On 5/10/15 12:52 AM, Marco Leise wrote:
* "zeroesAllocations"
I called it "elementsAreInited" and as the name suggests, it
tells whether new elements receive their T.init
automatically.
Now std.allocator deals mostly with raw memory, so zeroing
is the only option, but I can see some friction coming up
when typed allocators are built on those.
Typed allocators backed by a "zeroesAllocations"
allocator could overwrite the memory 3 times: zeroing,
T.init, ctor assignments.
a) Could the parent allocator be informed that it does not
need to zero initialize because we will blit T.init over
the memory anyways?
b) If we get zeroed memory, can the typed allocator know at
compile-time that T.init is all zeroes and elide the blit?
Yah, about that... I introduced this property as an OS interface
courtesy - there are functions that already zero memory so it's wasteful
to zero it again if that's all that's needed.
Sadly I found little if any interesting properties of this, and for
anything that's not zero there's no gain - either the allocator does it
or emplace() etc. does.
Long story short is I'm considering just dropping that.
* "roundingFunction"
This is like a growth function, right? I often have some
default lying around, but it is sure good to be able to
provide your own. My default is: n' = n + (n >> 1) + 4
One thing that could go wrong is that on 32-bit you pick a
new size like n'=2*n and you just don't find enough
contiguous virtual memory.
a) On allocation failure the growth function is ignored and
the minimal required size used.
b) The growth function needs to be designed more carefully,
putting the effort on the user.
This is not growth. Growth is "I have an array of size n and plan to
append an unspecified number of elements to it, what's a good function
that takes n and gives me a larger number?"
Quantization is different. What's happening here is you're reducing a
large set of integer to a smaller number of quantas. That way you are
overallocating (so indeed growth properties are nicer) but also reducing
the total set of distinct sizes that the allocator needs to deal with.
Andrei
