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2018 Dec 19
5
[RFC] Adding thread group semantics to LangRef (motivated by GPUs)
...e key issue is that *the set of threads participating in the exchange of data is implicitly defined by control flow*. Two examples to demonstrate the resulting problem and the limitation of the existing LLVM IR semantics. The first one: bool value = ...; if (condition) { bitmask0 = ballot(value); foo(bitmask0); } else { bitmask1 = ballot(value); bar(bitmask1); } The semantics of high-level languages demand that `bitmask0` only contains set bits for threads (lanes) for which `condition` is true, and analogously for `bitmask1`. Howe...