There are a few recommended ways to clear a register, all of which have been stated above: For GPRs: XR/XGR - designating the same source register SR, SGR - designating the same source register LHI/LGHI - Can load constants 0-7 on a fast path
For FPRs: LZER/LZDR For VRs: VGBM/VZERO - VGBM with a 0 immediate field. This information can be found in the IBM Z / LinuxOne Optimization Primer here: https://community.ibm.com/HigherLogic/System/DownloadDocumentFile.ashx?DocumentFileKey=d1cdb394-0159-464c-92a3-3f74f8c545c4
