Hi all,

Inspired by Gareth aka. Kit's infectious enthusiasm, and the recent long discussion on exceptions on this list, I wrote up an article on Exceptions on the FPC wiki. (Strangely enough we didn't have one before.)

http://wiki.freepascal.org/Exceptions

I hope it'll be useful, and not entirely inaccurate! Particularly the part about performance. If a programmer knows exactly what each exception statement inserts in the code, that should help in deciding when to worry about the performance, and when to embrace the convenience.

~Kirinn

Infectious enthusiasm - I like that! I'm honoured.

It's a good start on the Wiki article.  For more advanced things, you might want to mention how you can use "raise" to re-raise an exception in an except block, and the fact that SysUtils has code that wraps interrupts like SIGSEGV and SIGFPE onto exception objects.

Gareth aka. Kit

On Wed 01/08/18 01:32 , Kirinn sent:
Hi all,

Inspired by Gareth aka. Kit's infectious enthusiasm, and the recent long discussion on exceptions on this list, I wrote up an article on Exceptions on the FPC wiki. (Strangely enough we didn't have one before.)

http://wiki.freepascal.org/Exceptions
[1]">http://wiki.freepascal.org/Exceptions

I hope it'll be useful, and not entirely inaccurate! Particularly the part about performance. If a programmer knows exactly what each exception statement inserts in the code, that should help in deciding when to worry about the performance, and when to embrace the convenience.

~Kirinn

Thanks Kirinn for making this new wiki page. I just added a single line at the bottom for "Further reading".

When you add a component to your program, are you made aware about all of the exceptions that might be raised by the component? When writing control software this can be *very* important.

There is a little more to the old story here (maybe should be added to the wiki page):
https://community.embarcadero.com/blogs/entry/delphi-s-involvement-with-the-esa-rosetta-comet-spacecraft-project-1

I have these links here:
http://turbocontrol.com/embeddedfreepascal.htm

Best regards,
Paul
www.ControlPascal.com

Kirinn schrieb am Mi., 1. Aug. 2018, 23:30:

Please note that you can't generalize the performance impact as it depends on the used exception handling mechanism. By default FPC uses setjmp/longjmp which exhibit the behavior you described. On Win64 however SEH is used which - in that platform - relies on metadata inside the binary to mark the areas for a protected block (no matter whether it's for "except" or "finally"). On Win32 with SEH enabled (which isn't the default currently) it is again different. And should we decide to support DWARF exception frames that would again be different.

Regards,
Sven

Op 01-08-18 om 02:32 schreef Kirinn:

We had some discussion about memory-leaks and exceptions. All condensed into this bug report: https://bugs.freepascal.org/view.php?id=33650

Maybe something to add?

Regards,

Joost.

The information in the Wiki about how exception frames are set up is pretty nice. I didn't know that 'raise' creates an exception frame for the procedure it appears in, but explains why it's often shipped off to another local procedure. Very useful information!
True though, it's hard to generalise the performance impact of exception handlers and try...finally blocks, although I think the simple rule of thumb is that there is a performance hit no matter what you try.

Gareth aka. Kit

On Thu 02/08/18 06:54 , Sven Barth via fpc-devel
sent:
Kirinn schrieb am Mi., 1. Aug. 2018, 23:30:
Hi all,

Inspired by Gareth aka. Kit's infectious enthusiasm, and the recent long discussion on exceptions on this list, I wrote up an article on Exceptions on the FPC wiki. (Strangely enough we didn't have one before.)

http://wiki.freepascal.org/Exceptions [2]

I hope it'll be useful, and not entirely inaccurate! Particularly the part about performance. If a programmer knows exactly what each exception statement inserts in the code, that should help in deciding when to worry about the performance, and when to embrace the convenience.

Please note that you can't generalize the performance impact as it depends on the used exception handling mechanism. By default FPC uses setjmp/longjmp which exhibit the behavior you described. On Win64 however SEH is used which - in that platform - relies on metadata inside the binary to mark the areas for a protected block (no matter whether it's for "except" or "finally"). On Win32 with SEH enabled (which isn't the default currently) it is again different. And should we decide to support DWARF exception frames that would again be different. 
Regards, Sven  _______________________________________________

J. Gareth Moreton wrote

The information in the Wiki about how exception frames are set up is pretty nice. I didn't know that 'raise' creates an exception frame for the procedure it appears in, but explains why it's often shipped off to another local procedure. Very useful information!

Implicit exception frames in procedures with 'raise' clause come from string manipulations that are used to compose non-trivial exception messages, not from the 'raise' itself.

Regards,
Sergei