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Date: Sun, 3 Jan 2021 16:37:50 +0000
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To: Sara Golemon <pollita@php.net>
Cc: PHP internals <internals@lists.php.net>
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Subject: Re: [PHP-DEV] Does PHP ever stack allocate?
From: olleharstedt@gmail.com (=?UTF-8?Q?Olle_H=C3=A4rstedt?=)

2021-01-03 16:15 GMT, Sara Golemon <pollita@php.net>:
> 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
>> the
>> memory when the scope ends. If PHP uses the stack, can this be seen in
>> the
>> 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.
>

Thanks Sara! I realize I should have been more precise: Can PHP
allocate non-reference counted memory that automatically is freed when
leaving scope, similar to what Go does with escape analysis?

Article describing the Go mechanism:
https://segment.com/blog/allocation-efficiency-in-high-performance-go-servi=
ces/

Olle