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How often does branchless programming actually matter?
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Yes. GPUs don't have per-core branching, they have dozens of cores running the same instructions. So if some cores should run the if branch and some run the else branch, all cores in the group will execute both branches, and mask out the one they shouldn't have run. I also think they they don't have the advanced branch prediction CPUs have.
https://en.wikipedia.org/wiki/Single_instruction,_multiple_threads
Makes sense. The most programming I've ever done for a GPU was a few simple shaders for a toy project.