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<base href="https://bugs.freedesktop.org/" />
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<th>Priority</th>
<td>medium
</td>
</tr>
<tr>
<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - GLSL: vectorize optimization does not take branches into account"
href="https://bugs.freedesktop.org/show_bug.cgi?id=74113">74113</a>
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<th>Assignee</th>
<td>idr@freedesktop.org
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</tr>
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<th>Summary</th>
<td>GLSL: vectorize optimization does not take branches into account
</td>
</tr>
<tr>
<th>QA Contact</th>
<td>intel-3d-bugs@lists.freedesktop.org
</td>
</tr>
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<th>Severity</th>
<td>normal
</td>
</tr>
<tr>
<th>Classification</th>
<td>Unclassified
</td>
</tr>
<tr>
<th>OS</th>
<td>All
</td>
</tr>
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<th>Reporter</th>
<td>aras@unity3d.com
</td>
</tr>
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<th>Hardware</th>
<td>All
</td>
</tr>
<tr>
<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Version</th>
<td>git
</td>
</tr>
<tr>
<th>Component</th>
<td>glsl-compiler
</td>
</tr>
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<th>Product</th>
<td>Mesa
</td>
</tr></table>
<p>
<div>
<pre>"Vectorize multiple scalar assignments" optimization (added in 4bd6e0d7c69 on
master) does not take possible branches into account.
For example, on a fragment shader like this:
uniform sampler2D maintex;
uniform float factor;
varying vec2 uv;
void main()
{
vec4 c = texture2D (maintex, uv);
vec2 coord = c.xy;
bool cond = c.w >= 0.5;
if (!cond)
coord.x += factor;
if (cond)
coord.y += factor;
gl_FragColor = vec4(coord,0,0);
}
It does try to merge both coord.x and coord.y assignments into one, even if
they are under different branch conditions. Effectively it ends up with "if
(cond) coord.xy += ..." which is wrong.
Real-life shaders like FXAA3.11 have a code very similar to the above, and it
regresses under this optimization.
As a simple stop-gap solution the optimization could just never go into any
branch-like structures (return visit_continue_with_parent from
visit_enter(ir_if) etc.</pre>
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