<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/58600>58600</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            Clang misdiagnoses UB (emits unreachable IR) when incrementing unspecified value
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
      </td>
    </tr>

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

    <tr>
      <th>Reporter</th>
      <td>
          bcsb1001
      </td>
    </tr>
</table>

<pre>
    Clang, with -O1 or higher, seems to think the following C code invokes undefined behaviour. The IR output given by `clang -std=c17 -O1 -emit-llvm -S` has `main` as just `unreachable`.
```c
#include <stdio.h>

int main(void) {
    unsigned char x;
    (void) &x;
    while (++x) putchar('a');
    putchar('\n');
}
```
My understanding is that this is _not_ UB under C17 because the address of `x` is taken and `unsigned char` can't have trap representations, so the value of `x` is unspecified until the first increment. Interestingly, if you replace `++x` with `x+=1`, Clang compiles the code correctly, even though these are equivalent according to the standard. The same bug happens with `x++`, `--x` or `x--` instead of `++x`.
</pre>
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