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To: Aran Reeks <cdtreeks@gmail.com>, Mike Schinkel <mike@newclarity.net>,
	Nikita Popov <nikita.ppv@gmail.com>
CC: PHP internals <internals@lists.php.net>
Thread-Topic: [PHP-DEV] Typed array properties V2
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Date: Fri, 17 Jan 2020 23:19:37 +0000
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Subject: Re: [PHP-DEV] Typed array properties V2
From: tysonandre775@hotmail.com (tyson andre)

> I'm unsure of exactly how this might work so defer to an Internals export=
,=0A=
> but having previously read @Nikita Popov <nikita.ppv@gmail.com>'s great=
=0A=
> post on PHP's arrays, I did wonder if by knowing the data type within an=
=0A=
> array and that it'd conform to a strict structure, could the array itself=
=0A=
> be stored in an alternative way in C? Perhaps a more memory efficient way=
,=0A=
> or one that's faster to iterate over rather than just a hash table?=0A=
=0A=
> Link to this article for reference:=0A=
> https://nikic.github.io/2012/03/28/Understanding-PHPs-internal-array-impl=
ementation.html=0A=
=0A=
A significant improvement that could be made to memory efficiency is to red=
uce the minimum array size for *dynamically* created arrays.=0A=
Currently, it's always 8 and must be a power of 2, even if there's only one=
 element or the array is packed (i.e. a list without gaps)=0A=
- Array constants cached in opcache already have this optimization.=0A=
=0A=
I have a PR that reduces the minimum size to 2 - the largest blockers were =
getting a realistic idea of whether commonly used applications=0A=
would see a decrease or increase in runtime, and needing code review.=0A=
("I'd want to know how this would be benchmarked before adding more changes=
, in case additional optimizations somehow turn out to be a performance reg=
ression.")=0A=
=0A=
> For example, Phan's memory usage for self-analysis (./phan --print-memory=
-usage-summary)=0A=
> went from 444MB/576MB to 387MB/475MB (end memory/max memory) with this pa=
tch.=0A=
> (This is a static analyzer for PHP which heavily uses small arrays)=0A=
=0A=
----=0A=
=0A=
> I did wonder if by knowing the data type within an=0A=
> array and that it'd conform to a strict structure=0A=
> could the array itself be stored in an alternative way in C?=0A=
=0A=
It might theoretically be possible to optimize the memory of an `int[]` (64=
 bits per element) without references or gaps in elements.=0A=
If a reference in an element got added, or it stopped being a packed array =
without gaps (i.e. a list),=0A=
then such an implementation would use an unoptimized version that also enfo=
rces the type constraint=0A=
(128 bits per list element with zvals).=0A=
It would probably require changing a lot of APIs and macros, and I'd have n=
o idea how to implement it.=0A=
=0A=
I think JavaScript engines do something similar by having specializations f=
or JS arrays of integers,=0A=
arrays of floats, arrays of small (e.g. 16-bit) integers, etc.=