Keil uVision5 armcc.exe: if语句中无符号短裤的问题不能正常工作？

Question

armcc.exe V5.06 (750)请告诉我为什么一旦val16溢出并变得比start_count小，标记为有故障的代码就不能工作。循环不再退出。

HAL_GetTick()返回uint32_t (或int)

unsigned short start_count;

unsigned short val16;
unsigned short result;

start_count = HAL_GetTick(); 

//This does not work
do
{
    val16 = HAL_GetTick();
    result = val16 - start_count;
}
while ((val16 - start_count)  < 100);  //this part does not work correctly 
                                       //when val16 overflows

//This works!  same code logic
do
{
    val16 = HAL_GetTick();
    result = val16 - start_count;
}
while (result  < 100);

`

Answer 1

首先，不要在没有特殊需要的情况下使用较短的类型，因为这通常会增加32位RISC目标的额外开销。

第一个while不起作用，因为算术是使用int完成的，而不是无符号的短值，从而产生意外的结果:)。这里有一个演示程序：

unsigned HAL_GetTick(void)
{
    static unsigned tick = 65535 - 5;
    return tick++;
}

int main(void)
{
    unsigned short val16;
    unsigned short result;
    unsigned short start_count;

    start_count = HAL_GetTick(); 

    //This does not work
    do
    {
        val16 = HAL_GetTick();
        result = val16 - start_count;
        printf("%d %d\n", (val16 - start_count)  < 100, (val16 - start_count));
    }
    while ((val16 - start_count)  < 100);  //this part does not work correctly 
                                        //when val16 overflows

    //This works!  same code logic
    do
    {
        val16 = HAL_GetTick();
        result = val16 - start_count;
    }
    while (result  < 100);
}

https://godbolt.org/z/fa5M7W

第二个循环使用无符号短类型，并且在计算result时不会隐式转换该类型。

https://godbolt.org/z/6TshP8

现在，使用ARM Cortex内核的非本机整数的问题。

示例程序：

volatile uint32_t tick;

uint32_t HAL_GetTick(void)
{
    return tick;
}

void foo(void)
{
    unsigned short val16;
    unsigned short result;
    unsigned short start_count;

    start_count = HAL_GetTick(); 


    //This works!  same code logic
    do
    {
        val16 = HAL_GetTick();
        result = val16 - start_count;
    }
    while (result  < 100);
}

void bar(void)
{
    uint32_t val16;
    uint32_t result;
    uint32_t start_count;

    start_count = HAL_GetTick(); 


    //This works!  same code logic
    do
    {
        val16 = HAL_GetTick();
        result = val16 - start_count;
    }
    while (result  < 100);
}

以及生成的代码：

foo:
        ldr     r1, .L9
        ldr     r2, [r1]
        lsl     r2, r2, #16
        lsr     r2, r2, #16
.L6:
        ldr     r3, [r1]
        sub     r3, r3, r2
        lsl     r3, r3, #16
        lsr     r3, r3, #16
        cmp     r3, #99
        bls     .L6
        bx      lr
.L9:
        .word   tick
bar:
        ldr     r2, .L15
        ldr     r1, [r2]
.L12:
        ldr     r3, [r2]
        sub     r3, r3, r1
        cmp     r3, #99
        bls     .L12
        bx      lr
.L15:
        .word   tick

正如您所看到的，使用16位整数的版本比使用32位本机大小整数的版本效率要低。