<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/105341>105341</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            [Coverage] false counter for branch coverage underflows when there's data race
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            tools:llvm-cov
      </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/#g:!((g:!((g:!((h:codeEditor,i:(filename:'1',fontScale:14,fontUsePx:'0',j:1,lang:c%2B%2B,selection:(endColumn:1,endLineNumber:30,positionColumn:1,positionLineNumber:30,selectionStartColumn:1,selectionStartLineNumber:1,startColumn:1,startLineNumber:1),source:'%23include+%3Cstdio.h%3E%0A%23include+%3Cstdlib.h%3E%0A%23include+%3Cpthread.h%3E%0A%0A%23define+NUM_THREADS+10000%0A%0Aint+x%3B%0Aint+y%3B%0A%0Avoid*+f(void*+thread_id)+%7B%0A++++x%2B%2B%3B%0A++++for+(int+i+%3D+0%3B+i+%3C+100%3B+i+%2B%2B)%0A++++++++if+(x+%25+10000)%0A++++++++++++%2B%2By%3B%0A++++pthread_exit(NULL)%3B%0A%7D%0A%0Aint+main()+%7B%0A++++pthread_t+threads%5BNUM_THREADS%5D%3B%0A++++long+t%3B%0A%0A++++for(t+%3D+0%3B+t+%3C+NUM_THREADS%3B+t%2B%2B)%0A++++++++pthread_create(%26threads%5Bt%5D,+NULL,+f,+(void*)t)%3B%0A%0A++++for(t+%3D+0%3B+t+%3C+NUM_THREADS%3B+t%2B%2B)+%0A++++++++pthread_join(threads%5Bt%5D,+NULL)%3B+++%0A%0A++++return+0%3B%0A%7D%0A'),l:'5',n:'1',o:'C%2B%2B+source+%231',t:'0')),k:33.333333333333336,l:'4',n:'0',o:'',s:0,t:'0'),(g:!((h:compiler,i:(compiler:clang_trunk,filters:(b:'0',binary:'1',binaryObject:'1',commentOnly:'0',debugCalls:'1',demangle:'0',directives:'0',execute:'1',intel:'1',libraryCode:'0',trim:'1',verboseDemangling:'0'),flagsViewOpen:'1',fontScale:14,fontUsePx:'0',j:1,lang:c%2B%2B,libs:!(),options:'-fprofile-instr-generate+-fcoverage-mapping+-fprofile-update%3Dsingle',overrides:!(),selection:(endColumn:1,endLineNumber:1,positionColumn:1,positionLineNumber:1,selectionStartColumn:1,selectionStartLineNumber:1,startColumn:1,startLineNumber:1),source:1),l:'5',n:'0',o:'+x86-64+clang+(trunk)+(Editor+%231)',t:'0')),k:33.333333333333336,l:'4',n:'0',o:'',s:0,t:'0'),(g:!((h:tool,i:(args:'--show-branches%3Dcount',argsPanelShown:'0',compilerName:'x86-64+clang+(trunk)',editorid:1,fontScale:14,fontUsePx:'0',j:1,monacoEditorHasBeenAutoOpened:'1',monacoEditorOpen:'1',monacoStdin:'1',stdin:'',stdinPanelShown:'1',toolId:llvm-covtrunk,wrap:'1'),l:'5',n:'0',o:'llvm-cov+(clang-only)+x86-64+clang+(trunk)+(Editor+%231,+Compiler+%231)',t:'0')),k:33.33333333333333,l:'4',n:'0',o:'',s:0,t:'0')),l:'2',n:'0',o:'',t:'0')),version:4). It's possible to get the following results:

```c
   13|   986k| if (x % 10000)
  ------------------
  |  Branch (13:13): [True: 986k, False: 18.4E]
  ------------------
   14|   986k| ++y;
```

The cause is:

1. Concurrent counter updates `mov` -- `add` -- `mov` interleave with each other
2. The counter for "then" clause could be greater than the counter for "if" clause: $C_\text{then} > C_\text{if}$ 
3. $C_\text{else} = C_\text{if} - C_\text{then} < 0$, resulting in `0xffffffffffffff..` thus `18.4E`

I understand we can apply flag `-fprofile-update=atomic`. But otherwise, in the report we should probably report the negative value as is, which is also gcov's practice:

```c
   997744:   13:        if (x % 10000)
branch  0 taken 997755 (fallthrough)
branch  1 taken -11
   997755:   14: ++y;
```

or report 0, which is [`lcov`'s practice](https://github.com/linux-test-project/lcov/blob/d465f73117ac3b66e9f6d172346ae18fcfaf0f69/lib/lcovutil.pm#L3590-L3593).

cc @evodius96
</pre>
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