<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/61770>61770</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            [AArch64]: 128-bit Const Atomic Load implemented using Store Pair instruction, induces Runtime Crash on Arm AArch64
        </td>
    </tr>

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

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

    <tr>
      <th>Reporter</th>
      <td>
          lukeg101
      </td>
    </tr>
</table>

<pre>
    Consider the following C test:
```
// test.c - fails when compiled and run with either LLVM or GCC
#include <stdatomic.h>
const _Atomic __int128 x = 0;
int main() {
__int128_t y = atomic_load_explicit(&x,memory_order_seq_cst);
}
// Fails when run as x is put into read-only section // $ clang -O0 test.c -o test.o
// $ ./test.o
/ / [1] 55891 bus error ./test.o
```
When compiled and run with clang (observed using apple clang on an M1 machine) targeting Arm AArch64, we observe a run-time crash.

As far as I can tell this fault is due to the fact that `const`-qualified data is stored in read-only section, and the compare-and-swap loop (https://godbolt.org/z/GaGKhT4f9) implemented using the `LDAXP`, `STLXP`, and `CBNZ` emits a store to the read-only memory.

It appears this issue is fixed by the patch [1] when targeting armv8.4-a and above, but it is not clear there is a (lock-free) fix for this crash for Armv8.3-a or below (or for any Arm arch that doesn't implement single-copy atomicity for 16-byte aligned 128-bit loads or stores). GCC emits a function call [2] instead. Is it possible to emit a function call in LLVM in a similar way without an ABI break?

[1] https://reviews.llvm.org/rG13aa102e07695297fd17f68913c343c95a7c56ad
[2] https://godbolt.org/z/7nqPW6Yb6


</pre>
<img width="1px" height="1px" alt="" src="http://email.email.llvm.org/o/eJx0VVFz4jYQ_jXiZceMLWODH3ggUDKZ5tqb3k2v7QuzltZYPVniJDmE_vqOZEIul3YmASTtrna_79sVeq-OhmjNqjtW7WY4ht66tR6_0rHIi1lr5WW9tcYrSQ5CT9BZre1ZmSNsIZAPrNywfMfyDavz69-05HvG98lkLiCDDpX2cO7JgLDDSWmSgEaCGw2cVeiBVOjJwePj7x_AOrjfbl8ClcoIPUoCVm59kBjsoMS8Z-VPk4Wwxgc4bNI-HA7KhIKv4BlYuYOclXeTmTIBBlSG8RXjDbDldf_F4RDgklymCw7aojzQ80kroUJyqp8Z3w40WHc5WCfJHTx9OwgfGG9u17Dl7g0C-9fCY63o4RmUh9MYQJlgwRHKzBp9AU8iKGvg6sj4AoRGc4Ts1_wGpJ1-2Td3RNM54_sfjiD9V3cFq3ZQVaumgHb0QM5Z997-LX9f_p-pKSnGV7b15J5IwuijIPB00nQ9tQbQwIcCBhS9MhQRD-iOFKLlxg2w2TjR1wvGt3AmuIYCjNdkQQ0EwqHv59fk0ufGQ4cuQvgAAg0E0hpCr-L2qEOEVY4EwU5KRREg9BiA1XnSCKvz7NuIWnWKJEgMGF18sI4kKPOeiphcLD2Gi1igowyNzPwZT6CtPUUU-hBOPrZB4uJoZWt1mFt3ZHz_D-P7e7z_uf-86JqIgRpOmgYy4YZajM3q_HG3-eNjRJ9v4_LT58fbMmbA6nx798tfrM6BBhU84JT3S7GvqU_6fIPbQ4jcEDo_oaW8HymW3qlnktBeUogTBtHf5JIE-8oYuuFpNV9kmLLB1j5RTK2NKk7AGxtAaMI0JVyKjhEdbcXXrHOUFNCpZ-ism7JI_KblJgUvM4yN35K256Qulw7RXJJe0Il-olNa8obxZXhFEyKUmjJhT5dr_6pwSf5FnbWXQIA6TjoJBV9lrQoQ29vHCxOOnvFmHofODd5uNFM7CtQ6wsIjLMr4QCjn8OBj4SfrvWp1oiE6vvNTZppoykTG1KA0OjjjJXWSHUPsks3dA7SO8Csr99_T9kLFW4E5elJ09nOtn4arytx9USIWOad8WTcVb5adLJZdvWqKUpSLUjQVLkVVo7wF5u8Dv1fu0nz7-KX-s62_z2om16VsygZntC7qVZ4XzbJsZv1atk3b8aYqsF1ybLumoE7mKFdSrPJqWc3Umue8zEu-Kup8VTXzoiyrVi7KvKxFTo1ki5wGVPpW2ywpdV0Xy2U-09iS9uml4tzQeZIx47GUmVtHn6wdj54tcq18eEVoFlTQ6Yl7mTnVjpWbmxK26f24Ph-PFuV_dOmn1GwfUbkkATfe5oMychTk4bfRpLG1TbK25vshNxudXv-Atgr92M6FHRjfx0yvX9nJ2b9JBMb3qT7P-D7V_28AAAD___iPb3A">