Comment # 16
on bug 93594
from Nicolai Hähnle
Interesting information about D3D10, thank you. The GLSL 4.50 spec says: "The discard keyword is only allowed within fragment shaders. It can be used within a fragment shader to abandon the operation on the current fragment. This keyword causes the fragment to be discarded and no updates to any buffers will occur. Control flow exits the shader, and subsequent implicit or explicit derivatives are undefined when this exit is non-uniform." One annoying aspect of this language is that one can reasonably read it as non-uniformity only being relevant for non-helper fragments. If a pixel quad is partial covered by the original primitive, and discard is used in a way that keeps the covered pixels but discard the helper ones, should derivatives be defined or not? As Michel said, I am indeed currently working on a patch changing exec mask behavior in LLVM for stores and atomics in pixel shaders. While what I have so far does not fix this bug yet, it already requires switching back and forth between WQM and non-WQM/"exact" modes. Extending this to keep full quads alive after KILL_IF should not add much more overhead.
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