What is the Runtime Cost of RTTI in Embedded Systems?

The Cost of RTTI

While it's widely acknowledged that Runtime Type Information (RTTI) carries a performance cost, it's often difficult to find specific measurements or quantitative data. This becomes even more crucial when considering embedded systems with limited resources.

RTTI Performance Impact

The implementation of RTTI is compiler-dependent, leading to varying performance overhead. However, certain general observations can be made:

• GCC's preferred ABI: If your target platform uses the "vendor-neutral" C ABI preferred by GCC, RTTI support has a negligible space overhead as it leverages existing vtables. Additionally, typeid(a) == typeid(b) comparisons are exceptionally fast.
• Other platforms: For platforms not using the preferred ABI, RTTI support may incur additional runtime overhead.

Avoiding RTTI

For design reasons, it's recommended to avoid using RTTI whenever possible. However, there are instances when its use is necessary. In such cases, it's important to assess the resource implications carefully.

Quantitative Benchmarks

Despite claims that RTTI is expensive, reliable benchmarks are hard to come by. One study suggests that the memory overhead for RTTI is minimal, while the processor time overhead is implementation-specific.

Static vs. Dynamic Casting

To reduce runtime costs, consider leveraging static casting whenever possible:

<code class="cpp">if (typeid(a) == typeid(b)) {
  B* ba = static_cast<B*>(&a);
}</code>

This eliminates the need for inheritance traversal and unnecessary comparisons.

Ultimately, the cost-effectiveness of RTTI depends on the specific implementation and the constraints of your system. However, by understanding the underlying mechanisms and adopting appropriate coding practices, you can minimize the runtime impact of RTTI in embedded systems.

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