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Date: Sun, 3 Jan 2021 10:15:10 -0600
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To: =?UTF-8?Q?Olle_H=C3=A4rstedt?= <olleharstedt@gmail.com>
Cc: PHP internals <internals@lists.php.net>
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Subject: Re: [PHP-DEV] Does PHP ever stack allocate?
From: pollita@php.net (Sara Golemon)

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On Fri, Jan 1, 2021 at 3:18 PM Olle H=C3=A4rstedt <olleharstedt@gmail.com> =
wrote:

> Or is everything reference counted with heap allocation? Since PHP has
> escape analysis, this could be used to use the stack instead, and kill th=
e
> memory when the scope ends. If PHP uses the stack, can this be seen in th=
e
> opcode?
>
>
Well, you're not going to like this answer, but.... yes and no.

A single PHP call frame holds a block of storage space for (among other
things) all* local variables.  This can be thought of analogously to "the
stack" as it's used by native applications.  Basic scalars (null, bool,
int, float) sit in this space with no additional pointers to anywhere.
Non-scalars use pointers to elsewhere in the heap to store the actual
payload.  This isn't unique to PHP, as these structures have runtime
determined size and thus can't** be stack allocated.

There's further asterii below all of those statements, but that's the
high-level generalized answer to your question as posed.

The implied question you asked is actually handled using another
mechanism.  PHP's internals have two separate memory allocation pools.
"Persistent" memory allocation, which creates blocks of memory for the
lifetime of the process, and "Engine" memory allocation, which are bulk
de-allocated*** at the end of every request (after relevant
destructors have fired).

-Sara

* All variables which are explicitly references as $foo (excluding
auto-globals).  ${'bar'} and $$baz style references to locals are special
and require their own separate conversation.
** C's alloca() and similar techniques in other languages can reserve
dynamic amounts of stack space, but let's ignore that power-move for the
sake of this argument.
*** Deallocated from the request handler's point of view, though the
runtime's memory manager (usually) doesn't give it back to the OS
immediately, since it's likely to be used by the next request anyway.

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