<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/64446>64446</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            Aliasing prevents optimization of redundant swaps
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            new issue
      </td>
    </tr>

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

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

<pre>
    ## Code to Reproduce
```cpp
template <typename T>
void swap(T &a, T &b) {
    T t = a;
    a = b;
 b = t;
}

void alias(int *x, int *y) {
    swap(*x, *y);
 swap(*x, *y);
}

void noalias(int * __restrict x, int * __restrict y) {
    swap(*x, *y);
    swap(*x, *y);
}
```
## Compiler Output (`clang++ -O3` trunk)
```asm
alias(int*, int*):
        mov eax, dword ptr [rdi]
        mov     ecx, dword ptr [rsi]
        mov dword ptr [rdi], ecx
        mov     dword ptr [rsi], eax
 mov     ecx, dword ptr [rdi]
        mov     dword ptr [rdi], eax
 mov     dword ptr [rsi], ecx
        ret
noalias(int*, int*):
 ret
```
*See [live example at Compiler Explorer](https://godbolt.org/z/qdhEErMx4)*

## Explanation
Even though `x` and `y` can alias each other in `alias()`, swapping twice should restore the original value. In fact, if they do alias each other, than `swap` would be a no-op. Essentially, there are two possibilities in this code:
- `x` and `y` point to distinct objects, meaning that they don't alias each other. If so, the codegen must be equivalent to that of `noalias`.
- `x` and `y` point to the same object, in which case a swap is a no-op. Two calls to `swap` are two no-ops.

Perhaps the analysis that's missing here is to identify swaps to be no-ops if aliasing takes place.
</pre>
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