https://bugs.freedesktop.org/show_bug.cgi?id=104492
Bug ID: 104492 Summary: Compute Shader: Wrong alignment when assigning struct value to structured SSBO Product: Mesa Version: 17.2 Hardware: x86-64 (AMD64) OS: Linux (All) Status: NEW Severity: normal Priority: medium Component: Drivers/Gallium/radeonsi Assignee: dri-devel@lists.freedesktop.org Reporter: florian.will@googlemail.com QA Contact: dri-devel@lists.freedesktop.org
Created attachment 136547 --> https://bugs.freedesktop.org/attachment.cgi?id=136547&action=edit Code to reproduce this issue. Requires SDL2.
Hi,
I'm trying to get the Banshee 3D engine https://github.com/BearishSun/BansheeEngine working on my hardware (HD 7870) using Mesa radeonsi (amdgpu kernel module). I use 17.2.4-0ubuntu1~17.10.1 from the Ubuntu artful-proposed archive.
It uses a compute shader that reads a texture cube and writes 6 sets of coefficients into a shader storage buffer object. Details are not important as I have isolated the (probably) incorrect driver behaviour and created a much simpler program that triggers the same issue.
In short: Let's say we have a GLSL struct and access the SSBO through a dynamically-sized array of that struct type using the std430 layout, like this:
struct ResultRecord { float a[10]; float b[10]; float c[10]; float weight; };
layout(std430) buffer gOutput { ResultRecord ssb[]; };
Then a simple assignment of a local variable of the same struct type to ssb[i] writes the floats into incorrect buffer offsets, because b, c and "weight" are placed as if the array elements in a, b and c were vec4-aligned instead of float-aligned, tripling the size of a, b and c.
Code that triggers it is like this, which should hopefully be valid GLSL (and makes more sense the way it is used in Banshee 3D): ResultRecord result; // ... populate result ... ssb[gl_LocalInvocationIndex] = result;
The test program is available here and contains a (maybe too) detailed explanation and three program variations that fix the issue: https://gist.github.com/w-flo/b1a5791749eea5f36cb54628037ba2bf But I'll also attach it to this bug report.
Looking at the RADEON_DUMP_SHADERS=1 output, I think that the bug is already visible in the TGSI dump, as explained in the comment at the top of my reproducer program. I'll attach the output (it's possibly based on an older version of the reproducer).