LGTM. I thought we already handled the no-SSE case, but looks like only for the 32-bit subtarget.

Superceded by D15567.

LGTM too. Thanks for putting in the legwork since I was busy!

Peter, how do you suggest to go further from here? Do you want to make the needed changes and upload a new patch?

the frame parameter is passed in C as a pointer argument.
However, in the assembly it should be passed by value on the stack rather than a pointer on the stack.
Do we assume that this will be handled by the clang-FE ?

It's worth noting that I opted to implement this as a calling convention because it's relatively easy to support in the Rust compiler, where I really want this feature. While something similar might be useful in a C frontend as well, historically there has been no great need.

The hardware provides only up to one parameter, unless I've missed some major point. I think software interrupt calling conventions would be best left to some other work (if even they're well-defined enough to be worth trying to support).