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Attributes [[likely]] and
[[unlikely]]
선언문을 제외한 label 과 statements 에서 최적화를 위해 사용
(https://wikidocs.net/51311)
[[likely]] : 빈도가 높음
[[unlikely]] : 빈도가 낮음
case 2 인 경우에는 최적화가 되지만 case 1 인 경우에는 최적화되지 않는다.
(fallthrough 무시)
성능 비교
https://en.cppreference.com/w/cpp/language/attributes/likely

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