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SubjectRe: Subtleties of __attribute__((packed))
On Wed, Dec 06, Phil Endecott wrote:

> I don't think so. Example:
>
> struct test {
> int a __attribute__((packed));
> int b __attribute__((packed));
> };
>
> char c = 1;
> struct test t = { .a=2, .b=3 };
>
> $ arm-linux-gnu-gcc -O2 -S -W -Wall test1.c
>
> .file "test2.c"
> .global c
> .data
> .type c, %object
> .size c, 1
> c:
> .byte 1
> .global t
> .align 2 <<<<<<<<===== t is aligned
> .type t, %object
> .size t, 8
> t:
> .word 2
> .word 3
> .ident "GCC: (GNU) 4.1.2 20061028 (prerelease) (Debian 4.1.1-19)"
>
>
> Compare with:
>
> struct test {
> int a;
> int b;
> } __attribute__((packed));
>
> char c = 1;
> struct test t = { .a=2, .b=3 };
>
> $ arm-linux-gnu-gcc -O2 -S -W -Wall test2.c
>
> .file "test1.c"
> .global c
> .data
> .type c, %object
> .size c, 1
> c:
> .byte 1
> .global t <<<<<< "align" has gone, t is unaligned
> .type t, %object
> .size t, 8
> t:
> .4byte 2
> .4byte 3
> .ident "GCC: (GNU) 4.1.2 20061028 (prerelease) (Debian 4.1.1-19)"
>

Maybe the arm backend is somehow broken. AFAIK (and I verfied it on S390 and
i386) the alignment shouldn't change.

struct foo {
int a;
char b;
int c;
};

struct bar1 {
char a __attribute__((__packed__));
struct foo b __attribute__((__packed__));
};

struct bar2 {
char a;
struct foo b;
} __attribute__((__packed__));

struct bar3 {
char a;
struct foo b;
};

struct bar1 packed1 = { 10, { 20, 30, 40 } };
struct bar2 packed2 = { 50, { 60, 70, 80 } };
struct bar3 unpacked = { 90, { 100, 110, 120 } };

s390x-linux-gcc -S packed2.c:
packed2.c:9: warning: '__packed__' attribute ignored for field of type 'char'

.file "packed2.c"
.globl packed1
.data
.align 2
.type packed1, @object
.size packed1, 13
packed1:
.byte 10
.4byte 20
.byte 30
.zero 3
.4byte 40
.globl packed2
.align 2
.type packed2, @object
.size packed2, 13
packed2:
.byte 50
.4byte 60
.byte 70
.zero 3
.4byte 80
.globl unpacked
.align 4
.type unpacked, @object
.size unpacked, 16
unpacked:
.byte 90
.zero 3
.long 100
.byte 110
.zero 3
.long 120
.ident "GCC: (GNU) 4.1.2 20060531 (prerelease) (SUSE Linux)"
.section .note.GNU-stack,"",@progbits
-
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