Sony is ahead of most other OEMs when it comes to its support of open source. It contributes significantly to AOSP and even releases binaries for many of its devices so developers can build AOSP ROMs for them. Today, Sony is announcing support for the first three 64-bit devices in the Open Device project.
The new devices are the Xperia Z3+, Xperia Z4 Tablet, and Xperia Z4 Tablet WiFi. Technically, we're talking about a phone and two versions of the same tablet, but it's nice they're supporting the mobile data version specifically. All three devices are powered by the 64-bit octa-core Snapdragon 810. Note, Sony isn't making full ROMs available, just the drivers that make the hardware work with AOSP code.
If you're of the developer persuasion, you can get the binaries and follow the instructions to integrate them with an AOSP build. You may notice there's also a GitHub for the 64-bit Xperia M4 Aqua, but that phone can't be unlocked yet, so it's not officially supported at this time.