PHP 8.6 Closure Optimizations

A new RFC for PHP 8.6 introduces two significant optimizations for closures and arrow functions, aiming to enhance performance and memory management. These changes, set to land in PHP 8.6, address common performance bottlenecks associated with closure instantiation and reference cycles.

• Static Closure Inference: Closures that are guaranteed not to use $this will automatically be inferred as static. This prevents implicit capture of $this, breaking reference cycles and allowing earlier garbage collection of objects.
  • Specific conditions prevent inference, such as using $$var (if $var could be 'this'), hidden instance calls (e.g., Foo::bar(), $f()), or declaring other non-inferable non-static closures. Despite these rules, tests on Symfony Demo showed successful inference for 78% of explicitly static closures.
• Stateless Closure Caching: static closures that capture no variables and declare no static variables will be cached and reused across multiple calls. This dramatically reduces the overhead of creating identical closure instances.
  • A synthetic benchmark showed an 80% performance improvement, and practical tests in Laravel templates indicated a 3% performance gain by avoiding thousands of closure instantiations.
• Backward Incompatible Changes: While generally beneficial, the RFC notes a few minor BC breaks:
  • ReflectionFunction::getClosureThis() will return NULL for closures inferred as static.
  • Stateless cached closures from the same lexical location will now be strictly identical (===).
  • Objects that previously formed reference cycles may be garbage collected earlier, potentially altering destructor timing.
  • Closure::bind() and Closure::bindTo() will discard objects passed to inferred static closures to maintain compatibility with explicit static closures.

Despite minor backward compatibility considerations, these proposed changes are poised to deliver substantial performance improvements and more efficient resource utilization in PHP applications, particularly those heavily reliant on closures.