Re: MPS: weak hash tables

["Gerd Möllmann" <gerd.moellmann@gmail.com>]

Helmut Eller <eller.helmut@gmail.com> writes:

> On Tue, Jul 02 2024, Pip Cet wrote:
>
>> Also, there's the whole caution thing about weak objects containing
>> only unaligned words or words pointing directly to a base object,
>> which is only relevant on Unix/i386, IIRC. (MPS emulates instructions
>> to simulate fine-grained barriers, which is a really cool idea; I'd
>> still like an option to turn it off though...). That would mean we
>> have to replace Lisp_Objects and use the ptr member of our union (and
>> that's the reason I'm using fixnums rather than plain integers for the
>> hash).
>
> Why do you think that the restriction only applies to 32-bit systems?
> My interpretation of
>
>   Section 7.4. Caution
>   ...
>   “Aligned pointer” means a word whose numeric value (that is, its value
>   when treated as an unsigned integer) is a multiple of the size of a
>   pointer. If you’re using a 64-bit architecture, that means that an
>   aligned pointer is a multiple of 8 and its bottom three bits are zero.
>   ...
>
> is that it applies to 64-bit machines as well.

OTOH, when I see this in a bit broader context, namely

  7.3
  ...
  
  Emulation of accesses to protected objects happens when all of the
  following are true:

  The object is a weak object allocated in an AWL pool.
  
  The MPS is running on Linux/IA-32 or Windows/IA-32. Extending this
  list to new (reasonable) operating systems should be tolerable (for
  example, macOS/IA-32). Extending this to new processor architectures
  requires more work.
  
  The processor instruction that is accessing the object is of a
  suitable simple form. The MPS doesn’t contain an emulator for all
  possible instructions that might access memory, so currently it only
  recognizes and emulates a simple MOV from memory to a register or
  vice-versa.
  
  Contact us if you need emulation of access to weak references for new
  operating systems, processor architectures, or memory access
  instructions.

  7.4. Caution

  Because of the instruction emulation described in Protection faults
  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  above, AWL places the following restriction on the format of objects
  allocated in it:

  Each slot in an object must either be a valid word-aligned reference,
  or else the bottom bits of the word must be non-zero so that it does
  not look like an aligned pointer.

  “Aligned pointer” means...

I'd bet that these restrictions don't matter when emulation is not done,
which is the case for 64 bit processors, not IA32 etc. And indeed, on my
machine with arm64 splatting in the marker vectors works just fine.

I can understand that Ravenbrook documents these restrictions, for
future (payed) developments and so on, but, you know... implementing
them gets pretty ugly pretty quickly. (And I wonder if the emulation
brings enough to warrant the effort.)

It would be nice if the ugliness could be encapsulated so that one
doesn't have to see it all the time, as far as that it possible in C
:-). Or conditionalized, maybe, because with Helmut's idea (which I find
the right one), we're using and additional word for weak references.