Reading from and writing to whole structures qualified with
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
In contrast, when individual fields in a struct are declared
__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
has the advantage of making it clearer to the programmer which fields
of the struct are not naturally aligned.