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Performance penalty associated with marking whole structures as packed

Reading from and writing to whole structures qualified with __packed requires unaligned accesses and can therefore incur a performance penalty.

When optimizing a struct that is packed, the compiler tries to deduce the alignment of each field, to improve access. However, it is not always possible for the compiler to deduce the alignment of each field in a __packed struct. In contrast, when individual fields in a struct are declared as __packed, fast access is guaranteed to naturally aligned members within the struct. Therefore, when the use of a packed structure is required, ARM recommends that you always pack individual fields of the structure, rather than the entire structure itself.


Declaring individual non-aligned fields of a struct as __packed also has the advantage of making it clearer to the programmer which fields of the struct are not naturally aligned.