[Bug gcov-profile/116452] New: Inconsistent way of reporting negative counter values

[wentaoz5 at illinois dot edu via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116452

            Bug ID: 116452
           Summary: Inconsistent way of reporting negative counter values
           Product: gcc
           Version: 15.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: gcov-profile
          Assignee: unassigned at gcc dot gnu.org
          Reporter: wentaoz5 at illinois dot edu
  Target Milestone: ---

Currently branch coverage will report negative counter values as is [1] but
line coverage will report 0 [2].

[1]
https://github.com/gcc-mirror/gcc/blob/ec9d6d45191f639482344362d048294e74587ca3/gcc/gcov.cc#L3186
[2]
https://github.com/gcc-mirror/gcc/blob/ec9d6d45191f639482344362d048294e74587ca3/gcc/gcov.cc#L3276

As a result, the following could happen:

   995861:   11:void g() {
   995861:   12:    if (x % 10000)
branch  0 taken 995986 (fallthrough)
branch  1 taken -125
   995986:   13:        ++y;
        -:   14:    else
    #####:   15:        --y;
   995861:   16:}

Branch 1 is reportedly taken "-125 times" and line 15 is executed 0 time.

---

How to reproduce:

$ gcc --version

gcc (GCC) 15.0.0 20240808 (experimental)
Copyright (C) 2024 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

$ cat > test.c << EOF

#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>

#define NUM_THREADS 10000

int x;
int y;

void g() {
    if (x % 10000)
        ++y;
    else
        --y;
}

void* f(void* thread_id) {
    x++;
    for (int i = 0; i < 100; i++)
        g();
    pthread_exit(NULL);
}

int main() {
    pthread_t threads[NUM_THREADS];
    long t;

    for(t = 0; t < NUM_THREADS; t++)
        pthread_create(&threads[t], NULL, f, (void*)t);

    for(t = 0; t < NUM_THREADS; t++)
        pthread_join(threads[t], NULL);

    return 0;
}

EOF
$ gcc --coverage -fprofile-update=single test.c -o test
$ ./test
$ gcov -b -c test
$ cat test.c.gcov