opcache.jit won't write to opcache.file_cache

[kkmuffme]

Description

The following code:

<?php

function foo() {
    echo "hello";
}

foo();

php -d opcache.enable_cli=1 -d opcache.jit=1255 -d opcache.file_cache=/tmp/opcache test.php
php -d opcache.enable_cli=1 -d opcache.jit=disable -d opcache.file_cache=/tmp/opcache test.php

When jit is enabled, nothing is written to /tmp/opcache
When jit is disabled, /tmp/opcache is populated

Why?

PHP Version

8.4

Operating System

No response

[nielsdos]

Why?

Because of this:

php-src/ext/opcache/zend_file_cache.c

Lines 1102 to 1107 in d3da2a2

	#ifdef HAVE_JIT
	/* FIXME: dump jited codes out to file cache? */
	if (JIT_G(on)) {
	return FAILURE;
	}
	#endif

Support for dumping in combination with JIT is incomplete.

[abcbarryn]

So, I tried commenting out that code just as a test and got a SIGSEGV. I am currently running with JIT disabled and file cache enabled because that is faster for me than having JIT enabled and file cache disabled. It looks like these two options are mutually exclusive for now, is that right? What is file cache caching? Is it caching machine code or PHP byte code?

[nielsdos]

They are mutually exclusive right now indeed.
file caching predates the JIT, it caches PHP byte code. Caching machine code on disk is harder because some of the addresses in the generated machine code are fixed addresses that depend on the currently running process that may differ in the next run due to ASLR.

[abcbarryn]

Well in one of my other apps I found a way to crash PHP. Turn opcache.file_cache on, set opcache.jit=off then have a PHP file that does this:

ini_set('opcache.jit', 'function');
use somedir/somefile

BOOM! PHP crashes. If I move the ini_set('opcache.jit', 'function'); after the use, it is ok.

[abcbarryn]

I was reviewing the options for JIT trigger and the code for file cache:

T: Trigger
0: Compile all functions on script load.
1: Compile all functions on first execution.
2: Profile first request and compile the hottest functions afterwards.
3: Profile on the fly and compile hot functions.
4: Currently unused.
5: Use tracing JIT. Profile on the fly and compile traces for hot code segments.

and

#ifdef HAVE_JIT 
	/* FIXME: dump jited codes out to file cache? */ 
	if (JIT_G(on)) { 
		return FAILURE; 
	} 
#endif

I am wondering if rather than the file cache code checking as it does now for JIT being off if it could check for the trigger flag being set to greater than 0 and then cache the parsed byte code on script load if it is? Maybe even force the trigger flag to a value of 1 if opcache.file_cache is enabled? Instead of if (JIT_G(on)) how could I test for this? I might need to modify other sections of code too, but I think this could be a MAJOR performance improvement when the server is restarted! Right now I am simulating this by setting opcache.jit=off in the php.ini which allows file cache to work and then editing the PHP source file and adding ini_set('opcache.jit', 'function'); to the beginning of the PHP file. This generally seems to work and the system loads and runs very fast! The effect is to cache the parsed PHP byte code to disk and then run JIT at run time in shared memory. This is similar to how java works.

[abcbarryn]

So, interesting update, I applied this patch which I wrote:

--- php-8.4.5/ext/opcache/zend_file_cache.c     2025-04-12 00:39:42.711942765 -0400
+++ php-8.4.5/ext/opcache/zend_file_cache.c     2025-04-12 00:49:16.020482017 -0400
@@ -1096,5 +1096,6 @@
 #ifdef HAVE_JIT
        /* FIXME: dump jited codes out to file cache? */
-       if (JIT_G(on)) {
+       if (JIT_G(trigger) == ZEND_JIT_ON_SCRIPT_LOAD) {
+       // For now at least don't cache machine code, only cache before JIT runs
                return FAILURE;
        }

I am setting the following in php.ini:

[opcache]
; Determines if Zend OPCache is enabled
opcache.enable=1
opcache.jit_buffer_size=256M
opcache.jit=1215
opcache.file_cache = "/var/lib/wwwrun/phpcache"

The first time the PHP page is accessed I get:

Service unavailable!
The server is temporarily unable to service your request due to maintenance downtime or capacity problems. Please try again later.

If you think this is a server error, please contact the [webmaster](mailto:webmaster@star2.abcm.com).

Error 503
[redacted.com](https://redacted.com/)
Apache/2.4.63 (Linux/SUSE)

But the PHP byte code does get cached in the file cache! And the next and subsequent times I access the PHP page it works and shows that JIT is on! Any ideas how I can fix the first access issue? I get the same behavior with opcache.jit= tracing. With opcache.jit=function the file cache is disabled.

[abcbarryn]

So, on first access of the PHP file it is throwing a SIGSEGV...
[12-Apr-2025 02:32:31] WARNING: [pool www] child 8827 exited on signal 11 (SIGSEGV) after 56.781718 seconds from start
But subsequent accesses work!

[abcbarryn]

I have found a very good work around.
in /usr/local/autojit.php I put

<?php
if (opcache_is_script_cached($_SERVER['SCRIPT_FILENAME']))
        ini_set('opcache.jit', 'function');
?>

in php.ini I put

[opcache]
; Determines if Zend OPCache is enabled
opcache.enable=1
opcache.jit_buffer_size=256M
opcache.jit=off
auto_prepend_file = "/usr/local/autojit.php"
opcache.file_cache = "/var/lib/wwwrun/phpcache"

So the first time the script is run or if it is modified JIT is off and the script is added to the file cache. The next time it is run JIT is turned on and the machine code is generated in shared memory and file cache writing is turned off. When the server is restarted the cached PHP byte code is loaded from file cache, and after the script loads once JIT is turned on. It is a work around but it works! Now maybe someone can get PHP to do this by itself without the funky config? For now, at least it works...