Re: __atomic_test_and_set() and mips o32 - help wanted

[Jason Thorpe <thorpej%me.com@localhost>]

+ port-alpha

> On Nov 18, 2025, at 10:00 PM, Maciej W. Rozycki <macro%orcam.me.uk@localhost> wrote:
> 
> On Tue, 18 Nov 2025, Mouse wrote:
> 
>>> Just trap and emulate LL/SC in the kernel for user software;
>> 
>> I'm not sure the smartest option is to place the emulation load most on
>> the machines with the least computational resources to spare.
> 
> Well, you need to trap into the kernel anyway to emulate the operation 
> required, so you might as well make it benefit hardware such as MIPS III, 
> which (as David has also observed) is very much legacy now too, having 
> reached 30+ years of age.  And with the alternative syscall approach all 
> the parties lose, as even hardware that has atomics support available has 
> to trap.

There are actually other ways to do this.  This is a situation where an ifunc would be beneficial, or some other run-time fix-up to the correct implementation.  A restartable atomic sequence is incredibly cheap.

> I have also saved the VAX GCC backend from being dropped and now non-BWX 
> (pre-EV56) Alpha support is at risk as the old register allocator is being 
> removed right away, along with all the backends that still rely on it, and 
> sadly I've been running out of resources to get that sorted, so you are 
> welcome (as is anyone) to step in and assist.

No everyone knows about the impending dooms, alas.  It’s a real shame that non-BWX support is on the chopping block (aside: how on earth can that have such a huge impact on the register allocator???) … there are quite a few non-BWX machines still out there (at least at chez moi, there are 3 + a parts machine), and for someone wanting to dabble in Alpha hardware, the BWX-supporting systems fetch top dollar on the used market (compared to merely more-than-I-should-spend for an EV4-class system) - people still gaga to run VMS I guess - making the non-BWX systems a lot more accessible.

Emulation of BWX would be ***incredibly*** slow.  Like, slow enough that pre-BWX systems would be unusable, I’m afraid.

I’m not in a position to directly contribute to the typing-of-code effort to help save the pre-BWX Alpha back-end, but what else can I (and others) do to help?

-- thorpej