Hi,

I don't understand why exceptions are so slow in D. They should be very fast, because, (in my imagination, at least), they don't have to do much. But I just wrote a piece of code yesterday which used pretty much used exceptions like a fancy goto (thanks for that description, guys) and it ran unbelievebly slowly. When I recoded it without exceptions, the speedup was phenomenal.

But in my imagination, all you have to do to throw an exception in D is load the stack pointer with a previously stashed value, stash a pointer to the /next/ exception frame down (ready for next time), and perform a jump (through some sort of lookup table based on the type of object thrown). I'm obviously missing something though - it must be doing much more than that or it wouldn't take so long.

What am I missing? What else does it have to do?

Arcane Jill

"Arcane Jill" <Arcane_member@pathlink.com> wrote in message news:cagtdm$1ug5$1@digitaldaemon.com...
> Hi,
>
> I don't understand why exceptions are so slow in D. They should be very
fast,
> because, (in my imagination, at least), they don't have to do much. But I
just
> wrote a piece of code yesterday which used pretty much used exceptions
like a
> fancy goto (thanks for that description, guys) and it ran unbelievebly
slowly.
> When I recoded it without exceptions, the speedup was phenomenal.
>
> But in my imagination, all you have to do to throw an exception in D is
load the
> stack pointer with a previously stashed value, stash a pointer to the
/next/
> exception frame down (ready for next time), and perform a jump (through
some
> sort of lookup table based on the type of object thrown). I'm obviously
missing
> something though - it must be doing much more than that or it wouldn't
take so
> long.
>
> What am I missing? What else does it have to do?

It has to be compatible with Microsoft's "Windows Structured Exception Handling" which is far too complicated to explain in a simple post.

But it shouldn't matter if it is relatively slow. Exceptions should be for exceptional cases, not the ordinary program flow.

"Walter" <newshound@digitalmars.com> wrote in message news:cagu40$1vnd$1@digitaldaemon.com...
>
> "Arcane Jill" <Arcane_member@pathlink.com> wrote in message news:cagtdm$1ug5$1@digitaldaemon.com...
> > Hi,
> >
> > I don't understand why exceptions are so slow in D. They should be very
> fast,
> > because, (in my imagination, at least), they don't have to do much. But I
> just
> > wrote a piece of code yesterday which used pretty much used exceptions
> like a
> > fancy goto (thanks for that description, guys) and it ran unbelievebly
> slowly.
> > When I recoded it without exceptions, the speedup was phenomenal.
> >
> > But in my imagination, all you have to do to throw an exception in D is
> load the
> > stack pointer with a previously stashed value, stash a pointer to the
> /next/
> > exception frame down (ready for next time), and perform a jump (through
> some
> > sort of lookup table based on the type of object thrown). I'm obviously
> missing
> > something though - it must be doing much more than that or it wouldn't
> take so
> > long.
> >
> > What am I missing? What else does it have to do?
>
> It has to be compatible with Microsoft's "Windows Structured Exception Handling" which is far too complicated to explain in a simple post.

You're not wrong, about either the need for SEH compatibility, or its complexity.

> But it shouldn't matter if it is relatively slow. Exceptions should be for exceptional cases, not the ordinary program flow.

Exactamundo. Theoretically, I concur. Practically, D is not Java, does not have a byte-code interpreter, and therefore must interact with the machine for things like /0 and *NULL. Hence, it is slow. QED


-- 
The Dr.

da-da-da-dum, da-da-da-dum, da-da-da-dum, daaah
da-da-da-dum, da-da-da-dum, da-da-da-dum, daaah
woo-ooooo

>It has to be compatible with Microsoft's "Windows Structured Exception Handling" which is far too complicated to explain in a simple post.

Heres a good article http://www.codetools.com/cpp/exceptionhandler.asp

> pretty much used exceptions
>like a
>> fancy goto

Perverse!

Charlie

In article <cagu40$1vnd$1@digitaldaemon.com>, Walter says...
>
>
>"Arcane Jill" <Arcane_member@pathlink.com> wrote in message news:cagtdm$1ug5$1@digitaldaemon.com...
>> Hi,
>>
>> I don't understand why exceptions are so slow in D. They should be very
>fast,
>> because, (in my imagination, at least), they don't have to do much. But I
>just
>> wrote a piece of code yesterday which used pretty much used exceptions
>like a
>> fancy goto (thanks for that description, guys) and it ran unbelievebly
>slowly.
>> When I recoded it without exceptions, the speedup was phenomenal.
>>
>> But in my imagination, all you have to do to throw an exception in D is
>load the
>> stack pointer with a previously stashed value, stash a pointer to the
>/next/
>> exception frame down (ready for next time), and perform a jump (through
>some
>> sort of lookup table based on the type of object thrown). I'm obviously
>missing
>> something though - it must be doing much more than that or it wouldn't
>take so
>> long.
>>
>> What am I missing? What else does it have to do?
>
>It has to be compatible with Microsoft's "Windows Structured Exception Handling" which is far too complicated to explain in a simple post.
>
>But it shouldn't matter if it is relatively slow. Exceptions should be for exceptional cases, not the ordinary program flow.
>
>

"Charlie" <Charlie_member@pathlink.com> wrote in message news:cahflb$2onn$1@digitaldaemon.com...
> >It has to be compatible with Microsoft's "Windows Structured Exception Handling" which is far too complicated to explain in a simple post.
>
> Heres a good article http://www.codetools.com/cpp/exceptionhandler.asp

That's a good introduction to SEH, thanks.