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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 - Adding a single-iteration do-while loop in a GLSL shader leads to a different image"
href="https://bugs.freedesktop.org/show_bug.cgi?id=101648">101648</a>
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<th>Summary</th>
<td>Adding a single-iteration do-while loop in a GLSL shader leads to a different image
</td>
</tr>
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<th>Product</th>
<td>Mesa
</td>
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<th>Version</th>
<td>17.1
</td>
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<th>Hardware</th>
<td>x86-64 (AMD64)
</td>
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<th>OS</th>
<td>Linux (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>Mesa core
</td>
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<th>Assignee</th>
<td>mesa-dev@lists.freedesktop.org
</td>
</tr>
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<th>Reporter</th>
<td>h.evrard@imperial.ac.uk
</td>
</tr>
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<th>QA Contact</th>
<td>mesa-dev@lists.freedesktop.org
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</tr></table>
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<div>
<pre><a href="https://github.com/mc-imperial/shader-compiler-bugs/issues/75">https://github.com/mc-imperial/shader-compiler-bugs/issues/75</a>
Steps to Reproduce:
-------------------------------
0. This is reproducable with Mesa version 17.1.3, on Intel intergated GPUs.
1. Obtain and build the latest release of get-image-glsl, a simple tool
that creates a .png image from a fragment shader. We need the
`get_image_glfw` executable.
<a href="https://github.com/graphicsfuzz/get-image-glsl">https://github.com/graphicsfuzz/get-image-glsl</a>
2. From a terminal, execute (this must be done in the directory where
both .frag and .json files are):
`/path/to/get_image_glfw original.frag`
This will create output.png, which should look like original.png
3. From a terminal, execute (same remark as above):
`/path/to/get_image_glfw variant.frag`
This will create output.png, which should look like variant.png
Expected Results:
-------------------------------
Both shaders should render an image that looks like original.png. This
is because the only difference between the .frag files is a 'do-while'
loop that should execute only once:
```
$ diff -u original.frag variant.frag
--- original.frag 2017-06-28 11:04:21.278754777 +0100
+++ variant.frag 2017-06-28 11:23:16.925591708 +0100
@@ -42,10 +42,12 @@
{
s += _FLOAT_CONST[2];
}
+ do {
if(float(i) >= limit)
{
return s;
}
+ } while(gl_FragCoord.x < 0.0);
}
return s;
}
```
Actual Results:
-------------------------------
The shaders render completely different images, which they should not.
Mesa code coverage:
-------------------------------
Unzip the `coverage.html.zip` file to get a web page that present the
comparative coverage of Mesa source code between the execution of the
original and the variant shader.
Additional Information:
-------------------------------
Some info on the system where the bug was found:
CPU: Intel(R) Core(TM) i7-2600 CPU @ 3.40GHz
GPU: "2nd Generation Core Processor Family Integrated Graphics Controller"
OS: Linux 4.8.0-56-generic #61~16.04.1-Ubuntu SMP
Mesa version: 17.1.3</pre>
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