On 03/31/2011 04:14 PM, Russell King wrote:
> On Thu, Mar 31, 2011 at 04:09:32PM +0200, Jiri Slaby wrote:
>> On 03/31/2011 03:51 PM, Russell King wrote:
>>> On Thu, Mar 31, 2011 at 03:48:20PM +0200, Jiri Slaby wrote:
>>>> How it can? Packed only tells the compiler to have one byte alignment
>>>> steps. And it should not matter here as all the members are 32-bit long.
>>>
>>> It also tells the compiler that it may be misaligned, so to avoid
>>> alignment faults it will use byte loads/stores.
>>
>> No, it should not (if offsetof % sizeof == 0). Otherwise it's a bug in
>> the compiler.
>>
>> Am I missing something?
>
> $ cat t1.c
> struct foo {
> unsigned long bar;
> } __attribute__((packed));
>
> unsigned long baz(struct foo *f)
> {
> return f->bar;
> }
> $ arm-linux-gcc -O2 -S -o - t1.c
> ...
> .global baz
> .type baz, %function
> baz:
> @ args = 0, pretend = 0, frame = 0
> @ frame_needed = 0, uses_anonymous_args = 0
> @ link register save eliminated.
> ldrb r3, [r0, #0] @ zero_extendqisi2
> ldrb r2, [r0, #1] @ zero_extendqisi2
> ldrb r1, [r0, #2] @ zero_extendqisi2
> orr r3, r3, r2, asl #8
> ldrb r0, [r0, #3] @ zero_extendqisi2
> orr r3, r3, r1, asl #16
> orr r0, r3, r0, asl #24
> mov pc, lr
> .size baz, .-baz
> .ident "GCC: (GNU) 4.3.5"
> .section .note.GNU-stack,"",%progbits
>
> It does this because with the packed attribute, it can't make assumptions
> about the alignment of 'f'.
I don't see why it couldn't like without the packed attribute. (The
manual says packed attribute for a struct is equivalent to specifying
packed to each member of that struct. It doesn't say anything about the
structure alignment itself.)
Ok, not that I'm fully convinced, but it doesn't matter.
So let's NOT mark it packed.
Anyway the info you sent was useful for me.
thanks,
--
js
suse labs
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