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Date: Tue, 4 Mar 2025 22:14:05 +0100
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Subject: Re: [PHP-DEV] Re: RFC: Marking return values as important
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To: "Marc B." <marc@mabe.berlin>, Volker Dusch <volker@tideways-gmbh.com>
Cc: php internals <internals@lists.php.net>
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From: tim@bastelstu.be (=?UTF-8?Q?Tim_D=C3=BCsterhus?=)

Hi Marc

On 3/3/25 17:14, Marc B. wrote:
> In this thread, I only found the information that currently OPcache does not
> discard such unused assignments. It would be good to know if such optimization
> could be considered to not end up in a situation that such optimization would be
> useful but can't be applied as of this feature.

Unless it knows for sure that the function is unable to return an object 
(this includes arrays, which could contain objects), OPcache is already 
disallowed to elide the dead store. Otherwise this would result in an 
observable behavioral change, since destructors are called immediately 
rather than when the variable goes out of scope / is overwritten. For 
practically all modern code this means that this optimization only 
affects internal functions, since OPcache only looks at a single file. 
Thus it does not know about the return type of userland functions 
defined in a different file.

In fact, if you call `get_defined_vars()` or similar functions the dead 
store elimination is also disabled, since that would result in an 
observable behavioral change.

Should the `(void)` cast not be accepted, we will only special case the 
assignment to `$_` to not be elided, even if OPcache knows that the 
function will never return an object. The behavior for other variables 
will remain unchanged.

Best regards
Tim Düsterhus