<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/74248>74248</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            backends will turn OOB InsertElement into OOB store
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            backend:AArch64,
            llvm:codegen,
            miscompilation
      </td>
    </tr>

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

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

<pre>
    here's a slightly modified version of one of our unit tests:
```llvm
define void @insert_store_nonconst_index_not_known_valid_by_and(ptr %p, i8 zeroext %val, i32 %idx) {
 %x4 = load <16 x i8>, ptr %p, align 16
 %x6 = insertelement <16 x i8> %x4, i8 %val, i32 %idx
 store <16 x i8> %x6, ptr %p, align 16
 ret void
}
```

the issue is what happens when `%idx` is out of bounds. LangRef says that in this case the insertelement poisons the vector. however, here's what we get on AArch64 (other backends seem similar):
```
insert_store_nonconst_index_not_known_valid_by_and:
 strb    w1, [x0, w2, uxtw]
        ret
```
this is, of course, much worse than a vector full of poison. there would seem to be two solutions. first, bounds-checking on vector element access indices. second, we change LangRef to make this situation immediate UB.

cc @nunoplopes @tanmaytirpankar
</pre>
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