Optimize in-memory XMLWriter

[nielsdos]

We're currently using a libxml buffer, which requires copying the buffer to zend_strings every time we want to output the string. Furthermore, its use of the system allocator instead of ZendMM makes it not count towards the memory_limit and hinders performance.

This patch adds a custom writer such that the strings are written to a smart_str instance, using ZendMM for improved performance, and giving the ability to not copy the string in the common case where flush has empty set to true.

[Girgias]

Looks sensible

[staabm]

This patch adds a custom writer such that the strings are written to a smart_str instance, using ZendMM for improved performance

do we have an idea how much faster this is?

[nielsdos]

This patch adds a custom writer such that the strings are written to a smart_str instance, using ZendMM for improved performance

do we have an idea how much faster this is?

It highly depends on the workload.
On a simple test like this:

<?php

$writer = XMLWriter::toMemory();
var_dump($writer);

for ($i = 0; $i < 10000; $i++) {
    xmlwriter_start_element($writer, 'foo');
    xmlwriter_write_cdata($writer, 'some cdata');
    xmlwriter_end_element($writer);
}

for ($i = 0; $i < 10000; $i++)
    $writer->flush(false);

I get:

Benchmark 1: ./sapi/cli/php x.php
  Time (mean ± σ):     148.7 ms ±   3.2 ms    [User: 144.6 ms, System: 3.8 ms]
  Range (min … max):   144.3 ms … 156.5 ms    19 runs
 
Benchmark 2: ./sapi/cli/php_old x.php
  Time (mean ± σ):     212.8 ms ±   4.5 ms    [User: 207.7 ms, System: 4.6 ms]
  Range (min … max):   203.8 ms … 220.1 ms    14 runs
 
Summary
  ./sapi/cli/php x.php ran
    1.43 ± 0.04 times faster than ./sapi/cli/php_old x.php

Further speed improvements likely possible by switching to fast ZPP.