Re: [PATCH] Add atomic-box-update! function to (ice-9 atomic)

[Philip McGrath]

On Thu, Jun 22, 2023, at 5:02 AM, Andrew Tropin wrote:
> On 2023-06-22 01:21, Philip McGrath wrote:
>>
>> In any case, the current documentation for
>> atomic-box-compare-and-swap! is clear that the comparison is eq?: it
>> just means that, when the behavior of eq? is unreliable, so is the
>> behavior of atomic-box-compare-and-swap!.
>
> Good.  Than implementation of atomic-box-update! looks correct to me.
> Any thoughts on including it in (ice-9 atomic)?
>
>>
>> Tangentially, does atomic-box-compare-and-swap! handle spurious
>> failures on architectures like ARM, or does it expose machine
>> semantics to the programmer? Maybe that's covered by details of the
>> C11 memory model, but I don't think "sequential consistency" alone is
>> enough to answer the question.
>>

I think your implementation is correct. If atomic-box-compare-and-swap! does 
not handle spurious failures, I think your implementation is still correct, but 
a more efficient implementation could retry without an extra call to the update 
procedure.

Someone with a better sense of Guile performance might have a view about 
whether `apply` is likely to be expensive: alternatively, the update procedure 
could be restricted to a single argument, or `case-lambda` could provide a 
specialized entry-point.

-Philip