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"Using #pragma once instead of include guards will typically increase
compilation speed since it is a higher-level mechanism; the compiler
itself can compare filenames or inodes without having to invoke the C
preprocessor to scan the header for #ifndef and #endif.

Some compilers such as GCC and Clang include special speedup code to
recognize and optimize the handling of include guards, and thus little
or no speedup benefit is obtained from the use of #pragma once."