<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/107025>107025</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            [Coverage][MC/DC] negative counter value ("18.4E") reported for nested ternary expressions
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            clang:codegen,
            coverage
      </td>
    </tr>

    <tr>
      <th>Assignees</th>
      <td>
      </td>
    </tr>

    <tr>
      <th>Reporter</th>
      <td>
          whentojump
      </td>
    </tr>
</table>

<pre>
    Compiler explorer [link](https://godbolt.org/#z:OYLghAFBqd5TKALEBjA9gEwKYFFMCWALugE4A0BIEAZgQDbYB2AhgLbYgDkAjF%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%2Bw9MJhgEHEc7iV4OBrg2BDBgZ1mkHEmTYJ82DGcA9CRKA2ECEYwBKAAagQ2DThuaTMO5faiOIkjDnlY4aGu%2Bg8IYp5oLa1geXEMaQOs6BpIx3BkgAGiedZkpgfhpGSGAhSwwDYGSxxpPm3U0GkpB7gWZKRHMpBkiNURzhx5I0INiTDaN42TWeZJbegQ0jWNqCYKgm0zXNjBkj4aSYCwHEMp8z2EQQqCWiFXY4A1QF1A0rgQO40wBBV4SLJU1RSAAnOkmTZEIYMGMUSNMMM0NjDw8OA30AxTN4HQGHjjTzJjoxJDjEyDCjFVzIMFPLFTsPrA6Ww7FIy5cKakFrpGB7lpW1bUeel6JIxd6Ns25wQPgxBkO%2BroIUhEHGiBQY%2BjwAB0yRaLDQTSIbxtG0bfPoZGsFxgh5B4RAKAkN42YQC0yVtWSShIOg05kl5xioL%2BSivRgTaHGSHtez7bAXV95Cu0oqjGD0IiR1B8mERRhbFtwpZC8eosQCxLZth2ZDdr28gDoVsjFSopXoeVYFCCwGDy52AASF5WNgzB3Jm6BZcw7buWwzet6XpCD%2Bacmj6wGBNIQ0/RkQi8wRMi%2BJ1E9Ap97S%2Bfug3gAJI9vQ9CWWwA0nUXn6qtg5d%2BrO87CdzvNp5Gp/n5flnYl1wBkkIqlzi5yPCLU8YsGI3ilg%2BWWbdFYggqgcZ8mc3zwI/F%2BD05Bfz/jGEBdWoFgxG21u8WGWgeACnIWQyh5twyW3sHBdBiFyDejVtwVC1DoKYQYdzCwaEaFry4eQEKWRXDSCAA%3D%3D%3D)

Program reduced from https://elixir.bootlin.com/linux/v6.11-rc6/source/fs/proc/internal.h#L70

```c
int main() { 
    int x = 1       //   #0
          ? 128     //   #1
 : (
                1   //  (#0 - #1)
              ? 256 // #2
              : 512 // ((#0 - #1) - #2)
          );
    return x;
}
```

Coverage report

```text
    1|      1|int main() { 
    2|      1|    int x = 1       //   #0
 ------------------
  |  Branch (2:13): [Folded - Ignored]
 ------------------
    3|      1|          ? 128     //   #1
    4| 1|          : (
    5|      0|                1   //  (#0 - #1)
 ------------------
  |  Branch (5:17): [Folded - Ignored]
 ------------------
    6|      1|              ? 256 //   #2
    7| 18.4E|              : 512 // ((#0 - #1) - #2)
    8|      0| );
    9|      1|    return x;
   10|      1|}
```

Direct cause:

1. Affected by the constant at line 5, `inc #2` wrongly instrumented for unexecuted line 6
2. Counter for line 7 is calculated by `((#0 - #1) - #2)` $= ((1 - 1) - 1) = -1$, showing up as 18.4E (18446744073709551615).

The problem only appears if `-fcoverage-mcdc` is supplied.

cc @evodius96  


</pre>
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