[Bug 111006] Adding a uniform-dependent if-statement in shader renders a different image

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=111006

            Bug ID: 111006
           Summary: Adding a uniform-dependent if-statement in shader
                    renders a different image
           Product: Mesa
           Version: git
          Hardware: x86-64 (AMD64)
                OS: Linux (All)
            Status: NEW
          Severity: normal
          Priority: medium
         Component: glsl-compiler
          Assignee: mesa-dev at lists.freedesktop.org
          Reporter: abelbriggs1 at hotmail.com
        QA Contact: intel-3d-bugs at lists.freedesktop.org

Created attachment 144639
  --> https://bugs.freedesktop.org/attachment.cgi?id=144639&action=edit
Reproduction shader_test files, images and diff

The attached archive contains two shaders, a reference shader and a variant
shader, that by construction should render the same image. On the build and PC
specified below, however, these shaders render different images. Images of the
shaders’ output are also supplied in the archive, as well as the minor diff
between the source code of the two shaders.

The difference between the two shaders is that the variant saves the current
value of GLF_color, then sets it to a random value. Then, depending on a
condition that is always true at runtime, GLF_color is overwritten with its
original value.

11a12,13
> uniform vec2 injectionSwitch;
> 
20,22c22,23
<  float width = 256.0;
<  float c_re = 0.8 * (xCoord - width / 2.0) * 4.0 / width - 0.4;
<  float c_im = 0.8 * (yCoord - 256.0 / 2.0) * 4.0 / width;
---
>  float c_re = 0.8 * (xCoord - 256.0 / 2.0) * 4.0 / 256.0 - 0.4;
>  float c_im = 0.8 * (yCoord - 256.0 / 2.0) * 4.0 / 256.0;
32a34,40
>      vec4 _GLF_outVarBackup_GLF_color;
>      _GLF_outVarBackup_GLF_color = _GLF_color;
>      _GLF_color = vec4(2799.7886, 1762.3462, - 8612.3800, - 7.8);
>      if(((0.0 < injectionSwitch.y)))
>       {
>        _GLF_color = _GLF_outVarBackup_GLF_color;
>       }
79a88
> 
80a90
> uniform vec2 injectionSwitch 0.0 1.0

The value of injectionSwitch is set to (0.0, 1.0), ensuring that the
conditional (0.0 < injectionSwitch.y) is always true at runtime. Notably, if
you remove injectionSwitch and replace the condition with ‘true’, the variant
renders the same image as the reference.

Steps to reproduce:
-------------------------------------------------------------------------------
Obtain and build piglit, the Mesa OpenGL test suite runner:
https://gitlab.freedesktop.org/mesa/piglit
Download the attached archive.
>From a terminal, execute the supplied tests with the piglit GLES3 shader
runner: 
$ bin/shader_runner_gles3 reference.shader_test
$ bin/shader_runner_gles3 variant.shader_test

Expected results:
-------------------------------------------------------------------------------
Both tests should produce an image like reference.png.

Actual results:
-------------------------------------------------------------------------------
The variant produces a different image even though it should run exactly the
same as the reference shader. Even though the variant swaps out GLF_color and
swaps it back in, the two shaders are semantically equivalent, and there should
be no difference in their rendering. Despite this, the variant renders many of
the colored pixels in the image as transparent.

Build & PC specs:
-------------------------------------------------------------------------------
CPU: Intel Core i7-5820k 
GPU: nVIDIA GTX 970

OS: Ubuntu 19.04
libdrm: git-5db0f7692d1fdf05f9f6c0c02ffa5a5f4379c1f3
Mesa: git-d4575c3071 (most recent as of this writing)
Xf86-video-nouveau: 1.0.16
Linux kernel version: 5.0.0-16-generic

This bug was found with GraphicsFuzz: https://github.com/google/graphicsfuzz

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