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Date: Fri, 2 May 2025 14:35:30 +0200
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Subject: Re: [PHP-DEV] Deferred backtrace generation algorithm.
To: Edmond Dantes <edmond.ht@gmail.com>,
 php internals <internals@lists.php.net>
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From: bobwei9@hotmail.com (Bob Weinand)

Hey Edmond,

On 2.5.2025 10:14:37, Edmond Dantes wrote:
> Good day, everyone.
>
> In the neighboring thread Concept: Lightweight error channels, the
> topic of exception performance was discussed.
>
> The main issue is the immediate backtrace generation in the
> constructor, which can cause up to a 50% performance loss compared to
> return (as I understood from the discussion — I may be mistaken).
It basically depends on the stack depth at that point. So it _can_ be 
more, or less than that.
> I have a small idea on how to improve the situation in cases where
> exceptions are caught and a backtrace isn't needed.
>
> Let’s assume we delay backtrace generation. In PHP, you can’t just
> keep a reference to a stack frame, since it may be destroyed. You
> could copy it, of course (which is relatively inexpensive compared to
> generating the full backtrace).
>
> Based on that, there are two possible implementations:
>
> * Generate the backtrace at the moment the stack is freed
>
> * Clone the stack frame when the stack is freed (this is roughly what
> happens in Python)
>
> This would require changes to the functions
> `zend_vm_stack_free_extra_args_ex` and
> `zend_vm_stack_free_extra_args`.

What you actually want is, I think, a proper new call_info flag, which 
you can attach: "frame has associated exceptions to check" (you can 
stuff that into the same branch than ZEND_CALL_FREE_EXTRA_ARGS in 
leave_helper though).

There can be many exceptions referencing the same (and different) stack 
frames in flight. So you effectively will have to manage a weakmap of 
next-stack-frame to exception objects.

This certainly seems doable.


Of note is also that $trace and $string are actually private properties. 
So access via reflection works there. And it might be changed too. It 
possibly should be converted to a property hook then, which materializes 
on first access.


But otherwise, I think, this is mostly a matter of implementation. If 
you're interested in providing a patch, you're definitely welcome.


Bob