ARM Compiler toolchain v4.1 for µVision Using the Compiler

Compiler Coding Practices
5.39 Detailed comparison of an unpacked struct, a __packed struct, and a struct
with individually __packed fields
The differences between not packing a struct, packing an entire struct, and packing individual
fields of a struct are illustrated by the three implementations of a struct shown in Table 5-10.
In the first implementation, the struct is not packed. In the second implementation, the entire
structure is qualified as __packed. In the third implementation, the __packed attribute is removed
from the structure and the individual field that is not naturally aligned is declared as __packed.
Table 5-10 C code for an unpacked struct, a packed struct, and a struct with individually
packed fields
Unpacked struct __packed struct __packed fields
struct foo
{
char one;
short two;
char three;
int four;
} c;
__packed struct foo
{
char one;
short two;
char three;
int four;
} c;
struct foo
{
char one;
__packed short two;
char three;
int four;
} c;
Table 5-11 shows the corresponding disassembly of the machine code produced by the compiler
for each of the sample implementations of Table 5-10, where the C code for each
implementation has been compiled using the option -O2.
Note
The -Ospace and -Otime compiler options control whether accesses to unaligned elements are
made inline or through a function call. Using -Otime results in inline unaligned accesses. Using
-Ospace results in unaligned accesses made through function calls.
Table 5-11 Disassembly for an unpacked struct, a packed struct, and a struct with
individually packed fields
Unpacked struct __packed struct __packed fields
; r0 contains address of c
; char one
LDRB r1, [r0, #0]
; short two
LDRSH r2, [r0, #2]
; char three
LDRB r3, [r0, #4]
; int four
LDR r12, [r0, #8]
; r0 contains address of c
; char one
LDRB r1, [r0, #0]
; short two
LDRB r2, [r0, #1]
LDRSB r12, [r0, #2]
ORR r2, r12, r2, LSL #8
; char three
LDRB r3, [r0, #3]
; int four
ADD r0, r0, #4
BL __aeabi_uread4
; r0 contains address of c
; char one
LDRB r1, [r0, #0]
; short two
LDRB r2, [r0, #1]
LDRSB r12, [r0, #2]
ORR r2, r12, r2, LSL #8
; char three
LDRB r3, [r0, #3]
; int four
LDR r12, [r0, #4]
In the disassembly of the unpacked struct in Table 5-11, the compiler always accesses data on
aligned word or halfword addresses. The compiler is able to do this because the struct is padded
so that every member of the struct lies on its natural size boundary.
In the disassembly of the __packed struct in Table 5-11, fields one and three are aligned on their
natural size boundaries by default, so the compiler makes aligned accesses. The compiler
always carries out aligned word or halfword accesses for fields it can identify as being aligned.
For the unaligned field two, the compiler uses multiple aligned memory accesses
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