<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/63564>63564</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            Instcombine miscompile removing memcpy from constant memory to alloca
        </td>
    </tr>

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

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

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

<pre>
    https://alive2.llvm.org/ce/z/4Qv6c5

```
$ cat /tmp/a.ll
@g = external constant [128 x i8]

define void @f() {
  %a = alloca [128 x i8]
  call void @bar(ptr %a) readonly
  call void @llvm.memcpy.p0.p0.i64(ptr %a, ptr @g, i64 128, i1 false)
  ret void
}

declare void @llvm.memcpy.p0.p0.i64(ptr nocapture, ptr nocapture, i64, i1)
declare void @bar(ptr)

$ opt -p instcombine -S /tmp/a.ll
@g = external constant [128 x i8]

define void @f() {
 call void @bar(ptr nonnull @g) #1
  ret void
}

; Function Attrs: nocallback nofree nounwind willreturn memory(argmem: readwrite)
declare void @llvm.memcpy.p0.p0.i64(ptr noalias nocapture writeonly, ptr noalias nocapture readonly, i64, i1 immarg) #0

declare void @bar(ptr)

attributes #0 = { nocallback nofree nounwind willreturn memory(argmem: readwrite) }
attributes #1 = { memory(read) }
```

I don't understand what UB alive2 claims the optimized function is triggering. But separately this does seem wrong in the context of poison vs undef. Previously we'd hand `bar` an alloca containing undef, but now we pass `bar` `g` which may contain poison.

https://github.com/llvm/llvm-project/blob/3a4e9f7ae50980ce8f103cbe86d24c574c8c6cac/llvm/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp#L525 is where this happens.

@nunoplopes 
</pre>
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