What is the Performance Cost of Runtime Type Information (RTTI) in C ?

The Cost of RTTI: An In-Depth Analysis

RTTI, or Runtime Type Information, is a feature in C that allows you to determine the type of an object at runtime. While it is a powerful tool, it comes with a certain performance overhead. However, quantifying this overhead has proven elusive.

Performance Impact

The cost of RTTI varies depending on the implementation. However, some general observations can be made.

• Space Overhead: The RTTI structures themselves are usually small, and their memory footprint is negligible in most practical scenarios.
• Lookup Cost: The primary performance concern with RTTI is the time it takes to retrieve type information. This lookup involves traversing inheritance trees and comparing type identifiers.

Implementation Differences

GCC, the most widely used C compiler, has a vendor-neutral ABI that provides stable std::type_info objects across dynamic linking boundaries. This means that type comparisons using typeid(a) == typeid(b) are very fast on Linux, BSD, and other supported embedded platforms.

In contrast, mingw32-gcc follows the Windows C ABI, which does not guarantee the stability of std::type_info objects across DLLs. As a result, type comparisons in this case rely on strcmp, which is significantly slower.

Development Considerations

While RTTI provides certain advantages, it is generally advisable to avoid it for design reasons. However, if you have specific requirements that necessitate its use, understanding its performance characteristics can help you make informed decisions.

GCC-Specific Insights

In GCC, the use of RTTI increases the binary size of a simple test program by a few hundred bytes. This may seem counterintuitive, but it is likely due to adjustments made within the STL code in the absence of RTTI.

Conclusion

The cost of RTTI is implementation-specific and should be carefully considered when making design choices. By understanding the underlying mechanics and performance implications, developers can make informed decisions about whether and how to employ RTTI in their applications.

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