Re: Summary on what's blocking full Rust support on m68k

[John Paul Adrian Glaubitz <glaubitz%physik.fu-berlin.de@localhost>]

Hi,

On Thu, 2026-04-23 at 09:39 +0200, John Paul Adrian Glaubitz wrote:
> Once the alignment issue has been sorted out, the next issue to address is the lack of the
> M68k backend in LLVM to handle 32-bit offsets for variables and jumps [4]. Currently, the
> backend will always generate 16-bit offsets which is not enough to compile all Rust code,
> although some Rust code compiles also with 16-bit offsets.
> 
> I have created a rough patch to add support for 32-bit offsets with the help of Gemini but
> the patch has multiple problems as outlined in the GitHub issue in [4]. One of the problems
> is that it creates the 32-bit offsets in an inefficient way as if the target was a ColdFire
> CPU. However, the original 680x0 CPUs can actually use more efficient code to address 32-bit
> variables and jumps. The inefficient way would still be used when ColdFire support gets added
> to LLVM though [5].

I have used Google Gemini Code Assist now [1] in order to create a patch to add support for 32-bit
relocations to the LLVM M68k backend. This time, the code makes use of the full capabilities of
68020+ CPUs and does not the inefficient four-instruction method to implement 32-bit accesses.

I'm hesitant to send this in for review as this was basically written by Gemini under my supervision
with a lot of testing of build tests.

Adrian

> [1] https://github.com/llvm/llvm-project/issues/181481#issuecomment-4476933700

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