Arena/Segment allocator and zero initialized MemorySegment

[Maurizio Cimadamore]

Hi Remi,
this is a bit of a controversial area, and it is rather easy to end up 
in a slippery slope. Segment allocator is a rather general API, you can 
think of it as a general allocation trait (e.g. all the ways in which a 
segment can be allocated).

As such, allocators don't provide many guarantees. As you noticed, 
there's no guarantee that the memory you get back if you do e.g. 
allocate(100) will be zero-initialized. There's also no guarantee that, 
if you call allocate(100) twice you will get back _different_ memory 
segments (e.g. aliasing). The only think SegmentAllocator promises is 
that when you get back a segment, the segment will be (a) big enough and 
(b) be aligned consistently with what you requested.

I'd say that, for most APIs, this behavior is too general to be useful. 
This is why Arena for example promises that the segments it returns will 
never be aliased (e.g. if you call allocate(100) twice on an arena, you 
will get back two different segments). But Arena still doesn't make any 
promises when it comes to zeroing (e.g. if you call allocate(100) the 
API won't say whether the returned segment will be zero-initialized or 
not). And I don't think it should make that promise - how much zeroing 
an application is willing to live with is something that is ultimately 
application-dependent, so making a choice for all applications seems 
questionable.

Can you describe a bit more what kind of stuff your method will do with 
the allocator? How many segments is it allocating (one or more?), and 
what should happen if, say, the provided allocator doesn't feature the 
required characteristics (e.g. throw, or workaround, e.g. by calling 
allocateFrom with a zeroed array, which will also zero the returned 
segment) ?

Cheers
Maurizio




On 05/06/2024 10:03, Remi Forax wrote:
> Hello,
> for one of my API, I want to take a SegmentAllocator as parameter, and this allocator whould provides zero initialized MemorySegment, but it seems there is no way to know if a SegmentAllocator or an Arena will intialize the MemorySegment with zeroes when allocate() is callled.
>
> I believe what is missing is a default method isZeroInitialized() in SegmentAllocator that returns false by default and true for the 4 global(), ofAuto(), ofConfined() and ofShared() implementations.
>
> regards,
> Rémi
>