Tipster @UniverseIce claims that only Qualcomm's Snapdragon 875 will be utilizing ARM's high-performance X1 cores this generation. This is bad news for Samsung fans, as it means that the Exynos 2100 might fall behind considerably in single-threaded performance.
https://www.notebookcheck.net/The-Qualcomm-Snapdragon-875-might-have-a-secret-weapon-against-the-Samung-Exynos-2100-ARM-s-latest-X1-cores.496860.0.html