Adding 0.0 to an expression causes significant visual difference

[afd]

[Shaders, screenshots, error log, reproduction steps, etc.] [ZIP]

Recipient:

Variant:

The recipient and variant images are rendered by identical shaders, except in the variant shader this line:

    vec3 b = fract(sin(co.x * 0.3e-3 + co.y) * vec3(8.1e5, 1.0e5, 0.1e5));

is replaced by:

    vec3 b = fract(sin(co.x * 0.3e-3 + co.y) * (vec3(8.1e5, 1.0e5, 0.1e5) + vec3(injectionSwitch.x, 0.0, 0.0)));

At runtime, we set injectionSwitch to (0.0, 1.0), so adding vec3(injectionSwitch.x, 0.0, 0.0) should be like adding a vector of zeroes; i.e., it should have no effect. Yet making this change causes a visible difference in the image that is rendered.

[afd]

Reported to ARM here:

https://community.arm.com/message/39861

[afd]

Via email discussions with ARM, we agreed that this was a false alarm due to floating-point issues. We found this shader's variants led to different images on many platforms.