How Much Performance Does RTTI Cost in C ?

RTTI: A Quantitative Perspective

While it is widely acknowledged that using RTTI in C incurs a performance hit, specific details regarding its magnitude have remained elusive. This article aims to shed light on RTTI's resource consumption, addressing the need for quantitative data.

To optimize runtime performance, it is recommended to use static_cast instead of dynamic_cast whenever possible. The former involves a single comparison of std::type_info, while the latter necessitates traversing an inheritance tree and performing additional comparisons.

RTTI Usage in GCC

In GCC, RTTI follows a specific ABI (Application Binary Interface) for Linux and BSD platforms. This ABI provides consistent and unique typeid() objects for each type, enabling efficient type comparisons. The runtime memory usage for RTTI is minimal, as the class vtable typically includes a pointer to a per-type RTTI structure.

An experiment involving GCC 4.4.3 revealed that disabling RTTI using -fno-rtti actually increased the binary size of a test program. This suggests that GCC's STL implementation behaves differently without RTTI, potentially due to the reliance on exceptions.

Implications for Embedded Systems

For embedded systems with limited RAM, RTTI usage should be carefully considered. While the memory overhead is negligible in GCC's preferred ABI, the performance impact of traversing inheritance trees and comparing std::type_info objects can be significant.

In such scenarios, it is crucial to evaluate the necessity of RTTI and explore alternative design approaches that minimize its usage. If RTTI is deemed essential, consider implementing static factory methods or virtual function dispatch to reduce the runtime overhead.

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