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Are Exceptions in C++ really slow

Posted by: admin November 30, 2017 Leave a comment

Questions:

I was watching Systematic Error Handling in C++—Andrei Alexandrescu he claims that Exceptions in C++ are very very slow.

I want to know is this still true for C++98

Answers:

The main model used today for exceptions (Itanium ABI, VC++ 64 bits) is the Zero-Cost model exceptions.

The idea is that instead of losing time by setting up a guard and explicitly checking for the presence of exceptions everywhere, the compiler generates a side table that maps any point that may throw an exception (Program Counter) to the a list of handlers. When an exception is thrown, this list is consulted to pick the right handler (if any) and stack is unwound.

Compared to the typical if (error) strategy:

  • the Zero-Cost model, as the name implies, is free when no exception occurs
  • it cost around 10x/20x an if when an exception does occur

The cost, however, is not trivial to measure:

  • The side-table is generally cold, and thus fetching it from memory takes a long time
  • Determining the right handler involves RTTI: many RTTI descriptors to fetch, scattered around memory, and complex operations to run (basically a dynamic_cast test for each handler)

so, mostly cache misses, and thus not trivial compared to pure CPU code.

Note: for more details, read the TR18015 report, chapter 5.4 Exception Handling (pdf)

So, yes, exceptions are slow on the exceptional path, but they are otherwise quicker than explicit checks (if strategy) in general.

Note: according to NoSenseEtAl, Andrei Alexandrescu seems to question this “quicker”. I personally measured a speed-up in my programs and I have yet to see a proof about the loss of optimizability.


Does it matter ?

I would claim it does not. A program should be written with readability in mind, not performance (at least, no as first criterion). Exceptions are to be used when one expects that the caller cannot or will not wish to handle the failure on the spot, and pass it up the stack. Bonus: in C++11 exceptions can be marshalled between threads using the Standard Library.

This is subtle though, I claim that map::find should not throw but I am fine with map::find returning a checked_ptr which throws if an attempt to dereference it fail because it’s null: in the latter case, as in the case of the class that Alexandrescu introduced, the caller chooses between explicit check and relying on exceptions. Empowering the caller without giving him more responsibility is usually a sign of good design.

Questions:
Answers:

When the question was posted I was on my way to the doctor, with a taxi waiting, so I only had time then for a short comment. But having now commented and upvoted and downvoted I’d better add my own answer. Even if Matthieu’s answer already is pretty good.


Are exceptions especially slow in C++, compared to other languages?

Re the claim

“I was watching Systematic Error Handling in C++—Andrei Alexandrescu he claims that Exceptions in C++ are very very slow.”

If that’s literally what Andrei claims, then for once he’s very misleading, if not downright wrong. For a raised/thrown exceptions is always slow compared to other basic operations in the language, regardless of the programming language. Not just in C++ or more so in C++ than in other languages, as the purported claim indicates.

In general, mostly regardless of language, the two basic language features that are orders of magnitude slower than the rest, because they translate to calls of routines that handle complex data structures, are

  • exception throwing, and

  • dynamic memory allocation.

Happily in C++ one can often avoid both in time-critical code.

Unfortunately There Ain’t No Such Thing As A Free Lunch, even if the default efficiency of C++ comes pretty close. 🙂 For the efficiency gained by avoiding exception throwing and dynamic memory allocation is generally achieved by coding at a lower level of abstraction, using C++ as just a “better C”. And lower abstraction means greater “complexity”.

Greater complexity means more time spent on maintenance and little or no benefit from code reuse, which are real monetary costs, even if difficult to estimate or measure. I.e., with C++ one can, if so desired, trade some programmer efficiency for execution efficiency. Whether to do so is largely an engineering and gut-feeling decision, because in practice only the gain, not the cost, can be easily estimated and measured.


Are there any objective measures of C++ exception throwing performance?

Yes, the international C++ standardization committee has published a Technical Report on C++ performance, TR18015.


What does it mean that exceptions are “slow”?

Mainly it means that a throw can take a Very Long Time™ compared to e.g. an int assignment, due to the search for handler.

As TR18015 discusses in its section 5.4 “Exceptions” there are two principal exception handling implementation strategies,

  • the approach where each try-block dynamically sets up exception catching, so that a search up the dynamic chain of handlers is performed when an exception is thrown, and

  • the approach where the compiler generates static look-up tables that are used to determine the handler for a thrown exception.

The first very flexible and general approach is almost forced in 32-bit Windows, while in 64-bit land and in *nix-land the second far more efficient approach is commonly used.

Also as that report discusses, for each approach there are three main areas where exception handling impacts on efficiency:

  • try-blocks,

  • regular functions (optimization opportunities), and

  • throw-expressions.

Mainly, with the dynamic handler approach (32-bit Windows) exception handling has an impact on try blocks, mostly regardless of language (because this is forced by Windows’ Structured Exception Handling scheme), while the static table approach has roughly zero cost for try-blocks. Discussing this would take a lot more space and research than is practical for an SO answer. So, see the report for details.

Unfortunately the report, from 2006, is already a little bit dated as of late 2012, and as far as I know there’s not anything comparable that’s newer.

Another important perspective is that the impact of use of exceptions on performance is very different from the isolated efficiency of the supporting language features, because, as the report notes,

“When considering exception handling, it must be contrasted to alternative ways of
dealing with errors.”

For example:

  • Maintenance costs due to different programming styles (correctness)

  • Redundant call site if failure checking versus centralized try

  • Caching issues (e.g. shorter code may fit in cache)

The report has a different list of aspects to consider, but anyway the only practical way to obtain hard facts about the execution efficiency is probably to implement the same program using exception and not using exceptions, within a decided cap on development time, and with developers familiar with each way, and then MEASURE.


What is a good way to avoid the overhead of exceptions?

Correctness almost always trumps efficiency.

Without exceptions, the following can easily happen:

  1. Some code P is meant to obtain a resource or compute some information.

  2. The calling code C should have checked for success/failure, but doesn’t.

  3. A non-existent resource or invalid information is used in code following C, causing general mayhem.

The main problem is point (2), where with the usual return code scheme the calling code C is not forced to check.

There are two main approaches that do force such checking:

  • Where P directly throws an exception when it fails.

  • Where P returns an object that C has to inspect before using its main value (otherwise an exception or termination).

The second approach was, AFAIK, first described by Barton and Nackman in their book *Scientific and Engineering C++: An Introduction with Advanced Techniques and Examples, where they introduced a class called Fallow for a “possible” function result. A similar class called optional is now offered by the Boost library. And you can easily implement an Optional class yourself, using a std::vector as value carrier for the case of non-POD result.

With the first approach the calling code C has no choice but to use exception handling techniques. With the second approach, however, the calling code C can itself decide whether to do if based checking, or general exception handling. Thus, the second approach supports making the programmer versus execution time efficiency trade-off.


What is the impact of the various C++ standards, on exception performance?

“I want to know is this still true for C++98”

C++98 was the first C++ standard. For exceptions it introduced a standard hierarchy of exception classes (unfortunately rather imperfect). The main impact on performance was the possibility of exception specifications (removed in C++11), which however were never fully implemented by the main Windows C++ compiler Visual C++: Visual C++ accepts the C++98 exception specification syntax, but just ignores exception specifications.

C++03 was just a technical corrigendum of C++98. The only really new in C++03 was value initialization. Which has nothing to do with exceptions.

With the C++11 standard general exception specifications were removed, and replaced with the noexcept keyword.

The C++11 standard also added support for storing and rethrowing exceptions, which is great for propagating C++ exceptions across C language callbacks. This support effectively constrains how the current exception can be stored. However, as far as I know that does not impact on performance, except to the degree that in newer code exception handling may more easily be used on both sides of a C language callback.

Questions:
Answers:

It depends on the compiler.

GCC, for example, was known for having very poor performance when handling exceptions, but this got considerably better in the past few years.

But note that handling exceptions should – as the name says – be the exception rather than the rule in your software design. When you have an application which throws so many exceptions per second that it impacts performance and this is still considered normal operation, then you should rather think about doing things differently.

Exceptions are a great way to make code more readable by getting all that clunky error handling code out of the way, but as soon as they become part of the normal program flow, they become really hard to follow. Remember that a throw is pretty much a goto catch in disguise.

Questions:
Answers:

Like in silico said its implementation dependent, but in general exceptions are considered slow for any implementation and shouldn’t be used in performance intensive code.

EDIT: I’m not saying don’t use them at all but for performance intensive code it is best to avoid them.

Questions:
Answers:

Yes, but that doesn’t matter.
Why?
Read this:
https://blogs.msdn.com/b/ericlippert/archive/2008/09/10/vexing-exceptions.aspx

Basically that says that using exceptions like Alexandrescu described (50x slowdown because they use catch as else) is just wrong.
That being said for ppl who like to do it like that
I wish C++22 🙂 would add something like:
(note this would have to be core language since it is basically compiler generating code from existing one)

result = attempt<lexical_cast<int>>("12345");  //lexical_cast is boost function, 'attempt'
//... is the language construct that pretty much generates function from lexical_cast, generated function is the same as the original one except that fact that throws are replaced by return(and exception type that was in place of the return is placed in a result, but NO exception is thrown)...     
//... By default std::exception is replaced, ofc precise configuration is possible
if (result)
{
     int x = result.get(); // or result.result;
}
else 
{
     // even possible to see what is the exception that would have happened in original function
     switch (result.exception_type())
     //...

}

P.S. also note that even if the exceptions are that slow… it is not a problem if you dont spend a lot of time in that part of the code during execution… For example if float division is slow and you make it 4x faster that doesnt matter if you spend 0.3% of your time doing FP division…