Adding zero vector causes major change to image

[Alastair_D_1]

We think we've found a GLSL shader compiler bug.

This issue describes the problem in full and links to the relevant files:

https://github.com/mc-imperial/shader-compiler-bugs/issues/41

System Setup Information:
-----------------------------------------

System Used: Dell XPS
CPU SKU: i7
GPU SKU: HD 520
Processor Line: U-Series
System BIOS Version: Dell Inc. 1.0.0, 10/09/2015
CMOS settings: I can check on request
Graphics Driver Version: 21.20.16.4542
GOP/VBIOS Version: Intel Video BIOS
Operating System: Windows 10
OS Version: Pro
API: OpenGL
Occurs on non-Intel GPUs?: Haven't tried

Steps to Reproduce:
-------------------------------

1. Obtain the latest release of get-image, a simple tool that creates
   a .png image from a fragment shader (alternatively, use some other
   method observing the effect of a fragment shader)

2. From a command prompt, execute:

   C:\path\to\get_image\bin\Windows\get_image.exe recipient_reduced_final.frag

   This will create output.png, which should look like expected.png

3. From a command prompt, execute:

   C:\path\to\get_image\bin\Windows\get_image.exe variant_52_reduced_29_simplified.frag

   This will create output.png, which should look like actual.png

Expected Results:
-------------------------------
Both shaders should render an image that looks like expected.png.  This is because the only difference between the .frag files is:

- Statement

    return ObjUnion(obj0(p), obj1(p));

  has been replaced with

    return ObjUnion(obj0(p + vec3(0.0, injectionSwitch.x, 0.0)), obj1(p));

  in variant_52_reduced_29_simplified.frag

  Because injectionSwitch.x is set to 0.0, this should not have an effect.

Actual Results:
-------------------------------
The shaders render completely different images, which they should not.

Additional Information:
-------------------------------
N/A

[Michael_C_Intel2]

Hi Alastair,

Thanks, I reported the issue and and will see what the driver team finds. 

-Michael