c/inc_loop.c

Ubuntu 25.04 GCC 14.2 -O0 x86_64 produces a horrendous:

    11c8:       48 83 45 f0 01          addq   $0x1,-0x10(%rbp)
    11cd:       48 8b 45 f0             mov    -0x10(%rbp),%rax
    11d1:       48 3b 45 e8             cmp    -0x18(%rbp),%rax
    11d5:       72 f1                   jb     11c8 <main+0x7f>

To do about 1s on P14s we need 2.5 billion instructions:

time ./inc_loop.out 2500000000

and:

time ./inc_loop.out 2500000000

gives:

          1,052.22 msec task-clock                       #    0.998 CPUs utilized             
                23      context-switches                 #   21.858 /sec                      
                12      cpu-migrations                   #   11.404 /sec                      
                60      page-faults                      #   57.022 /sec                      
    10,015,198,766      instructions                     #    2.08  insn per cycle            
                                                  #    0.00  stalled cycles per insn   
     4,803,504,602      cycles                           #    4.565 GHz                       
        20,705,659      stalled-cycles-frontend          #    0.43% frontend cycles idle      
     2,503,079,267      branches                         #    2.379 G/sec                     
           396,228      branch-misses                    #    0.02% of all branches

With -O3 it manages to fully unroll the loop removing it entirely and producing:

    1078:       e8 d3 ff ff ff          call   1050 <strtoll@plt>
}
    107d:       5a                      pop    %rdx
    107e:       c3                      ret

to is it smart enough to just return the return value from strtoll directly as is in rax.

c/inc_loop.c

#include <stdlib.h>

int main(int argc, char **argv) {
    unsigned long long max;
    if (argc > 1) {
        max = strtoll(argv[1], NULL, 0);
    } else {
        max = 1;
    }
    unsigned long long ret;
    for (ret = 0; ret < max; ret++) {}
    return ret;
}

c/inc_loop.c

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