On Sun, 2021-04-18 at 17:10 +0300, Matwey V. Kornilov via Gcc-help
wrote:
>
> Hi,
>
> When a structure is declared as `__attribute__ ((packed))' what should
> it alignment be? Unfortunately, I have not found clear statement in
> the
> docs, but I've found the following very inconsistent behavior instead.
> It is not clear to me to what extent `__attribute__ ((packed))' should
> break alignment rules.
It depends on platform ABI.
> Imagine the following code:
>
> typedef struct {
> unsigned char a;
> unsigned char b;
> unsigned short c;
> unsigned int d;
> unsigned long e;
> } __attribute__ ((packed)) s;
>
> s x = {
> .a = 1,
> .b = 2,
> .c = 3,
> .d = 4,
> .e = 5
> };
/* snip */
> Then, using `objdump -x test.o', I examine `.data.x` section
> Alignment:
>
> A. 3 .data.x 00000010 0000000000000000 0000000000000000
> 00000040 2**3
>
> B. 3 .data.x 00000010 0000000000000000 0000000000000000
> 00000040 2**3
>
> C. 3 .data.x 00000010 0000000000000000 0000000000000000
> 00000040 2**3
>
> D. 3 .data.x 00000010 0000000000000000 0000000000000000
> 00000040 2**0
>
>
> Note, that -Os on gcc 11 forces the Aligment to 1 byte. I've found
> this
> when tried to build some vendor tree of u-boot bootloader. Bumping
> version from gcc 7 to gcc 11 breaks the firmware image.
>
The Arm Architecture ABI [says][1]:
> There is no minimum alignment required for a section. Sections
> containing code must be at least 4-byte aligned. Platform standards may
> set a limit on the maximum alignment that they can guarantee (normally
> the minimum page size supported by the platform).
So -fdata-sections won't introduce more alignment.
[And][2]:
> The member alignment of an element of a composite type is the
> alignment of that member after the application of any language
> alignment modifiers to that member
and
> The alignment of an aggregate shall be the alignment of its most-
> aligned member.
so the alignment of a packed struct is 1 byte (as each of its member is
1-byte aligned).
[1]:
https://github.com/ARM-software/abi-aa/blob/master/aaelf64/aaelf64.rst#section-alignment
[2]:
https://github.com/ARM-software/abi-aa/blob/f689c71830bb7687745a16ec4a0dd22126093266/aapcs64/aapcs64.rst#composite-types
--
Xi Ruoyao <xry111@xxxxxxxxxxxxxxxx>
School of Aerospace Science and Technology, Xidian University
