<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/144736>144736</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            [DAGCombiner] trunc + zext on i64 to i32 folded into constant 0
        </td>
    </tr>

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

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

    <tr>
      <th>Reporter</th>
      <td>
          dlee992
      </td>
    </tr>
</table>

<pre>
    ### 💡 Example: Trunc + Zext in LLVM IR

The following function demonstrates a `trunc` followed by a `zext`:

```llvm
define i64 @bar(i64 %a) {
; CHECK-LABEL: bar:
; CHECK:       ; %bb.0:
; CHECK-NEXT:    ret 0
  %trunc = trunc nuw nsw i64 %a to i32
  %res = zext i32 %trunc to i64
  ret i64 %res
}
```
Ideally, LLVM should emit code that preserves the lower 32 bits of %a and zeros out the upper 32 bits. However, in the latest LLVM, it appears this truncation + zero-extension is incorrectly optimized into a constant 0. Since it's a new feature, I am not sure what's the correct semantics of nuw/nsw in trunc instruction.

This behavior is likely a regression and may be closely related to the following patch:
👉 llvm/llvm-project#113808
</pre>
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