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Thread-Topic: Packed Strings
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Date: Mon, 27 Nov 2017 23:11:02 +0000
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Subject: Packed Strings
From: dmitry@zend.com (Dmitry Stogov)

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Hi,


I spent some time, reviewing an old Andrea's idea about packed strings.

https://github.com/hikari-no-yume/php-src/tree/packedStrings

The idea is simple. In every place were we use zend_string*, we may store c=
haracters directly.

We use low byte to encode packed string marker and string length, we also n=
eed one byte for trailing zero, so we can keep up to 2-characters on 32-bit=
 system and up to 6 characters on 64-bit without allocation of additional m=
emory.


The refreshed dirty PoC implementation https://github.com/php/php-src/compa=
re/master...dstogov:packedStrings2?expand=3D1

You may take a quick look only into zend_string.h changes (the rest is almo=
st a monkey work).


I was able to run bench.php, and probably won't go forward.

Unfortunately, I got into two serious problems:


1) The original implementation used packed strings their selves as their ha=
sh value. This leaded to huge slowdown, because of hash collisions. (e.g. o=
n bench.php hash1()). I switched to hash recalculation on each usage, but t=
his negates the benefit of allocation elimination. Probably, we may use a c=
heaper hash function for packed strings...


2) PHP still uses char* in many places. When we take ZSTR_VAL() from a pack=
ed string stored in local variable (or function argument), we may very easy=
 get a dangling pointer. (e.g. INI directives processed by OnUpdateString, =
internal functions parameters received as char*, ...). Changing all this ch=
ar* into zend_string* would help, but looks unrealistic for PHP-7.3.


So, I gave up for now.

I decided, to share these results. May be someone would get related ideas.


Thanks. Dmitry.


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