On several architectures, some registers are considered reserved by LLVM. These registers effectively can't be used by LLVM's inline assembly instruction as in input/output/clobber.

rustc currently disallows using these registers in asm!. This seems to be in contradiction with a stated goal in the RFC:

The intent is that support for asm! should be independent of the rustc back-end used

As such, I'd expect there to be no backend-specific limitations on inline assembly.

Current LLVM-reserved registers are:

• x86: esi
• x86-64: rbx
• ARM: r7