ARM Compiler toolchain v4.1 for µVision Using the Compiler

Compiler Coding Practices
5.36 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, it is recommended that you always
pack individual fields of the structure, rather than the entire structure itself.
Note
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.
5.36.1 See also
Concepts
• Unaligned fields in structures on page 5-47.
Reference
Compiler Reference:
• #pragma pack(n) on page 5-56
• __packed on page 5-9.
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