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Summary: Due to the fact that kill is just a normal intrinsic, even though it's supposed to terminate the thread, we can end up with provably infinite loops that are actually supposed to end successfully. The AMDGPUUnifyDivergentExitNodes pass breaks up these loops, but because there's no obvious place to make the loop branch to, it just makes it return immediately, which skips the exports that are supposed to happen at the end and hangs the GPU if all the threads end up being killed. While it would be nice if the fact that kill terminates the thread were modeled in the IR, I think that the structurizer as-is would make a mess if we did that when the kill is inside control flow. For now, we just add a null export at the end to make sure that it always exports something, which fixes the immediate problem without penalizing the more common case. This means that we sometimes do two "done" exports when only some of the threads enter the discard loop, but from tests the hardware seems ok with that. This fixes dEQP-VK.graphicsfuzz.while-inside-switch with radv. Reviewers: arsenm, nhaehnle Subscribers: kzhuravl, jvesely, wdng, yaxunl, dstuttard, tpr, t-tye, hiraditya, llvm-commits Tags: #llvm Differential Revision: https://reviews.llvm.org/D70781
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