<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/123092>123092</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            [AMDGPU][Attributor] OpenMC Performance Regression on AMD GPU
        </td>
    </tr>

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

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

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

<pre>
    I have observed a performance regression in the OpenMC scientific simulation application (which uses OpenMP target offloading) on the AMD MI250 due to commit https://github.com/llvm/llvm-project/commit/7dbd6cd2946ec3a9b4ad2dfd7ead177baac15bd7, which is part of PR #114357. The regression is not observed on NVIDIA GPUs.

This commit causes OpenMC to be 20% slower overall on a typical benchmark problem, with the most expensive kernel in the simulation being about 2x slower.

OpenMC can be installed and its performance benchmark run using the following script: https://github.com/jtramm/openmc_offloading_builder/tree/main

FOM before this commit (higher is better):
```
Calculation Rate (inactive)       = 239870.0 particles/second
```

FOM after this commit:

```
 Calculation Rate (inactive)       = 192707.0 particles/second
```

Main kernel timing before this commit: 
```
     XS lookups (Fuel)             = 1.8720e+01 seconds
```

Main kernel after this commit:
```
     XS lookups (Fuel)             = 3.5231e+01 seconds
```

Rocprof shows similar slowdown for this kernel, which it lists as:

```
__omp_offloading_25_7c638f81__ZN6openmc32process_calculate_xs_events_fuelEv_l256.kd
```
</pre>
<img width="1" height="1" alt="" src="http://email.email.llvm.org/o/eJyclVFv2zYQxz8N_XKYIVKWZD34wY2nIA9ujawZhr0IJHWy2FCkQFJ2-u0HSjbsdFvbTQhiGebxfvz_j3fce3U0iBuSfSDZbsHH0Fm3-RIc7_uFsM3XzRN0_IRghUd3wgY4DOha63puJILDo0PvlTWgDIQO4dOAZv8AXio0QbVKglf9qHmIa_gwaCXnd8LW507JDkaPfg47QODuiAFs22rLG2WOhJVg5523-x3sn1iWQDMiBAvS9r0K0IUweJJuCasIq44qdKNYStsTVml9un78Mjj7BWUgrJrjCKuKRjS5bFi5ylGmvBQr3rCmbQrkDS0KwbmkmWgKwh5gZlUeBu4iIByegbCU0lWaFUv43L0Xw4Ox4aaaNfDx96fd0xYeDy9-SZItSbafO-Wvh5D8JsNDPJxAYAlhGXhtz-jAntBxreNOHMLXQUmuQaCRXc_dKwzOCo39RKpCNwnWWx8A3wY0Xp0QXtEZ1Fef7lwRqMwRuLBjAPZ2SXhhvABJHpeBMj5wrWMZmAZU8O-K4UbjRgOjj7vGVK3V2p7jNy-dGgJJt98zba4-wio7oOllfSuGWoxKN-gIq4JDJKzquTIzaPVpDwJb6xDCna6ErTt17NBFSwSGEKPLmDjZkjy5_CXbB67lVY9nHjAGKsNlUCeMAQ-MbJOf-p_ugKXlukiWyVQrSmr0hFUepTXNN2kv4LwN6O65L4DvF8MPIGF-IgEtWZEUP0mw58pcqyOoPhr1dyWjad_yxOeP30Bb-zoOPtJUI-obyR3Pcl2wBAn7kFCYOfz3Qf5Nkv-XP11mLKU_yv9s5eBsC76zZx9viNLcTfehsWcDrb0AzYh3XSGAVj544P4fjatr2w_3dcyyupB5um7XtK7__JjPlZ6ywVmJ3tfyYjTWb77GE5rg63ZE_eup1izLl6_vXFw0m7Qp05IvcEOLNC9zmiXpots0AtOS0lyWyaooUrEq1jktGEohBc0FW6gNS1iWUJrRgq4ZW6YpEzmlRdbmDOmKklWCPVd6GVvo0rrjQnk_4oayNCnZQnOB2k_TgzGDZ5h-JYzFYeI2U98V49GTVTIJdNsmqKCnsbPd7x4PLyTbxfcQnBJjsI5ku2svPNx1mOdbh7VmGgiPh5fF6PTmPw-BiTTeistRThv2VwAAAP__KxY5cw">