Static memory alignment benchmark on Pi 4 (Arm 64)

main.c

#include <stdint.h>
#include <stdio.h>
#include <stdalign.h>
#include <assert.h>
#include <sys/mman.h>
//#include <x86intrin.h>

#define COUNT 4096

struct __attribute__((packed)) {
//struct {
  uint32_t a;
  uint8_t b;
  uint16_t c;
} test_mem[COUNT];
//} alignas(64) test_mem[COUNT];

static inline uint64_t read_cntvct(void) {
    uint64_t cnt;
    asm volatile("mrs %0, cntvct_el0" : "=r"(cnt));
    return cnt;
}

//static inline uint64_t read_cycle_counter(void) {
//    uint64_t value;
//    asm volatile("mrs %0, pmccntr_el0" : "=r"(value));
//    return value;
//}

int main() {
  unsigned int cpu;
  int err = mlockall(MCL_CURRENT|MCL_FUTURE);
  assert(err == 0);

  //auto tsc1 = __rdtscp(&cpu);
  uint64_t tsc1 = read_cntvct();
  //uint64_t tsc1 = read_cntvct();

  // do memory stuff here
  long sum = 0;
  for (int i = 0; i < COUNT; i++) {
    test_mem[i].a = i % (2<<16);
    test_mem[i].b = i % (2<<16) + 1;
    sum += test_mem[i].a - test_mem[i].b;
  }

  //auto tsc2 = __rdtscp(&cpu);
  uint64_t tsc2 = read_cntvct();
  //uint64_t tsc2 = read_cntvct();
  printf("tsc diff: %ld\n", tsc2 - tsc1);
  printf("sum: %ld\n", sizeof(test_mem));
}

perf.console

$ sudo perf stat -r10 ./main
tsc diff: 589
sum: 28672
tsc diff: 737
sum: 28672
tsc diff: 620
sum: 28672
tsc diff: 575
sum: 28672
tsc diff: 593
sum: 28672
tsc diff: 591
sum: 28672
tsc diff: 565
sum: 28672
tsc diff: 572
sum: 28672
tsc diff: 526
sum: 28672
tsc diff: 587
sum: 28672

 Performance counter stats for './main' (10 runs):

              3.33 msec task-clock                       #    0.631 CPUs utilized               ( +-  3.00% )
                 0      context-switches                 #    0.000 /sec
                 0      cpu-migrations                   #    0.000 /sec
                37      page-faults                      #   11.124 K/sec                       ( +-  0.70% )
           4613519      cycles                           #    1.387 GHz                         ( +-  0.60% )
           4474784      instructions                     #    0.97  insn per cycle              ( +-  0.32% )
   <not supported>      branches
             14479      branch-misses                                                           ( +-  1.20% )

          0.005268 +- 0.000134 seconds time elapsed  ( +-  2.54% )

# perf report --stdio # repeated multiple times

# Overhead  Command  Shared Object      Symbol
# ........  .......  .................  ...............................
#
    20.55%  main     [kernel.kallsyms]  [k] el0_da
    20.19%  main     [kernel.kallsyms]  [k] _raw_spin_unlock
    17.52%  main     [kernel.kallsyms]  [k] arch_local_irq_restore
    16.29%  main     [kernel.kallsyms]  [k] _raw_spin_unlock_irqrestore
    14.47%  main     [kernel.kallsyms]  [k] locks_remove_posix
    10.19%  main     [kernel.kallsyms]  [k] strnlen_user
     0.77%  main     [kernel.kallsyms]  [k] vma_expand
     0.03%  perf-ex  [kernel.kallsyms]  [k] perf_event_enable_on_exec

# Overhead  Command  Shared Object      Symbol
# ........  .......  .................  .............................
#
    39.82%  main     [kernel.kallsyms]  [k] arch_local_irq_restore
    25.53%  main     [kernel.kallsyms]  [k] handle_softirqs
    23.23%  main     [kernel.kallsyms]  [k] percpu_counter_add_batch
    10.62%  main     [kernel.kallsyms]  [k] strnlen_user
     0.77%  main     [kernel.kallsyms]  [k] mas_find
     0.03%  perf-ex  [kernel.kallsyms]  [k] perf_event_enable_on_exec

# Overhead  Command  Shared Object          Symbol
# ........  .......  .....................  .............................
#
    31.19%  main     ld-linux-aarch64.so.1  [.] 0x0000000000008a20
    27.40%  main     [kernel.kallsyms]      [k] __pi_clear_page
    25.11%  main     [kernel.kallsyms]      [k] release_pages
    15.19%  main     [kernel.kallsyms]      [k] kfree
     1.06%  main     [kernel.kallsyms]      [k] mas_wr_walk
     0.05%  perf-ex  [kernel.kallsyms]      [k] perf_event_enable_on_exec

# Overhead  Command  Shared Object      Symbol
# ........  .......  .................  .............................
#
    22.85%  main     [kernel.kallsyms]  [k] release_pages
    22.85%  main     [kernel.kallsyms]  [k] vma_interval_tree_remove
    19.47%  main     [kernel.kallsyms]  [k] _raw_spin_unlock
    18.07%  main     libc.so.6          [.] write
    16.05%  main     [kernel.kallsyms]  [k] __rcu_read_unlock
     0.68%  main     [kernel.kallsyms]  [k] get_random_u64
     0.03%  perf-ex  [kernel.kallsyms]  [k] perf_event_enable_on_exec

# Overhead  Command  Shared Object          Symbol
# ........  .......  .....................  .............................
#
    25.70%  main     ld-linux-aarch64.so.1  [.] 0x00000000000111d8
    23.23%  main     [kernel.kallsyms]      [k] next_uptodate_folio
    21.42%  main     [kernel.kallsyms]      [k] __pi_clear_page
    18.60%  main     [kernel.kallsyms]      [k] arch_local_irq_restore
    10.27%  main     [kernel.kallsyms]      [k] strnlen_user
     0.74%  main     [kernel.kallsyms]      [k] get_random_u16
     0.03%  perf-ex  [kernel.kallsyms]      [k] perf_event_enable_on_exec