This is spec text based on my previous uniformity analysis for WSL, you can see the previous version at:https://lists.w3.org/Archives/Public/public-gpu/2019Jul/0000.html (and the detailed version in the doc at the bottom of that email). The main difference is adding support for "discard", which somewhat complicates things, but still slots reasonably well into the overall framework.

The analysis currently does not support pointers (since it is a part of the spec that is still in flux). When we do add pointers, every read of a pointer will be considered non-uniform, and every taking of the adress of a variable will make that variable non-uniform (improvable later if it proves crippling, but it would be fairly complex).

It has a couple of other limitations:

• It cannot infer the uniformity of global variables, so it currently assumes that none of them is uniform. We can easily improve it by adding an annotation on global variables, declaring them as uniform. In that case, we would check that every store to them is uniform (preserving soundness), and consider every read of them to be uniform.
• It does not split the lifetimes of variables. What I mean by that, is that if there is any non-uniform store to a local variable, then every read of that variable will be considered non-uniform as well, even those that happen before the store. Correcting for that in the context of loops would be extremely painful, and I don't consider this limitation particularly major (in any case where it bites the programmer, they may easily split the variable into two).

There are still a couple of TODOs in the uniformity section of the spec:

• One about the treatment of infinite loops, I will do more research on that
• One about double-checking the rules for loop. I will test them on a bunch of examples in the next few days as I add tests to the test suite.
• One about the list of functions in the standard library that have interesting uniformity requirements. This one should be straightforward to fill.

I am very interested in getting feedback on this PR, both on the analysis itself and on its presentation. I've tried to explain it as clearly as I could, but I realize that I probably failed in quite a few places; I've just spent long enough working on this that I don't know which parts are hard to understand.

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