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List:       llvm-bugs
Subject:    [llvm-bugs] [Bug 63602] Miscompilation in LoopVectorizer
From:       LLVM Bugs via llvm-bugs <llvm-bugs () lists ! llvm ! org>
Date:       2023-06-29 14:25:16
Message-ID: 20230629142516.30bc28a2ebb25cfe () email ! llvm ! org
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<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/63602>63602</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            Miscompilation in LoopVectorizer
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            miscompilation,
            vectorization
      </td>
    </tr>

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

    <tr>
      <th>Reporter</th>
      <td>
          d-makogon
      </td>
    </tr>
</table>

<pre>
    Repro: https://godbolt.org/z/6MxeMx97G

The source IR is for the following code:
```java
int i = 1;
while ((i += 3) &lt; 383) {
  arr[i] = 1;
  arr[i - 1] &amp;= 1;
  arr[i] += 1;
}
```
The `arr` is filled with zeroes initially.
The program just stores `2` in each third element of the array starting from the \
index `4`. After the vectorizer pass however the part of the loop that is processed \
in the vector loop is filled with just ones.

The issue in the vector loop is the following:
First we load the elements for which we need to do the addition of `1` (corresponding \
to the line `arr[i] += 1`) into `%strided.vec3`. Then we store the ones into each \
third index: `store i32 1, ptr %21, align 4`;   `store i32 1, ptr %22, align 4`;   \
`...`. And then we do the addition of `1` to the **previously loaded** vector of \
values: `%54 = add &lt;8 x i32&gt; %strided.vec3, &lt;i32 1, i32 1, i32 1, i32 1, i32 \
1, i32 1, i32 1, i32 1&gt;`, and after that we perform the stores:  ```
%55 = extractelement &lt;8 x i32&gt; %54, i32 0
store i32 %55, ptr %21, align 4
```
```
%56 = extractelement &lt;8 x i32&gt; %54, i32 1
store i32 %56, ptr %22, align 4
```
...
So the issue is that the `+1` is done for the values that are loaded from the memory \
before the stores of `1`. So we end up storing `0+1=1` in each third index starting \
from 4. </pre>
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