[clang] [llvm] [MIPS] Add support for O64 Mips ABI (PR #223990)

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LunaTheFoxgirl wrote:

> I believe O64 ABI is a GCC extention that is dropped long ago? As far as I know Nintendo64 is actually using 32-bit C calling convention (i.e. O32), which uses 32-bit argument words.
> 
> SGI never implement O64 with their MipsPro, could you please elaborate a bit on your use case in homebrew?
> 
> Maybe what you actually want is `-mgp64` with `o32` ABI?

Specifically it is the homebrew SDKs which use o64, such as libdragon and co. This is a major blocker for homebrew game dev on the N64 w/ LLVM.

https://github.com/llvm/llvm-project/pull/223990