Real cost of C++ exception vs error-code checking

http://lazarenko.me/tips-and-tricks/c-exception-handling-and-performance

by zeugma • 15 years ago

71 62 15 years ago

lazarenko.me

McP• 15 years ago

I prefer Raymond Chen's take on exceptions: http://blogs.msdn.com/b/oldnewthing/archive/2005/01/14/35294...

Xurinos• 15 years ago

I didn't see any timings in his code.

bediger• 15 years ago

Yes. How can he possibly say that "exceptions are faster" without such timings? C++ style, unwind-the-stack-and-call-destructor exceptions must really have a high run-time cost when an exception occurs. Also, his instruction count misses the instructions that occur at run-time handling an exception. Even when an exception doesn't occur, it isn't obvious that some run-time code doesn't get excuted.

bostonpete• 15 years ago

"since you have to check every single line of code (indeed, every sub-expression) and think about what exceptions it might raise and how your code will react to it"

Yes, that's my concern with exceptions as well. It seems like the Java model (if I remember it right -- it's been a decade), which requires a method to either handle an exception that a sub-method throws or explicitly allow it to be thrown, would be preferable and help people avoid accidentally ignoring an exception.

I'd like to see support for exceptions like this in C++0X, but I haven't bothered to check to see if it's there...

snprbob862• 15 years ago

You're thinking of "checked exceptions" and C++ already has them (pre 0X) via the "throws" clause on method declarations.

Checked exceptions is a hotly debatted topic and I think the world has kinda finally come around to deciding that the are a bad idea overall. Google it and see for yourself.

SoftwareMaven2• 15 years ago

Requiring all exceptions to be checked is evil. It leads to horrendous abstraction leakage (I need to let every caller know I use FooBarWidget in the core of my application??), improper error handling (just catch whatever comes out and move on so I don't have to declare it), or ubiquitous error wrapping (every class has a try...catch that turns the called exceptions into new exception types to throw).

Ironic that C++ is moving towards that as the Java community is moving away (by relying more on RuntimeException, which isn't checked).

AshleysBrain3• 15 years ago

This reads to me like 'writing good code is hard, and writing even better code is even harder'. With modern C++0x smart pointers (like unique_ptr) and RAII, you can get very high performance and (more) easily exception-safe code. Maybe a lot of C++ exception criticism comes from people still trying to code like C in C++?

marshray• 15 years ago

Exactly.

svlla3• 15 years ago

"modern C++" has changed meanings so many times in the past 10 years that it's not funny anymore. there's always some "modern" solution in C++ that ends up causing more problems, leading to further "modern" solutions that end up... you get my point.

some folks have decided to get off the C++ feature treadmill and go back to, well... getting things done with solid languages (e.g. C) instead of learning about the latest C++ non-solutions to non-problems.

cpeterso3• 15 years ago

But your exception-safe C++ code soon becomes bloated with shared_ptr and unique_ptr templates and copy ctors. Almost any C++ function, overloaded operator, or some implicit temporary object's ctor can throw an exception without warning. You must use RAII everywhere for every "resource".

Exception-safe RAII is pretty much an all-or-nothing affair.

AshleysBrain3• 15 years ago

Today with terabyte harddrives, gigabytes of RAM and broadband connections, when is the binary size a more important factor than both execution speed and ease of development? Especially when the binary size difference is probably not huge?

Shouldn't the advice of this article just be "use exceptions"?

scott_s• 15 years ago

Not all development targets desktops or laptops.

svlla3• 15 years ago

Cache sizes have not seen gains proportional to RAM or HDs.

T-hawk• 15 years ago

Sure they have.

My 486 built in 1993 had 8 KB cache, 4 MB RAM, and a 120 MB HD.

My desktop built in 2009 has 2 MB cache, 2 GB RAM, and a 250 GB HD.

Okay, the cache has lagged behind by one or three doublings compared to the other storage types. But that's still pretty close to proportional in a world of exponential gains.

pmjordan2• 15 years ago

when is the binary size a more important factor than both execution speed and ease of development?

Binary size (or maybe more accurately in this case, binary code layout) can be highly relevant for speed due to the instruction cache.

As for ease of development, there are issues with C++ exceptions regarding this as well: some C++ libraries aren't exception safe, and neither are practically all C libraries. This is something you need to worry about whenever you pass a function pointer into a library, as there might be an exception-unsafe function higher up in the stack. Propagating an exception up through it is potentially extremely dangerous.

That said, using exceptions can still be a good idea, especially if your code doesn't need to be portable or if you know the platforms in advance, and you are careful about passing around function pointers. All you need to do is ensure that any of your code that might be called from third-party code with questionable exception semantics won't throw or propagate any exceptions, e.g. by installing a catch-all exception handler in it.

barrkel• 15 years ago

Binary working set sizes are often lower with exceptions than without, because exception handling code can be moved elsewhere by the compiler. Error-checking code, on the other hand, cannot be so easily detected, and hence moved.

I think the C++ implementation of exceptions has a lot to answer for though, in poisoning too many developers on the concept. It really is an awful implementation.

pilif3• 15 years ago

Symbian uses their own kind-of-exceptions (called TRAP, I think) and I've heard that decision to not use C++ exception was funded on binary size constraints.

cpeterso3• 15 years ago

Symbian LEAVEs and TRAPs predate C++ exceptions. Symbian/EPOC is old! :)

zwieback• 15 years ago

On my current platform (STM32L micro) we'll have 256K flash, 48K RAM, no hard drive. It's very reasonable to use C++ on such a processor but exception handling might not be something you want to pay for.

faragon3• 15 years ago

It is not. Unless for trivial things, using C++ for a system with 48KB of RAM is completely non sense (48KB is quite good amount of memory for plain C but not for C++).

Jach• 15 years ago

Space is time.

From the Gentoo wiki: -Os is very useful for large applications, like Firefox, as it will reduce load time, memory usage, cache misses, disk usage etc. Code compiled with -Os can be faster than -O2 or -O3 because of this. It's also recommended for older computers with a low amount of RAM, disk space or cache on the CPU. But beware that -Os is not as well tested as -O2 and might trigger compiler bugs.

I believe Apple compiles (a lot or all?) of their stuff with -Os.

Anyway C++ exceptions are awful. ;)

yason• 15 years ago

How many hundreds of megabytes or gigabytes is our operating system installation?

Size matters a lot because harddrives are stinkin' snails when compared to CPU and RAM. All that stuff needs to be loaded from somewhere, and while SSD's have changed the scheme a bit, there's still a major gap between storage and memory.

Quarrelsome2• 15 years ago

Nice article but as far as I'm concerned you don't need to _prove_ this as it is a logical fallacy to start with.

If an exception is being thrown then something is wrong, if something isn't wrong then you implemented your exceptions incorrectly as exceptions shouldn't exist in normal program flow.

So to recap, you're writing a crap ton of more code just so you can return your error code _slightly_ faster than it would take an exception. You're optimising your failure cases, which (in the _vast_ majority of cases) is UTTERLY ABSURD.

shin_lao• 15 years ago

It's not slightly faster, it can be an order of magnitude faster.

Example: a listen loop which handles disconnections through exceptions. This isn't stupid but it's not very efficient.

Quarrelsome2• 15 years ago

Why is it not stupid? If disconnections are part of normal application flow then why would you use an exception?

You are correct, I was somewhat disingenuous with _slightly_ faster. It is lots faster but lots faster in error cases, which from a philosophical angle is still absurd.

As long as you use your exceptions for "bad shit" (uncommon error conditions or completely unexpected failures or returns) then I still strongly believe that the performance comparison is silly.

anonymous246• 15 years ago

Maybe I'm missing something. Are you saying that in your application, handling rarely-occuring unanticipated disconnections via exceptions has such high overhead that its results in unacceptable performance?

aplusbi• 15 years ago

I think you partially missed the point - it's not that throwing exceptions is slow, is that even having them in your code is [allegedly] slow.

According to the article there are two methods used to implement exceptions in C++ - one that has higher overhead when you throw an exception (zero-cost) and one that has higher overhead when you call a function that might throw an exception (setjmp/longjmp).

Unfortunately the author didn't go over the latter method, which would have been more interesting.

pilif3• 15 years ago

I assume he was proving a point where main() of his full program using error code checking was not in fact checking the return value of foo()

Even in simple example code like this you can forget a check. In this case that result would be undefined if any call to devide failed.

I'd much rather have my program blow up with a readable stack trace pointing to where it happened than it working with a basically random value and then maybe blowing up somewhere totally unrelated or worse, destroying user data.

davidsiems3• 15 years ago

You can accomplish this by using asserts in your code.

You don't need exceptions to get a callstack and you can assert that values are valid and force a crash / callstack dump when they're not.

On top of that, you can compile the asserts out for release builds if you're confident they won't be hit.

pilif3• 15 years ago

Sure I can use asserts. But I'm as likely to forget the assert() as I am to forget to check the return value.

And even if I did: If you consider the faulty main() in the linked article: How would you use assert() there to make sure that result as used after the call to foo() is actually usable? If foo() returns -1 (because any of the calls to divide returned -1) then result is undefined.

dexen• 15 years ago

Normally you turn assert() off for production buids, so it's not a robust solution, as some error conditions may not get generated in testing.

The memory protection catches a lot of out-of-bounds memory references pretty well, and if you enable core dumps, you can extract neat backtrace from the core file (provided your routines fail-forward in case of invalid arguments). Moreover, some compilers can be instructed to instrument your code & data, including stack, with guard data, meant to trip process if it accesses wrong memory region. GNU malloc does some guardians if you sest $MALLOC_CHECK_.

If you aren't worried about vendor lock-in, you can use GCC's __attribute__((warn_unused_result)) [1]

[1] http://sourcefrog.net/weblog/software/languages/C/warn-unuse...

JoeAltmaier2• 15 years ago

Perhaps the worst problem with checking-vs-exceptions is, either solution dominates your code structure, obscuring the algorithm logic.

The holy grail would be some method of ensuring the code cannot fail e.g. weirdly constrained argument semantics. Thus separating algorithm from constraints instead of shuffling them together on the page like a deck of cards.

johnny5313• 15 years ago

If only c++ had some sort of static type system which could be leveraged to provide compile-time checks...

But seriously, this is a large part of the power of c++'s type system. Taking the article's example, if the argument types were of (user class) 'non_zero_float', there's no possibility for error.

You still have to check that your input is non-zero at some point, but you've now focused it into one place (the 'non_zero_float' class ctor), and other chunks of your program depending on those type semantics no longer need to worry about it.

JoeAltmaier2• 15 years ago

You can really make that type do a compile-time check on runtime values?

It would be better to have some way of getting the compiler to optimize constraints, perhaps by proving at compile time that the error is impossible.

adrianN• 15 years ago

You can't prevent exceptions when you do IO or dynamically allocate memory.

gersh• 15 years ago

A few issues. Does he actually benchmark? Things don't always work in practice the way you would think.

If you are going for ultra-high performance, do you even have error-checking? Do you write it in assembler?

Real cost of C++ exception vs error-code checking

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