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Thread-Topic: Q: Caching IS_REFERENCE zvals in PHP7?
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Date: Wed, 12 Aug 2015 21:16:03 +0000
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Subject: Q: Caching IS_REFERENCE zvals in PHP7?
From: ericsten@microsoft.com (Eric Stenson)

Good Morning, PHP Internals Folks!

I have a design question about caching of IS_REFERENCE zval values, and wan=
t to understand the most correct way to implement a memory Cache for zvals =
in PHP7.

Background:
-----------

WinCache's user cache does a 'deep copy' of zvals into a shared memory segm=
ent that can start at different offsets in the processes which share the me=
mory segment.  This means that all internal pointers must be converted into=
 offsets relative to the starting address of the shared memory segment.  Fu=
rther, flags are altered such that the cached zval is considered immutable =
and not ref counted.

So, for example, with an IS_STRING zval, a zval in shared memory is created=
 (zv_cached), the zend_string is cached, and then the Z_STR_P(zv_cached) is=
 set to the offset of the cached zend_string. =20

When the user fetches the cached IS_STRING zval, the reverse is performed: =
a zval is created in request-temporary memory (zv_fetched), and initialized=
 from zv_cached.  Z_STR_P(zv_fetched) is translated back into a pointer (to=
 the cached zend_string) which is correct in the current process.  The call=
er won't attempt to free or modify the cached zend_string since the type fl=
ags were modified when it was cached.

For IS_REFERENCE objects, I'm not clear on what I should do. Should I cache=
 the zend_reference struct, with its included zval?  Or should I "flatten" =
the IS_REFERENCE, and just convert it to the zval that's in the zend_refere=
nce struct?

Consider the following:

<?php

$bar  =3D "BAR";
$arr2 =3D array('black' =3D> &$bar, 'black2' =3D> &$bar );
wincache_ucache_add('foo', $arr2);
$fetched_arr =3D wincache_ucache_get('foo');

?>

The $fetched_arr won't have the internal references between elements (e.g. =
$fetched_arr['black'] and $fetched_arr['black2'] won't point at the same ac=
tual value).

Questions:
----------

* How should one persist/intern an IS_REFERENCE zval?
* Is it acceptable to "flatten" IS_REFERENCE zvals when persisting/internin=
g them?  If not, what's the reason for not doing so?
* If IS_REFERENCE zvals have to be maintained, does that mean that the zend=
_reference will need to be copied out of the shared memory cache each time =
it's fetched?
* Is there an example of a PHP7 extension which is doing something similar?=
  APC doesn't appear to have been updated for PHP7.

Thx!
    --E.



=20