[llvm-dev] [RFC] BasicAA considers address spaces?

escha via llvm-dev llvm-dev at lists.llvm.org
Sun Aug 9 05:46:26 PDT 2015

Personally I feel the most intuitive approach would be to have an equivalent of isTriviallyDisjoint for IR; we already have a model for how it would work, and it could be a TTI call. I’ve kind of wanted this for a while because there’s a lot of address-space-esque aliasing relationships that can’t be easily modeled on the IR level.

For example (in our model), we have some constraints like this:

Global memory can’t alias local memory.
Global writeable memory can’t alias global readonly memory (different address spaces).
Stack memory can’t alias global memory (different address spaces).
Texture reads cannot alias texture writes, because you can’t bind a texture as readable and writeable at the same time. Texture writes, however, can alias each other.
Vertex shader outputs can’t really alias each other, even though they are technically “stores”.
(there’s more where that came from)

These are all very trivial to express in code (the trivially disjoint function in our backend is like 50 lines of code to cover all the cases), but a few of them are slightly more complex than “address space A can’t alias B”, so having a generic callback might be nicer and more powerful than a “does address space A alias address space B” callback, I think.


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