On 13-01-2023 12:32, Ludovic Courtès wrote:
>> IIUC, if you use bytevector-slice iteratively, say:
>>
>> (let loop ((bv some-initial-value)
>>             (n large-number))
>>    (if (> n 0)
>>        (loop (bytevector-slice bv 0 (bytevector-length bv))
>>              (- n 1))
>>        bv))
>>
>> you will end up with a bytevector containing a reference to a
>> bytevector containing a reference to ... containing a reference to the
>> original reference, in a chain of length ≃ large-number.
> 
> The ‘parent’ word is here just so the backing memory isn’t reclaimed
> while the slice is still alive.
> 
> Whether there’s a long chain of ‘parent’ links or not makes no
> difference to GC performance nor to memory usage.

This is false, the opposite holds: memory usage is at least linear in 
the length of the chain, because the objects in the chain are pairwise 
non-eq?: for a chain (O_1, O_2, ..., O_n) that is alive, each object O_i 
in the chain is kept alive (because that's what the 'parent' link is 
for).  Because bytevector-slice does a fresh allocation, there then are 
n different objects.  Hence, memory usage is at least 
'SCM_BYTEVECTOR_HEADER_SIZE * sizeof(scm_t_bits) * n'.

Greetings,
Maxime.