PHP 8.6 Closure Optimization

Two closure optimizations were proposed as a PHP 8.6 RFC, taking a direction that accepts some BC break in exchange for performance gains.

• Static closure inference: non-static closures that don't actually use $this are automatically treated as static.

• This can reduce unnecessary $this capturing and reference cycles between objects and closures.

• However, the inference isn't complete, and closures are not converted to static when $this could indirectly flow in, such as in the following cases:

  • $$var
  • cases that look like static calls such as Foo::bar() but could be interpreted as instance calls
  • $f()
  • call_user_func()
  • cases where another non-static closure is declared inside
  • require, include, eval

• When tested on the Symfony Demo after removing explicit static, 68 out of 87 (about 78%) closures could be automatically inferred.

• Stateless closure caching: closures that are static, capture no variable values, and declare no static variables are stored in a reuse cache.

• Since identical closures created at the same lexical position are no longer newly created each time, an improvement of about 80% was observed in a very simple benchmark.

• In the Laravel template, closure instantiation was reduced by 2384 out of 3637, resulting in about 3% performance improvement based on local measurements.

Backward incompatible changes were also summarized.

• ReflectionFunction::getClosureThis() returns NULL for inferred static closures.
• Two stateless closures created at the same lexical position may now be treated as the same object.
• Objects that were previously collected late due to reference cycles may now be garbage collected sooner, which may also cause their destructor to run earlier.
• Closure::bind() and Closure::bindTo() still fail as before for explicitly static closures, but for closures that became static through inference, passing an object is allowed but ignored.

The vote passed 24:0:1, targeting PHP 8.6.