Templates

Templates

C++ templates can be used for compile-time optimizations, rather than merely as a programming convenience for algorithm generality and interface improvement. By specializing templated code for a particular type or constant parameter, the effect is that the resulting code is more specific, giving the compiler an opportunity for better optimizations.

For example, in AI we need vector and matrix classes. Rather than having our code dynamically check whether our precision is 32-bit float, or 8-bit quantized integers, or some other low-level type, we can use templated versions of the vector and matrix classes. This generates different functions for each type of data. At the cost of some extra code space, we've given the compiler the chance to do a much better job of optimizing the code for the specific low-level data types.

Going beyond just using template code to write the same algorithm for different types, there are various ways to optimize code that is templated to do more at compile-time:

• Template class and function specializations
• Constant template parameters
• Compile-time conditional tests on types (e.g. sizeof, type traits, etc.)
• if constexpr syntax
• Variadic templates
• Template Metaprogramming (TMP) techniques
• SFINAE techniques

Constants can be used to instantiate template code in a way that helps the compiler to optimize by evaluating constant expressions. Template parameters don't need to be types, but can also be constant variables or numbers, such as the size of an array. Using a template in this way is as efficient as hard-coding the array size, which helps the compiler to know exactly what it can optimize, such as if the array size is used in any computations.

If you think you can do better than the compiler's optimizer, remember that you can also override the generic template code. For example, you can instantiate your own specific version of a template class for a particular type. Similarly, you can provide a generic function declaration that instantiates a templated function with your explicit version.

An alternative to specializing a version of a template class or function is to use compile-time tests inside the generic template code. For example, you can use conditional tests involving compile-time operations:

• sizeof
• typeid
• std::is_same_v
• if constexpr conditional test syntax

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