define taskSELECT_HIGHEST_PRIORITY_TASK() \
{ \
UBaseType_t uxTopPriority = uxTopReadyPriority; \
\
/* Find the highest priority queue that contains ready tasks. */ \
while( listLIST_IS_EMPTY( &( pxReadyTasksLists[ uxTopPriority ] ) ) ) \
{ \
configASSERT( uxTopPriority ); \
--uxTopPriority; \
} \
\
/* listGET_OWNER_OF_NEXT_ENTRY indexes through the list, so the tasks of \
* the same priority get an equal share of the processor time. */ \
listGET_OWNER_OF_NEXT_ENTRY( pxCurrentTCB, &( pxReadyTasksLists[ uxTopPriority ] ) ); \
uxTopReadyPriority = uxTopPriority; \
} /* taskSELECT_HIGHEST_PRIORITY_TASK */
/*-----------------------------------------------------------*/
/* Define away taskRESET_READY_PRIORITY() and portRESET_READY_PRIORITY() as
* they are only required when a port optimised method of task selection is
* being used. */
#define taskRESET_READY_PRIORITY( uxPriority )
#define portRESET_READY_PRIORITY( uxPriority, uxTopReadyPriority )
#else /* configUSE_PORT_OPTIMISED_TASK_SELECTION */
/* If configUSE_PORT_OPTIMISED_TASK_SELECTION is 1 then task selection is
* performed in a way that is tailored to the particular microcontroller
* architecture being used. */
/* A port optimised version is provided. Call the port defined macros. */
#define taskRECORD_READY_PRIORITY( uxPriority ) portRECORD_READY_PRIORITY( uxPriority, uxTopReadyPriority )
/*-----------------------------------------------------------*/
#define taskSELECT_HIGHEST_PRIORITY_TASK() \
{ \
UBaseType_t uxTopPriority; \
\
/* Find the highest priority list that contains ready tasks. */ \
portGET_HIGHEST_PRIORITY( uxTopPriority, uxTopReadyPriority ); \
configASSERT( listCURRENT_LIST_LENGTH( &( pxReadyTasksLists[ uxTopPriority ] ) ) > 0 ); \
listGET_OWNER_OF_NEXT_ENTRY( pxCurrentTCB, &( pxReadyTasksLists[ uxTopPriority ] ) ); \
} /* taskSELECT_HIGHEST_PRIORITY_TASK() */
/*-----------------------------------------------------------*/
/* A port optimised version is provided, call it only if the TCB being reset
* is being referenced from a ready list. If it is referenced from a delayed
* or suspended list then it won't be in a ready list. */
#define taskRESET_READY_PRIORITY( uxPriority ) \
{ \
if( listCURRENT_LIST_LENGTH( &( pxReadyTasksLists[ ( uxPriority ) ] ) ) == ( UBaseType_t ) 0 ) \
{ \
portRESET_READY_PRIORITY( ( uxPriority ), ( uxTopReadyPriority ) ); \
} \
}
#endif /* configUSE_PORT_OPTIMISED_TASK_SELECTION */查找机制有两种：通用方法和基于特定处理器架构的优化方法
通用方法倒序遍历优先级链表，找出置位的最高优先级标志，进而拿到最高优先级就绪任务的控制块
优化方法使用的是处理器支持的计算前导零指令 clz
计算前导零
FreeRTOS\Source\portable\GCC\ARM_CM4F\portmacro.h
__attribute__( ( always_inline ) ) static inline uint8_t ucPortCountLeadingZeros( uint32_t ulBitmap )
{
uint8_t ucReturn;
__asm volatile ( "clz %0, %1" : "=r" ( ucReturn ) : "r" ( ulBitmap ) : "memory" );
return ucReturn;
}汇编代码说明：
__asm：c内嵌汇编
volatile：禁用编译器优化
"clz %0, %1"：汇编代码模板，0%对应第一个参数（ucReturn），1%对应第二个参数（ulBitmap）
"=r" ( ucReturn )：输出操作数，=r操作数使用通用寄存器
"r" ( ulBitmap )：输入操作数，r操作数使用通用寄存器
"memory"：寄存器
__asm volatile：用法详见 https://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html