[Bug 100303] Adding a single, meaningless if-else to a shader source leads to different image

[bugzilla-daemon@freedesktop.org]

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https://bugs.freedesktop.org/show_bug.cgi?id=100303

            Bug ID: 100303
           Summary: Adding a single, meaningless if-else to a shader
                    source leads to different image
           Product: Mesa
           Version: git
          Hardware: x86-64 (AMD64)
                OS: Linux (All)
            Status: NEW
          Severity: normal
          Priority: medium
         Component: Drivers/Gallium/radeonsi
          Assignee: dri-devel@lists.freedesktop.org
          Reporter: h.evrard@imperial.ac.uk
        QA Contact: dri-devel@lists.freedesktop.org

Created attachment 130352
  --> https://bugs.freedesktop.org/attachment.cgi?id=130352&action=edit
Zip archive with files to reproduce

See: https://github.com/mc-imperial/shader-compiler-bugs/issues/62

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

0. This bug seems to affect AMD GPU (tested on Tonga, see below for
   more config details)

1. Obtain and build the latest release of get-image, a simple tool that
   creates a .png image from a fragment shader
   https://github.com/mc-imperial/get-image

2. From a terminal, execute:

   /path/to/get_image/linux_out/install/bin/get_image original.frag

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

3. From a terminal, execute:

   /path/to/get_image/linux_out/install/bin/get_image variant.frag

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

Expected Results:
-------------------------------

Both shaders should render an image that looks like original.png. This
is because the only difference between the shader source files is the
addition of an if-else whose if block is empty and else block contains
the original code. We control the condition value to make sure it is
always false at runtime.

Diff:
```
27a28,29
> uniform vec2 injectionSwitch;
>
117a120,124
>     if(injectionSwitch.x > injectionSwitch.y)
>         {
>         }
>     else
>         {
129a137
>         }
```

The injectionSwitch uniform is always set to { 0.0, 1.0 }. The else
block contains the original code, untouched.

Actual Results:
-------------------------------
The shaders render different images, which they should not: the shadow
in the lower part of the ball disappears in the variant.

Additional Information:
-------------------------------
Some info on the system where the bug was found:

- CPU: AMD A10-7850K Radeon R7, 12 Compute Cores 4C+8G
- GPU: Advanced Micro Devices, Inc. [AMD/ATI] Tonga PRO [Radeon R9 285/380]
- Mesa: 17.1.0-devel (git-08df015)
- OS: Ubuntu 16.04, Linux 4.4.0-67-generic

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