alignment faults in 3.6
Måns Rullgård
mans at mansr.com
Thu Oct 11 06:46:11 EDT 2012
"David Laight" <David.Laight at ACULAB.COM> writes:
>> > It might be enough to use __attribute__((aligned(2))) on some structure
>> > members (actually does 'ldm' need 8 byte alignment?? - in which case
>> > aligned(4) is enough).
>>
>> The aligned attribute can only increase alignment.
>
> Not true.
> If you have:
>
> struct foo {
> short a;
> int b __attribute__((aligned(2)));
> short c;
> };
>
> You'll find sizeof (struct foo) is 8, and that, on arm/sparc etc,
> gcc will generate 2 16bit accesses (and shifts) for foo.b;
That is not what the manual says. Quoting:
When used on a struct, or struct member, the `aligned' attribute
can only increase the alignment
My gcc agrees with the manual:
$ cat foo.c
struct foo {
short a;
int b __attribute__((aligned(2)));
short c;
};
int foo(struct foo *f)
{
return f->b;
}
$ arm-unknown-linux-gnueabi-gcc-4.7.2 -O2 -o- -S foo.c
.text
.align 2
.global foo
.type foo, %function
foo:
ldr r0, [r0, #4]
bx lr
(compiler output edited for clarity)
This clearly says the 'b' member resides at an offset of 4 bytes into
the struct.
--
Måns Rullgård
mans at mansr.com
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