<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/93563>93563</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            [clang][codegen] clang generates bad code for placement new of aggregate with `[[no_unique_address]]` and `-O2`
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            clang
      </td>
    </tr>

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

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

<pre>
    Compiling the following code with `-O2` yields an executable that prints the incorrect result and then segfaults:

```c++
#include <new>
#include <cstdio>

template <class T>
struct box {
 [[no_unique_address]]
  T value;
};

template <class... Ts>
struct tuple : box<Ts>... {};

struct empty {};

int main() {
  using T = tuple<empty, int>;
  alignas(T) unsigned char buffer[sizeof(T)];

  std::puts("before"); std::fflush(stdout);

  auto* p = new(buffer) T{{empty{}}, {42}}; // bad codegen here

  std::puts("after"); std::fflush(stdout);

 p->~T();
}
```

Building and running this program with clang-trunk results in:

```
before
before
Segmentation fault (core dumped)
```

With ASAN, I get:

```
before
before
AddressSanitizer:DEADLYSIGNAL
=================================================================
==796460==ERROR: AddressSanitizer: SEGV on unknown address 0x000000000000 (pc 0x000000000000 bp 0x000000000001 sp 0x7ffc0b326b38 T0)
==796460==Hint: pc points to the zero page.
==796460==The signal is caused by a READ memory access.
==796460==Hint: address points to the zero page.
    #0 0x0 (<unknown module>)

AddressSanitizer can not provide additional info.
SUMMARY: AddressSanitizer: SEGV (<unknown module>) 
==796460==ABORTING
```

Interestingly, if I try to build with `-O0`, clang hangs and eventually aborts with:

```
fatal error: error in backend: SmallVector unable to grow. Requested capacity (4294967296) is larger than maximum value for size type (4294967295)
clang++-17: error: clang frontend command failed with exit code 70 (use -v to see invocation)
Ubuntu clang version 17.0.6 (++20231209124227+6009708b4367-1~exp1~20231209124336.77)
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin
clang++-17: note: diagnostic msg: 
********************

PLEASE ATTACH THE FOLLOWING FILES TO THE BUG REPORT:
Preprocessed source(s) and associated run script(s) are located at:
clang++-17: note: diagnostic msg: /tmp/hello_world-eac76f.cpp
clang++-17: note: diagnostic msg: /tmp/hello_world-eac76f.sh
clang++-17: note: diagnostic msg: 

********************
```



See: https://godbolt.org/z/jPr4e5Kco

</pre>
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