json: Improve performance of php_json_encode_array()

[TimWolla]

Instead of using a boolean flag to check for each element whether or not it is the first, we just unconditionally append a comma after each element and then remove the last comma at the end.

For:

<?php

$len = 0;
for ($i = 0; $i < 3_000_000; $i++) {
	$len += strlen(json_encode(array_fill(0, 20, [])));
}
var_dump($len);

This is ~1.06 faster for a gcc 13.3 release build on a Intel(R) Core(TM) i7-1365U.

Benchmark 1: /tmp/bench/before /tmp/bench/test6.php
  Time (mean ± σ):     819.6 ms ±   2.8 ms    [User: 816.4 ms, System: 2.4 ms]
  Range (min … max):   816.9 ms … 825.0 ms    10 runs

Benchmark 2: /tmp/bench/after /tmp/bench/test6.php
  Time (mean ± σ):     770.8 ms ±   5.8 ms    [User: 766.6 ms, System: 2.9 ms]
  Range (min … max):   765.3 ms … 785.8 ms    10 runs

Summary
  /tmp/bench/after /tmp/bench/test6.php ran
    1.06 ± 0.01 times faster than /tmp/bench/before /tmp/bench/test6.php

[nielsdos]

Tested on a much older CPU (i7-4790):

Benchmark 1: ./sapi/cli/php x.php
  Time (mean ± σ):      1.119 s ±  0.004 s    [User: 1.113 s, System: 0.003 s]
  Range (min … max):    1.112 s …  1.126 s    10 runs
 
Benchmark 2: ./sapi/cli/php_old x.php
  Time (mean ± σ):      1.100 s ±  0.007 s    [User: 1.095 s, System: 0.002 s]
  Range (min … max):    1.092 s …  1.115 s    10 runs
 
Summary
  ./sapi/cli/php_old x.php ran
    1.02 ± 0.01 times faster than ./sapi/cli/php x.php

Still seems worthwhile as the difference is negligible and my system has a noticeably slower memory subsystem which likely contributes to this result.

[nielsdos]

Anyway I looked at this and this seems legit. Nice one.