PHP: rfc:jit_config_defaults

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====== PHP RFC: Change how JIT is disabled by default ======
  * Date: 2023-11-15
  * Author: Daniil Gentili <daniil@daniil.it>
  * Status: Accepted


===== Introduction =====

This RFC proposes to change //how// JIT is disabled by default.

===== Proposal =====

Currently, JIT is disabled by default using the ''opcache.jit_buffer_size=0'' default, instead of ''opcache.jit=disable''.

For example, here are the defaults for these two configuration entries:

    opcache.jit=tracing
    opcache.jit_buffer_size=0

This effectively disables JIT not because ''jit=disable'', but rather because the buffer size is set to 0.

This RFC proposes to change these defaults to:

    opcache.jit=disable
    opcache.jit_buffer_size=64m

The chosen JIT buffer size default is 64 mb, a reasonable default, given that any smaller multiple of
2 causes the buffer to fill up when running moderately sized projects like Psalm or PHPStan.

Another possible option for the default is 128 mb, which is incidentally also the maximum allowed JIT buffer size on the aarch64 architecture.


===== Backward Incompatible Changes =====

This RFC does not change the default behavior, JIT was and still is disabled by default, however now it is now disabled by default using the ''opcache.jit'' setting, not the ''opcache.jit_buffer_size'' setting.  

This may affect users who enabled JIT by only setting a different ''opcache.jit_buffer_size'', without specifying a JIT mode with ''opcache.jit''.  

To enable JIT, users will also be required to populate the ''opcache.jit'' config value.

===== Proposed PHP Version(s) =====

Next PHP 8.4.

===== RFC Impact =====
==== To SAPIs ====

See Backwards Incompatible Changes section.

===== Proposed Voting Choices =====

2/3 required to accept. Voting started on 2023-12-03 and will end on 2023-12-18 00:00 GMT. 

<doodle title="Change how JIT is disabled by default?" auth="danog" voteType="single" closed="true" closeon="2023-12-18T00:00:00Z">
   * Yes
   * No
</doodle>

===== Patches and Tests =====

Pull request: https://github.com/php/php-src/pull/12678

===== References =====

Reference internals discussion: https://externals.io/message/121359
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