LLVM-reserved registers can't be used in inline assembly

[jethrogb]

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

[jethrogb]

At this time, solutions other than “fixing LLVM” seem non-exhaustive or non-trivial. See additional discussion at https://rust-lang.zulipchat.com/#narrow/stream/216763-project-inline-asm/topic/Handling.20clobbers

[bstrie]

Un-reserving registers can be done backwards-compatibly, yes? Presumably this does not block stabilization of asm!.

[Lokathor]

It seems that r6 is also reserved by LLVM on ARM with both T32 and A32.

r7 seems to now be available with A32 (but still not T32).