There's only one question for machine-language golfing, and it's for the ia32 and amd64 architectures. Here's one for a predecessor of them: the Zilog Z80!
The ISA is available here.
As usual, if your tip is only applicable to a certain environment/calling convention, please indicate that in your answer.
Only one tip per answer (see here).
You can also offer tips for the closely related Sharp SM83. The ISA for that is available here.1
Please mark your tips as to whether they're applicable to Z80, SM83, or both; the ISAs might be similar, but there are many differences.
- It's labeled as the LR35902; that's the SiC that was used in the DMG and CGB. It had an SM83 core.