[Bug gcov-profile/120478] New: Line coverage gets multiplied by a factor when invoking inline functions

[wentaoz5 at illinois dot edu via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=120478

            Bug ID: 120478
           Summary: Line coverage gets multiplied by a factor when
                    invoking inline functions
           Product: gcc
           Version: 16.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: gcov-profile
          Assignee: unassigned at gcc dot gnu.org
          Reporter: wentaoz5 at illinois dot edu
  Target Milestone: ---

Hit the issue when measuring
https://sources.debian.org/src/lz4/1.9.4-1/lib/lz4.c/#L985
https://sources.debian.org/src/lzo2/2.10-3/src/lzo1x_c.ch/#L176
etc.

How to reproduce:

$ gcc --version
gcc (GCC) 16.0.0 20250511 (experimental)
Copyright (C) 2025 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'
__inline__ __attribute__((__always_inline__))
int foo() { return 0; }

int main(void) {
    int x = 0, y = 0;
    for (int i = 0; i < 42; i++) {
        x++; y += foo();
        // x++;
    }
}
EOF
$ gcc --coverage test.c -o test
$ ./test
$ gcov test
$ cat test.c.gcov
        -:    0:Source:test.c
        -:    0:Graph:test.gcno
        -:    0:Data:test.gcda
        -:    0:Runs:1
        -:    1:__inline__ __attribute__((__always_inline__))
       42:    2:int foo() { return 0; }
        -:    3:
        1:    4:int main(void) {
        1:    5:    int x = 0, y = 0;
       43:    6:    for (int i = 0; i < 42; i++) {
       84:    7:        x++; y += foo();
        -:    8:        // x++;
        -:    9:    }
        -:   10:}

Line coverage for line 7 gets multiplied by 2 from its ground truth.
Uncommenting the statement that follows, or cancelling inline reverts 
the behavior:

       43:    6:    for (int i = 0; i < 42; i++) {
       42:    7:        x++; y += foo();
       42:    8:        x++;
        -:    9:    }


       43:    6:    for (int i = 0; i < 42; i++) {
       42:    7:        x++; y += foo_noinline();
        -:    8:        // x++;
        -:    9:    }