https://github.com/apple/swift/blob/b70cd7031fe7a2c400976007abbfff1597189174/stdlib/public/runtime/ImageInspectionMachO.cpp#L141-L145

void swift::initializeProtocolConformanceLookup() { REGISTER_FUNC( addImageCallback<TextSegment, ProtocolConformancesSection, addImageProtocolConformanceBlockCallback>); }

apple/swift

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#define REGISTER_FUNC(...) _dyld_register_func_for_add_image(__VA_ARGS__)

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http://mirror.informatimago.com/next/developer.apple.com/documentation/DeveloperTools/Conceptual/MachORuntime/5rt_api_reference/chapter_11_section_46.html

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When you call _dyld_register_func_for_add_image, the dynamic linker runtime calls the specified callback (func) once for each of the images that is currently loaded into the program. When a new image is added to the program, your callback is called again with the mach_header for the new image, and the virtual memory slide amount of the new image.