DEC Alpha support is somehow still in the mainline Linux kernel...
Take, for example, an 8-bit/byte store. Without BWX the sequence would be something like:
bic a0, #3, t1
and a0, #3, t4
ldl t2, (t1)
insbl a1, t4, t3
mskbl t2, t4, t2
bis t2, t3, t2
stl t2, (t1)
ret zero, (ra)
stb a1, 0(a0)
ret zero, (ra)People use and maintain them and they have very little impact outside arch/ nowadays so they're on the happy side of cost/benefit I guess.
I'm not sure if Alphas are even being made anymore, even 15-20 years ago.
Alpha's memory model has problems with providing atomic access to single bytes, which i'd imagine in a kernel is a bit annoying :-)
And then there's just the social aspect, m68k was used in the Amiga/Atari/Mac/QL/x68k, so there is a whole generation of us m68k fans who are willing to keep it alive.
Alpha has it's fans (me included!), but it's not exactly the same. So in a way it's no surprise it's slowly bitrotting away.
That required this smp_read_barrier_depends() through the kernel, but actually in recent years that has basically been subsumed by other concurrent access primitives that all the core kernel must use, so I think alpha is no longer much of a problem outside arch/alpha