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Date: Tue, 13 Sep 2016 18:30:00 +0200
To: Nikita Popov <nikita.ppv@gmail.com>,  	Dmitry Stogov <dmitry@zend.com>,  	Xinchen Hui <xinchen.h@zend.com>
Cc: PHP Internals <internals@lists.php.net>
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Message-ID: <20160913163000.D68A15FA84@mx.zeyos.com>
Subject: [PHP-DEV] Directly embed small strings in zvals
From: ben.coutu@zeyos.com (Benjamin Coutu)

Hello everyone,=0A=0AI was wondering if it would make sense to store small =
strings (length <=3D 7) directly inside the zval struct, thereby avoiding t=
he need to extra allocate a zend_string, which would also not entail any co=
stly indirection and refcounting for such strings.=0A=0AThe idea would be t=
o add a new sruct ``struct { uint8_t len; char val[7]; } sval`` to the _zen=
d_value union type in order to embed it directly into the zval struct and u=
se a type flag (zval.u1.v.type_flags) such as IS_SMALL_STRING to destinguis=
h between a regular heap allocated zend_string and the directly embedded co=
mpact representation.=0A=0ASmall strings are quite common IMHO. In fact qui=
ckly sampling my company's PHP code base I found well over 50% of the strin=
gs to be of length <=3D 7. It would save a lot of memory allocations as wel=
l as pointer indirection, and could also bypass refcounting logic. Also, co=
mparing small strings for equality would become a trivial operation (just c=
omparing two pre-aligned 64bit integers) - no more need to keep small strin=
gs interned.=0A=0AOf course it wouldn't longer be possible to also persiste=
ntly store the hash value of a small string, though calculating the hash va=
lue for small strings is less costly anyways because less characters equals=
 less iterations, so that might not be an issue in practice.=0A
I don't see such an idea in https://wiki.php.net/php-7.1-ideas and I was wo=
ndering: Has anybody experimented with that approach yet? Is it worth discu=
ssing?=0A=0APlease let me know your thoughts,=0A=0ABen

--=20

Bejamin Coutu
ben.coutu@zeyos.com

ZeyOS, Inc.
http://www.zeyos.com