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<base href="https://bugs.freedesktop.org/">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - radv: OpBitfieldUExtract returns incorrect result when count is zero"
href="https://bugs.freedesktop.org/show_bug.cgi?id=107276">107276</a>
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<th>Summary</th>
<td>radv: OpBitfieldUExtract returns incorrect result when count is zero
</td>
</tr>
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<th>Product</th>
<td>Mesa
</td>
</tr>
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<th>Version</th>
<td>git
</td>
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<th>Hardware</th>
<td>Other
</td>
</tr>
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<th>OS</th>
<td>All
</td>
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>normal
</td>
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<th>Priority</th>
<td>medium
</td>
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<th>Component</th>
<td>Drivers/Vulkan/radeon
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<th>Assignee</th>
<td>mesa-dev@lists.freedesktop.org
</td>
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<th>Reporter</th>
<td>philip.rebohle@tu-dortmund.de
</td>
</tr>
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<th>QA Contact</th>
<td>mesa-dev@lists.freedesktop.org
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</tr></table>
<p>
<div>
<pre>Created <span class=""><a href="attachment.cgi?id=140691" name="attach_140691" title="One of the affected shaders (GLSL version)">attachment 140691</a> <a href="attachment.cgi?id=140691&action=edit" title="One of the affected shaders (GLSL version)">[details]</a></span>
One of the affected shaders (GLSL version)
Hello,
when building Mesa with LLVM 7.0-svn, OpBitfieldUExtract sometimes returns the
original source operand instead of zero when the bit count is zero. With LLVM
6.0.1, it works as expected.
This causes the following bug in DXVK:
<a href="https://github.com/doitsujin/dxvk/issues/497">https://github.com/doitsujin/dxvk/issues/497</a>
Here's a renderdoc capture (recorded on Polaris) showing the issue:
<a href="https://mega.nz/#!8W5nDTxT!P7PI4BZ_gpmZDIoh1Iziq0PHpLp448hiUqJN9iEhukg">https://mega.nz/#!8W5nDTxT!P7PI4BZ_gpmZDIoh1Iziq0PHpLp448hiUqJN9iEhukg</a>
The attached shader is a GLSL version of the vertex shader used in the
bookmarked draw call. I added some comments around like 220 for observed vs.
expected behaviour of the shader code.
I tried to reproduce the problem outside of DXVK, but had no luck so far: A
small demo app testing the GLSL bitfieldExtract function in a compute shader
always returns the correct values.
Handling the count = 0 case explicitly in the NIR->LLVM translation fixes the
issue in Far Cry 5.
Regards,
- Philip</pre>
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