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'.
--
Russell King
Linux kernel 2.6 ARM Linux - http://www.arm.linux.org.uk/
maintainer of:
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