Are C Exceptions Still a Performance Bottleneck?

Are Exceptions in C Still Slow?

Andrei Alexandrescu's claim that C exceptions are very slow has raised concerns about their efficiency in C 98. However, the current exception handling model known as Zero-Cost Model Exceptions addresses this issue.

In the Zero-Cost model, the compiler creates a side table that maps points where exceptions may be thrown to a list of handlers. When an exception occurs, this table is used to determine the appropriate handler and unwind the stack. This process is significantly faster than explicitly checking for exceptions at every point.

Compared to the traditional if (error) strategy, the Zero-Cost model:

• Is free when no exceptions occur
• Costs approximately 10x-20x an if statement when an exception does occur

However, performance measurement can be challenging due to factors such as cache misses and RTTI overhead.

Slow On the Exceptional Path

Despite the efficiency improvements, exceptions remain slow on the exceptional path. However, this is generally outweighed by their speed advantage over explicit checks on the majority of code paths.

Readability vs. Performance

Regardless of performance considerations, code readability should be prioritized. Exceptions should be used when the caller cannot or does not wish to handle failures locally. However, the C 11 standard allows for checked references that provide a more controlled approach to exception handling.

Conclusion

While exceptions may not be blazingly fast, their performance impact is minimal compared to the benefits they provide for readability and code organization. Developers should prioritize code clarity and ease of maintenance over premature optimization concerns.

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