Is there an equivalent instruction to rdtsc in ARM?

Question

For my project I must use inline assembly instructions such as rdtsc to calculate the execution time of some C/C++ instructions.

The following code seems to work on Intel but not on ARM processors:

{unsigned a, d;asm volatile("rdtsc" : "=a" (a), "=d" (d)); t0 = ((unsigned long)a) | (((unsigned long)d) << 32);}
//The C++ statement to measure its execution time
{unsigned a, d;asm volatile("rdtsc" : "=a" (a), "=d" (d)); t1 = ((unsigned long)a) | (((unsigned long)d) << 32);}
time = t1-t0;

My question is:

How to write an inline assembly code similar to the above (to calculate the execution elapsed time of an instruction) to work on ARM processors?

Answer 1

You should read the PMCCNTR register of a co-processor p15 (not an actual co-processor, just an entry point for CPU functions) to obtain a cycle count. Note that it is available to an unprivileged app only if:

1. Unprivileged PMCCNTR reads are alowed:

   Bit 0 of PMUSERENR register must be set to 1 (official docs)

2. PMCCNTR is actually counting cycles:

   Bit 31 of PMCNTENSET register must be set to 1 (official docs)

This is a real-world example of how it`s done.

Answer 2

For Arm64, the system register CNTVCT_EL0 can be used to retrieve the counter from user space.

// SPDX-License-Identifier: GPL-2.0
u64 rdtsc(void)
{
    u64 val;

    /*
     * According to ARM DDI 0487F.c, from Armv8.0 to Armv8.5 inclusive, the
     * system counter is at least 56 bits wide; from Armv8.6, the counter
     * must be 64 bits wide.  So the system counter could be less than 64
     * bits wide and it is attributed with the flag 'cap_user_time_short'
     * is true.
     */
    asm volatile("mrs %0, cntvct_el0" : "=r" (val));

    return val;
}

Please refer this patch https://lore.kernel.org/patchwork/patch/1305380/ for more details.