"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.