Strange profiler behavior: same functions, different executions

Question

I learned to use gprof and then got weird results for this code:

int one(int a, int b)
{
    int i, r = 0;
    for (i = 0; i < 1000; i++)
    {
        r += b / (a + 1);
    }
    return r;
}

int two(int a, int b)
{
    int i, r = 0;
    for (i = 0; i < 1000; i++)
    {
        r += b / (a + 1);
    }
    return r;
}

int main()
{
    for (int i = 1; i < 50000; i++)
    {
        one(i, i * 2);
        two(i, i * 2);
    }
    return 0;
}

and this is the profiler output

  %   cumulative   self              self     total           
 time   seconds   seconds    calls  us/call  us/call  name    
 50.67      1.14     1.14    49999    22.80    22.80  two(int, int)
 49.33      2.25     1.11    49999    22.20    22.20  one(int, int)

If I name one, then two is the opposite, two is more time than one both are the same functions, but the first calls always take less time, and then the second

Why is this?

Note. The build code is exactly the same and the code compiles without optimization

Answer 1

I would suggest that this is some kind of accident during optimization at runtime - one is using the register, and the other is not, or something like that.

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Answer 2

: , mcount. mcount (monitor.h) 32- - 4 . : prof vs gprof mon.out. solaris 9 -

prof 
 %Time Seconds Cumsecs  #Calls   msec/call  Name
  46.4    2.35    2.3559999998      0.0000  .div
  34.8    1.76    4.11120000025      0.0000  _mcount
  10.1    0.51    4.62       1    510.      main
   5.3    0.27    4.8929999999      0.0000  one
   3.4    0.17    5.0629999999      0.0000  two
   0.0    0.00    5.06       1      0.      _fpsetsticky
   0.0    0.00    5.06       1      0.      _exithandle
   0.0    0.00    5.06       1      0.      _profil
   0.0    0.00    5.06      20      0.0     _private_exit, _exit
   0.0    0.00    5.06       1      0.      exit
   0.0    0.00    5.06       4      0.      atexit


gprof
   %  cumulative    self              self    total
 time   seconds   seconds    calls  ms/call  ms/call name
 71.4       0.90     0.90        1   900.00   900.00  key_2_text        <cycle 3> [2]
  5.6       0.97     0.07   106889     0.00     0.00  _findbuf [9]
  4.8       1.03     0.06   209587     0.00     0.00  _findiop [11]
  4.0       1.08     0.05                            __do_global_dtors_aux [12]
  2.4       1.11     0.03                            mem_init [13]
  1.6       1.13     0.02   102678     0.00     0.00  _doprnt [3]
  1.6       1.15     0.02                            one [14]
  1.6       1.17     0.02                            two [15]
  0.8       1.18     0.01   414943     0.00     0.00  realloc   <cycle 3> [16]
  0.8       1.19     0.01   102680     0.00     0.00  _textdomain_u     <cycle 3> [21]
  0.8       1.20     0.01   102677     0.00     0.00  get_mem [17]
  0.8       1.21     0.01                            $1 [18]
  0.8       1.22     0.01                            $2 [19]
  0.8       1.23     0.01                            _alloc_profil_buf [22]
  0.8       1.24     0.01                            _mcount (675)

Answer 3

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BTW: which optimization flags are compiled with?