Datasheet

ARM Compiler Reference

Example 3-2 Pointer to packed

__packed int *p

There are no packed array types. A packed array is an array of objects of

packed type. There is no padding in the array.

Note

On ARM processors, access to unaligned data can take up to seven

instructions and three work registers. Data accesses through packed

structures must be minimized to avoid increase in code size, and

performance loss.

The

__packed

qualifier is useful to map a structure to an external data

structure, or for accessing unaligned data, but it is generally not useful to

save data size because of the relatively high cost of access. The number

of unaligned accesses can be reduced by only packing fields in a structure

that requires packing.

When a packed object is accessed using a pointer, the compiler generates

code that works and that is independent of the pointer alignment

(Example 3-3).

Example 3-3 Packed structure

typedef __packed struct

{

char x; // all fields inherit the __packed qualifier

int y;

}X; // 5 byte structure, natural alignment = 1

int f(X *p)

{

return p->y; // does an unaligned read

}

typedef struct

{

short x;

char y;

__packed int z; // only pack this field

char a;

}Y; // 8 byte structure, natural alignment = 2

int g(Y *p)

{

return p->z + p->x; // only unaligned read for z

}