uptime wraps too soon

[pwittich]

Describe the bug
uptime seems to roll over after less than 1000 minutes (not clear about reason)

To Reproduce
Steps to reproduce the behavior:

1. look at uptime
2. it's never more than a few hundred minutes

It ought to be ok; TickType_t is AFAIK a signed 32 bit integer. The number is multiplied by some constants; hopefully the compiler avoids unneeded up- and down-multipliers. According to my estimate the overall scaling is (1000/100)*(1/60000). The tick rate is 100 Hz. So this should reduce to 1/6000, hopefully at compile time. Max is like 2x10^9 for a 32 bit signed integer, so the max number of minutes should be like 33k. What am I doing wrong here?

cm_mcu/projects/cm_mcu/CommandLineTask.c

Lines 1112 to 1118 in 36b40d7

	static BaseType_t uptime(int argc, char ** argv)
	{
	int s = SCRATCH_SIZE;
	TickType_t now = pdTICKS_TO_MS( xTaskGetTickCount())/1000/60; // time in minutes
	snprintf(m,s, "%s: MCU uptime %d minutes\r\n", argv[0], now);
	return pdFALSE;
	}

[pwittich]

Looking at the assembly output it looks like the compiler does not reduce this to 1/6000.

        bl      xTaskGetTickCount
        mov     r2, r0
        mov     r3, #1000
        mul     r3, r3, r2
        .loc 1 1116 14
        ldr     r2, .L354
        umull   r2, r3, r2, r3
        lsrs    r3, r3, #19

So we have an explicit multiplication by 1000 and then a division by a constant. The value stored at L354 is 375299969, so the umul followed by the shift corresponds to a multiplication of 1/5999999.999161332198245, i.e., 1/6e6.

[pwittich]

If I change the code in question to the following instead

  TickType_t now = xTaskGetTickCount()/(configTICK_RATE_HZ*60); // time in minutes

the generated assembly is now

        bl      xTaskGetTickCount
        mov     r2, r0
        .loc 1 1115 14
        ldr     r3, .L354
        umull   r2, r3, r3, r2
        lsrs    r3, r3, #7

where the value at 354 is now 91625969, which with the shift corresponds to multiplication by 1/6000.