Re: Remove last traces of HPPA support

On Tue, Jul 2, 2024 at 5:56 AM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Done at commit edadeb0710.

Here are some experimental patches to try out some ideas mentioned
upthread, that are approximately unlocked by that cleanup.

1. We could get rid of --disable-spinlocks. It is hard to imagine a
hypothetical new port that would actually be a useful place to run
PostgreSQL where you can't implement spinlocks. (This one isn't
exactly unlocked by PA-RISC's departure, it's just tangled up with the
relevant cruft.)

2. We could get rid of --disable-atomics, and require at least 32 bit
lock-free (non-emulated) atomics. AFAIK there are no relevant systems
that don't have them. Hypothetical new systems would be unlikely to
omit them, unless they are eg embedded systems that don't intend to be
able to run an OS.

Personally I would like to do this, because I'd like to be able to use
pg_atomic_fetch_or_u32() in a SIGALRM handler in my
latchify-all-the-things patch (a stepping stone in the multi-threading
project as discussed in the Vancouver unconference). That's not
allowed if it might be a locking fallback. It's not strictly
necessary for my project, and I could find another way if I have to,
but when contemplating doing extra work to support imaginary computers
that I don't truly believe in... and since this general direction was
suggested already, both on this thread and in the comments in the
tree...

Once you decide to do #2, ie require atomics, perhaps you could also
implement spinlocks with them, rendering point #1 moot, and delete all
that hand-rolled TAS stuff. (Then you'd have spinlocks implemented
with flag/u32 atomics APIs, but potentially also u64 atomics
implemented with spinlocks! Circular, but not impossible AFAICT.
Assuming we can't require 64 bit lock-free atomics any time soon that
is, not considered). 🤯🤯🤯But maybe there are still good reasons to
have hand-rolled specialisations in some cases? I have not researched
that idea and eg compared the generated instructions... I do
appreciate that that code reflects a lot of accumulated wisdom and
experience that I don't claim to possess, and this bit is vapourware
anyway.

3. While tinkering with the above, and contemplating contact with
hypothetical future systems and even existing oddball systems, it
practically suggests itself that we could allow <stdatomic.h> as a way
of providing atomics (port/atomics.h clearly anticipated that, it was
just too soon). Note: that's different from requiring C11, but it
means that the new rule would be that your system should have *either*
C11 <stdatomic.h> or a hand-rolled implementation in port/atomics/*.h.
This is not a proposal, just an early stage experiment to test the
waters!

Some early thoughts about that, not fully explored:
* Since C11 uses funky generics, perhaps we might want to add some
type checks to make sure you don't accidentally confuse u32 and u64
somewhere.
* I couldn't immediately see how to use the standard atomic_flag for
our stuff due to lack of relaxed load, so it's falling back to the
generic u32 implementation (a small waste of space). atomic_bool or
atomic_char should work fine though, not tried. I guess
pg_atomic_flag might be a minor historical mistake, assuming it was
supposed to be just like the standard type of the same name. Or maybe
I'm missing something.
* The pg_spin_delay_impl() part definitely still needs hand-rolled
magic still when using <stdatomic.h> (I'm not aware of any standard
way to do that). But I'm not sure it even belongs in the "atomics"
headers anyway? It's not the same kind of thing, is it?
* The comments seem to imply that we need to tell the compiler not to
generate any code for read/write barriers on TSO systems (compiler
barrier only), but AFAICS the right thing happens anyway when coded as
standard acquire/release barriers. x86: nothing. ARM: something.
What am I missing?
* It'd be interesting to learn about anything else that modern tool
chains might do worse than our hand-rolled wisdom.
* Special support for Sun's compiler could be dropped if we could just
use their <stdatomic.h>. The same applies for MSVC 2022+ AFAICS, so
maybe in ~3 years from now we could drop the Windows-specific code.
* Uhh, yeah, so that would also apply to any modern GCC/Clang, so in
effect everyone would be using <stdatomic.h> except any hand-rolled
special bits that we decide to keep for performance reasons, and the
rest would become dead code and liable for garbage collection. So
that would amount to a confusing policy like: "we require
<stdatomic.h> with at least lock-free int in practice, but we'd
consider patches to add a non-C11-way to do this stuff if you invent a
new kind of computer/toolchain and refuse to support C11". Hmm. (I
have another version of this type of thinking happening in another
pending patch, the pg_threads.h one, more on that shortly...)