Optimize performances of `str_pad()` #19272

alexandre-daubois · 2025-07-28T08:57:39Z

I would like to propose this optimization for str_pad(). Here is the benchmark code:

<?php

$iterations = 1000000;

for ($i = 0; $i < $iterations; $i++) {
    str_pad('hello', 2000, ' ', STR_PAD_RIGHT);
}

And the results with right padding:

Benchmark 1: ./sapi/cli/php.branch validation_benchmark.php
  Time (mean ± σ):      44.6 ms ±   0.8 ms    [User: 41.3 ms, System: 2.3 ms]
  Range (min … max):    43.2 ms …  47.4 ms    64 runs
 
Benchmark 2: ./sapi/cli/php.master validation_benchmark.php
  Time (mean ± σ):      2.977 s ±  0.076 s    [User: 2.962 s, System: 0.009 s]
  Range (min … max):    2.863 s …  3.061 s    10 runs
 
Summary
  ./sapi/cli/php.branch validation_benchmark.php ran
   66.75 ± 2.06 times faster than ./sapi/cli/php.master validation_benchmark.php

Left padding results:

alex@alex-macos php-src % hyperfine './sapi/cli/php.branch validation_benchmark.php' './sapi/cli/php.master validation_benchmark.php' --warmup 10
Benchmark 1: ./sapi/cli/php.branch validation_benchmark.php
  Time (mean ± σ):      43.8 ms ±   1.5 ms    [User: 40.5 ms, System: 2.3 ms]
  Range (min … max):    42.2 ms …  49.8 ms    58 runs
 
Benchmark 2: ./sapi/cli/php.master validation_benchmark.php
  Time (mean ± σ):      1.150 s ±  0.020 s    [User: 1.130 s, System: 0.009 s]
  Range (min … max):    1.125 s …  1.174 s    10 runs
 
Summary
  ./sapi/cli/php.branch validation_benchmark.php ran
   26.23 ± 1.01 times faster than ./sapi/cli/php.master validation_benchmark.php

The idea is to avoid modulo operation in loops and copying char by char. Instead, this PR prefers the bulk copy approach.

SakiTakamachi · 2025-07-28T12:10:59Z

ext/standard/string.c

@@ -5737,6 +5737,27 @@ PHP_FUNCTION(substr_count)
 }
 /* }}} */

+static inline void php_str_pad_fill(char *dest, size_t pad_chars, const char *pad_str, size_t pad_str_len) {


Suggested change

static inline void php_str_pad_fill(char *dest, size_t pad_chars, const char *pad_str, size_t pad_str_len) {

static zend_always_inline void php_str_pad_fill(char *dest, size_t pad_chars, const char *pad_str, size_t pad_str_len) {

Using zend_always_inline increases the likelihood that the function will actually be inlined.

Also, instead of passing char *dest and size_t pad_chars as separate arguments, how about accepting a zend_string and extracting the values within this inline function?

Good idea! PR updated with both suggestions

ext/standard/string.c

TimWolla

I feel that this makes the API of php_str_pad_fill() a little safer to use, because it makes less assumptions about the target pointer.

With regard to the inlining: I generally trust the compiler to make better than decisions than I can do myself. I would make the function just static void without any inlining hints and let the compiler decide.

ext/standard/string.c

TimWolla · 2025-07-28T13:19:46Z

ext/standard/string.c

-	for (i = 0; i < left_pad; i++)
-		ZSTR_VAL(result)[ZSTR_LEN(result)++] = pad_str[i % pad_str_len];
+	if (left_pad > 0) {
+		php_str_pad_fill(result, left_pad, pad_str, pad_str_len);


Suggested change

php_str_pad_fill(result, left_pad, pad_str, pad_str_len);

php_str_pad_fill(ZSTR_VAL(result) + ZSTR_LEN(result), left_pad, pad_str, pad_str_len);

@SakiTakamachi advised otherwise if I get it right?

It appears so, but the suggestion doesn't really make sense, since pad_chars cannot be piggy-backed on the zend_string. Perhaps Saki meant to pass pad_str and pad_str_len as a zend_string* (which would make sense to me).

It makes the code a bit slower, going from ~48ms per run to ~58ms. Do we still want this?

TimWolla · 2025-07-28T13:24:51Z

Please also see #19220 (comment) and #19220 (comment) for benchmarking advice.

alexandre-daubois · 2025-07-28T13:30:02Z

Thanks for the link, I'm having a look to hyperfine for this PR and #19276

alexandre-daubois · 2025-07-28T13:47:20Z

PR description updated to use hyperfine

TimWolla · 2025-07-29T09:29:00Z

PR description updated to use hyperfine

The php binary appears to be the system binary, it is not unlikely that it was compiled with different flags or a different compiler (version). You should make sure to test two binaries compiled with identical flags and compilers.

alexandre-daubois · 2025-07-29T09:43:25Z

I think that my system-wide install right now is PHP 8.5 nightly. Just to be sure, I rerun against master and the branch:

Benchmark 1: ./sapi/cli/php.branch validation_benchmark.php
  Time (mean ± σ):      44.6 ms ±   0.8 ms    [User: 41.3 ms, System: 2.3 ms]
  Range (min … max):    43.2 ms …  47.4 ms    64 runs
 
Benchmark 2: ./sapi/cli/php.master validation_benchmark.php
  Time (mean ± σ):      2.977 s ±  0.076 s    [User: 2.962 s, System: 0.009 s]
  Range (min … max):    2.863 s …  3.061 s    10 runs
 
Summary
  ./sapi/cli/php.branch validation_benchmark.php ran
   66.75 ± 2.06 times faster than ./sapi/cli/php.master validation_benchmark.php

github-actions · 2025-08-01T08:47:05Z

AWS x86_64 (c7i.24xl)

Attribute	Value
Environment	aws
Runner	host
Instance type	c7i.metal-24xl (dedicated)
Architecture	x86_64
CPU	48 cores
CPU settings	disabled deeper C-states, disabled turbo boost, disabled hyper-threading
RAM	188 GB
Kernel	6.1.144-170.251.amzn2023.x86_64
OS	Amazon Linux 2023.8.20250721
GCC	11.5.0
Time	2025-08-01 08:20:50 UTC

Laravel 11.1.2 demo app - 30 consecutive runs, 100 requests (sec)

PHP	Min	Max	Std dev	Average	Average diff %	Median	Median diff %	Memory
PHP - baseline@eaf2	0.46107	0.46896	0.00123	0.46674	0.00%	0.46677	0.00%	43.54 MB
PHP - str-pad-opt	0.46154	0.46938	0.00127	0.46765	0.20%	0.46767	0.19%	43.46 MB

Symfony 2.6.0 demo app - 30 consecutive runs, 100 requests (sec)

PHP	Min	Max	Std dev	Average	Average diff %	Median	Median diff %	Memory
PHP - baseline@eaf2	0.72773	0.73390	0.00138	0.72954	0.00%	0.72921	0.00%	39.69 MB
PHP - str-pad-opt	0.73090	0.73512	0.00087	0.73262	0.42%	0.73241	0.44%	39.77 MB

Wordpress 6.2 main page - 30 consecutive runs, 20 requests (sec)

PHP	Min	Max	Std dev	Average	Average diff %	Median	Median diff %	Memory
PHP - baseline@eaf2	0.57803	0.58039	0.00059	0.57886	0.00%	0.57869	0.00%	43.54 MB
PHP - str-pad-opt	0.58101	0.65067	0.02497	0.63512	9.72%	0.64595	11.62%	43.54 MB

bench.php - 25 consecutive runs (sec)

PHP	Min	Max	Std dev	Average	Average diff %	Median	Median diff %	Memory
PHP - baseline@eaf2	0.21721	0.22099	0.00093	0.21825	0.00%	0.21808	0.00%	26.53 MB
PHP - str-pad-opt	0.21425	0.21712	0.00077	0.21580	-1.12%	0.21585	-1.02%	26.53 MB

micro_bench.php - 25 consecutive runs (sec)

PHP	Min	Max	Std dev	Average	Average diff %	Median	Median diff %	Memory
PHP - baseline@eaf2	1.26166	1.28426	0.00498	1.26842	0.00%	1.26635	0.00%	20.82 MB
PHP - str-pad-opt	1.31617	1.33954	0.00573	1.32695	4.61%	1.32688	4.78%	20.82 MB

kocsismate · 2025-08-01T09:04:27Z

The above comment was due to test the real time benchmark :) Unfortunately, I guess most results should show zero difference, which is not currently the case :( The wordpress results are especially weird. I'll have to dig into it...

cc. @iluuu1994 @arnaud-lb

alexandre-daubois · 2025-08-01T09:54:37Z

If I can be of any help, don't hesitate to tell me! It is indeed surprising

github-actions · 2025-08-01T10:10:56Z

AWS x86_64 (c7i.24xl)

Attribute	Value
Environment	aws
Runner	host
Instance type	c7i.metal-24xl (dedicated)
Architecture	x86_64
CPU	48 cores
CPU settings	disabled deeper C-states, disabled turbo boost, disabled hyper-threading
RAM	188 GB
Kernel	6.1.144-170.251.amzn2023.x86_64
OS	Amazon Linux 2023.8.20250721
GCC	11.5.0
Time	2025-08-01 09:41:35 UTC

Laravel 11.1.2 demo app - 30 consecutive runs, 100 requests (sec)

PHP	Min	Max	Std dev	Average	Average diff %	Median	Median diff %	Instr count	Memory
PHP - baseline@eaf2	0.45780	0.46629	0.00139	0.46472	0.00%	0.46488	0.00%	177247194	43.48 MB
PHP - str-pad-opt	0.46514	0.46736	0.00050	0.46594	0.26%	0.46581	0.20%	177251261	43.48 MB

Symfony 2.6.0 demo app - 30 consecutive runs, 100 requests (sec)

PHP	Min	Max	Std dev	Average	Average diff %	Median	Median diff %	Instr count	Memory
PHP - baseline@eaf2	0.72645	0.72932	0.00071	0.72785	0.00%	0.72779	0.00%	288207116	39.78 MB
PHP - str-pad-opt	0.72983	0.73390	0.00103	0.73178	0.54%	0.73152	0.51%	288197741	39.78 MB

Wordpress 6.2 main page - 30 consecutive runs, 20 requests (sec)

PHP	Min	Max	Std dev	Average	Average diff %	Median	Median diff %	Instr count	Memory
PHP - baseline@eaf2	0.57872	0.58180	0.00064	0.57969	0.00%	0.57960	0.00%	1129301005	43.56 MB
PHP - str-pad-opt	0.57772	0.58038	0.00056	0.57900	-0.12%	0.57888	-0.12%	1129302558	43.56 MB

bench.php - 25 consecutive runs (sec)

PHP	Min	Max	Std dev	Average	Average diff %	Median	Median diff %	Instr count	Memory
PHP - baseline@eaf2	0.21719	0.22107	0.00105	0.21863	0.00%	0.21866	0.00%	1791734911	26.55 MB
PHP - str-pad-opt	0.21282	0.21641	0.00090	0.21439	-1.94%	0.21441	-1.94%	1794186569	26.55 MB

micro_bench.php - 25 consecutive runs (sec)

PHP	Min	Max	Std dev	Average	Average diff %	Median	Median diff %	Instr count	Memory
PHP - baseline@eaf2	1.25910	1.27670	0.00515	1.26772	0.00%	1.26679	0.00%	1269754366	20.84 MB
PHP - str-pad-opt	1.31593	1.33956	0.00516	1.32448	4.48%	1.32418	4.53%	1271643119	20.83 MB

alexandre-daubois requested a review from bukka as a code owner July 28, 2025 08:57

github-actions bot added the Extension: standard label Jul 28, 2025

SakiTakamachi reviewed Jul 28, 2025

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alexandre-daubois force-pushed the str-pad-opt branch from fbfa654 to 24d2823 Compare July 28, 2025 12:24

TimWolla reviewed Jul 28, 2025

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alexandre-daubois force-pushed the str-pad-opt branch from 24d2823 to ba1eb4c Compare July 28, 2025 13:12

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Optimize performances of str_pad()

78dd005

alexandre-daubois force-pushed the str-pad-opt branch from ba1eb4c to 78dd005 Compare July 28, 2025 13:46

	static inline void php_str_pad_fill(char dest, size_t pad_chars, const char pad_str, size_t pad_str_len) {
	static zend_always_inline void php_str_pad_fill(char dest, size_t pad_chars, const char pad_str, size_t pad_str_len) {

	php_str_pad_fill(result, left_pad, pad_str, pad_str_len);
	php_str_pad_fill(ZSTR_VAL(result) + ZSTR_LEN(result), left_pad, pad_str, pad_str_len);

Optimize performances of str_pad() #19272

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Optimize performances of str_pad() #19272

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github-actions bot commented Aug 1, 2025

AWS x86_64 (c7i.24xl)

Laravel 11.1.2 demo app - 30 consecutive runs, 100 requests (sec)

Symfony 2.6.0 demo app - 30 consecutive runs, 100 requests (sec)

Wordpress 6.2 main page - 30 consecutive runs, 20 requests (sec)

bench.php - 25 consecutive runs (sec)

micro_bench.php - 25 consecutive runs (sec)

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AWS x86_64 (c7i.24xl)

Laravel 11.1.2 demo app - 30 consecutive runs, 100 requests (sec)

Symfony 2.6.0 demo app - 30 consecutive runs, 100 requests (sec)

Wordpress 6.2 main page - 30 consecutive runs, 20 requests (sec)

bench.php - 25 consecutive runs (sec)

micro_bench.php - 25 consecutive runs (sec)

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Optimize performances of `str_pad()` #19272

Optimize performances of `str_pad()` #19272

alexandre-daubois commented Jul 28, 2025 •

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