Conferences certainly dominate journals, but most people publish their work on arxiv first, anyway.
I'm in CS (at the intersection of PL/compilers/HPC), and I've never heard of anyone in my field doing that. In fact, the only papers I've read on arxiv have been ones linked on HN.
Virtually every ML paper is posted on arxiv before conferences.
I'm in a similar intersection (hi!), and same goes for me. I want to change that, though. I have started publishing tech reports (I work in an industry research lab) whenever I submit a paper for review. I'm tired of work being stuck in endless review cycles, not public and not referenceable. Were I still in academia, I would submit to arxiv, and I have even recommended this to grad students.