[Bug 94073] Miscompilation of abs_vec3_vert_xvary_ref.vert in WebGL conformance

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=94073

            Bug ID: 94073
           Summary: Miscompilation of abs_vec3_vert_xvary_ref.vert in
                    WebGL conformance
           Product: Mesa
           Version: 11.0
          Hardware: Other
                OS: All
            Status: NEW
          Severity: normal
          Priority: medium
         Component: Drivers/DRI/i965
          Assignee: idr at freedesktop.org
          Reporter: bztdlinux at gmail.com
        QA Contact: intel-3d-bugs at lists.freedesktop.org

The test abs_vec3_vert_xvary.test.html fails due to the reference vertex shader
being miscompiled. The test can be reproduced by visiting this page with
Firefox (preferably Nightly).

https://www.khronos.org/registry/webgl/conformance-suites/1.0.2/conformance/ogles/GL/abs/abs_001_to_006.html

The test passes on Chromium, but the vertex shader submitted is the same,
checked with apitrace. It's unclear what is causing the behavior difference.

The error happens between the SSA and final NIR stages. Note that ssa_16 is
dropped and never used, leading to the blue channel never having the absolute
value applied:

https://pastebin.mozilla.org/8859127

Trivial modifications to the shader make it compile correctly. For example,
replacing

if((c[2] < 0.0)) c[2] *= -1.0;

with

float x = c.b;
if (x < 0.0) x *= -1.0;
c.b = x;

This is one of a series of similar failures on the Khronos test. The entire
test can be accessed here:
https://www.khronos.org/registry/webgl/conformance-suites/1.0.2/webgl-conformance-tests.html

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