On Wed, Jan 21, 2026, at 1:52 PM, Ilija Tovilo wrote:
> Hi everyone
>
> I've created a PR implementing two new optimizations for closures. [1]
> There are some theoretical BC breaks, so please let me know if any of
> them are of concern to you.
>
> 1. Static inference
>
> The engine will attempt to infer static for closures that are
> guaranteed not to make any use of $this. This has the benefit of
> avoiding a cycle when a closure is stored in a property of the
> declarator object. Frequently, such objects and their closures will
> not be collected for the entirety of the request, given the GC often
> doesn't run at all. But the real reason for this optimization is to
> enable optimization nr. 2.
>
> It's worth noting the rules for inferring static are slightly
> esoteric. You can read more about the rules in the PR description.
>
> 2. Stateless closure caching
>
> Stateless closures, i.e. those that are static, don't capture any
> variables and don't declare any static variables are now cached.
>
> function test() {
> $x = function () {};
> }
> for ($i = 0; $i < 10_000_000; $i++) {
> test();
> }
>
> Previously, this would have created 10 000 000 closure instances, even
> though all closures are effectively identical. Now, the first closure
> will be kept alive for reuse. With these two optimizations, this small
> benchmark improves by ~80%. Of course, this is a very synthetic
> benchmark. However, I could also measure improvements in real
> applications that instantiate many stateless closures. For example, in
> the Laravel template these two optimizations can avoid 2384 out of
> 3637 closure instantiations, improving performance by ~3% on my
> machine.
>
> The foreshadowed BC breaks come down to a three things:
>
> 1. For closures that are inferred as static,
> ReflectionFunction::getClosureThis() will now return NULL.
> 2. Objects that would previously have created cycles may be collected
> earlier, also triggering destructors earlier. IMO, this is a feature
> and more predictable than the current behavior.
> 3. Two stateless closures originating from the same lexical location
> will now be identical. I.e.:
>
> function test() {
> return function () {};
> }
> test() === test(); // true
>
> Of note is that Closure::bind() and Closure::bindTo() usually throw
> when attempting to bind an object to a static closure. In my PR, this
> is explicitly allowed only for closures that are inferred as static,
> but not those that are explicitly static.
>
> I'm looking forward to your feedback. If there are no concerns, I will
> merge this PR into master for PHP 8.6 in roughly two weeks.
>
> Ilija
>
> [1] https://github.com/php/php-src/pull/19941
This all looks great, Ilija! I am not concerned about the BC mentions, as they
seem to be accurate anyway.
My one question is about the bindTo() on an inferred static closure. What
purpose does that serve? I'm assuming that anything that mentions $this in its
body won't be marked static, and if it doesn't have $this I don't get what
value bindTo() would have. What's the use case for making an exception here?
--Larry Garfield
