Add a 1-char fastpath to implode()

[alexandre-daubois]

I think it's worth adding a 1 char fast path for implode(). A quick search on Github reveals the following stats:

• Implode usage with one char glue: /implode\(\s*["'][^"']{1}["']\s*,\s*/ language:PHP reveals 1.4 million occurrences
• Implode usage with 2+ char glue: /implode\(\s*["'][^"']{2,}["']\s*,\s*/ language:PHP reveals 1.1 million occurrences

Here is the benchmark code:

<?php

$iterations = 1000;
$array = array_fill(0, 10000, 'item');

for ($i = 0; $i < $iterations; $i++) {
    implode(',', $array);
}

And the results:

Benchmark 1: ./sapi/cli/php.branch bench.php
  Time (mean ± σ):      33.7 ms ±   1.2 ms    [User: 30.8 ms, System: 2.1 ms]
  Range (min … max):    32.7 ms …  41.4 ms    82 runs
 
Benchmark 2: ./sapi/cli/php.master bench.php
  Time (mean ± σ):      45.1 ms ±   6.7 ms    [User: 41.4 ms, System: 2.2 ms]
  Range (min … max):    42.9 ms …  97.5 ms    65 runs

Summary
  ./sapi/cli/php.branch bench.php ran
    1.34 ± 0.21 times faster than ./sapi/cli/php.master bench.php

No change for multi char glue, but a nice improved performance for single char glue especially one big arrays.

[nielsdos]

1-char bench on an i7-4790:

Benchmark 1: ./sapi/cli/php x.php 
  Time (mean ± σ):      47.9 ms ±   0.7 ms    [User: 45.5 ms, System: 2.0 ms]
  Range (min … max):    46.5 ms …  49.3 ms    60 runs
 
Benchmark 2: ./sapi/cli/php_old x.php
  Time (mean ± σ):      69.8 ms ±   1.1 ms    [User: 67.7 ms, System: 1.8 ms]
  Range (min … max):    67.4 ms …  73.0 ms    42 runs
 
Summary
  ./sapi/cli/php x.php  ran
    1.46 ± 0.03 times faster than ./sapi/cli/php_old x.php

Curiously a large difference with your measurement, did you benchmark a release build?

2-char glue is slightly slower:

Benchmark 1: ./sapi/cli/php x.php 
  Time (mean ± σ):      72.5 ms ±   1.9 ms    [User: 70.2 ms, System: 1.9 ms]
  Range (min … max):    70.2 ms …  82.4 ms    40 runs
 
Benchmark 2: ./sapi/cli/php_old x.php
  Time (mean ± σ):      66.4 ms ±   0.8 ms    [User: 63.9 ms, System: 2.0 ms]
  Range (min … max):    64.5 ms …  68.9 ms    45 runs
 
Summary
  ./sapi/cli/php_old x.php ran
    1.09 ± 0.03 times faster than ./sapi/cli/php x.php 

[alexandre-daubois]

I checked again and --enable-debug was used in configure. Removing it reveals results very close to yours. 2-char glue results on about the same perf on Apple M4:

Summary
  ./sapi/cli/php.branch bench.php ran
    1.01 ± 0.03 times faster than ./sapi/cli/php.master bench.php

Given the number of occurrences of 1 char glue as written in the description, I think this fast path would still worth it?

Description updated with the non-debug numbers for 1-char glue.

[nielsdos]

Ok for me

[mvorisek]

This is in loop, is ZSTR_LEN(glue) guaranteed to be evaluated only once?

What about moving the condition outside the loop, ie. "duplicating" the loop body?

Also what about the case when the glue is empty string? (we use that strongly in https://github.com/atk4/ui/blob/6.0.0/src/HtmlTemplate.php#L570 - with 10k calls per webpage request easily)

[nielsdos]

This is in loop, is ZSTR_LEN(glue) guaranteed to be evaluated only once?

No. But chances are that it doesn't matter if this is in a variable or not because you'll need a load from a spill slot in the variable case most likely.