[Bug 74113] New: GLSL: vectorize optimization does not take branches into account

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=74113

          Priority: medium
            Bug ID: 74113
          Assignee: idr at freedesktop.org
           Summary: GLSL: vectorize optimization does not take branches
                    into account
        QA Contact: intel-3d-bugs at lists.freedesktop.org
          Severity: normal
    Classification: Unclassified
                OS: All
          Reporter: aras at unity3d.com
          Hardware: All
            Status: NEW
           Version: git
         Component: glsl-compiler
           Product: Mesa

"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.

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