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

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