[U-Boot] __attribute__((__packed__)) switching to byte-access on ARM

[Scott Wood]

On Mon, Aug 11, 2008 at 05:48:21PM +0200, Guennadi Liakhovetski wrote:
> The respective assembly code is quite simple:
> 
> 00000000 <read_timer>:
>    0:	e59f3004 	ldr	r3, [pc, #4]	; c <timer_load_val>
>    4:	e5930040 	ldr	r0, [r3, #64]
>    8:	e12fff1e 	bx	lr
>    c:	7f006000 	swivc	0x00006000
> 
> Perfect. Now, if I make the two structs above packed, I get this assembly:
> 
> 00000000 <read_timer>:
>    0:	e59f301c 	ldr	r3, [pc, #28]	; 24 <.text+0x24>
>    4:	e5d30040 	ldrb	r0, [r3, #64]
>    8:	e5d32041 	ldrb	r2, [r3, #65]
>    c:	e5d31042 	ldrb	r1, [r3, #66]
>   10:	e1800402 	orr	r0, r0, r2, lsl #8
>   14:	e5d33043 	ldrb	r3, [r3, #67]
>   18:	e1800801 	orr	r0, r0, r1, lsl #16
>   1c:	e1800c03 	orr	r0, r0, r3, lsl #24
>   20:	e12fff1e 	bx	lr
>   24:	7f006000 	swivc	0x00006000
> 
> which converts all accesses to the structure to 8-bit... Ideas? Toolchain 
> eldk-4.1. U-Boot from nand/testing git with the patches I just sent to the 
> list, configured for smdk6400_config.

Packed implies a lack of alignment.  Since ARM can't do unaligned
accesses in a "normal" way, the compiler avoids them.

Why do you need to pack the struct?

-Scott