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

[Guennadi Liakhovetski]

On Mon, 11 Aug 2008, Scott Wood wrote:

> 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?

I don't. These structs are already filled with dummy bytes. So it also 
works without the packed attribute. Why I tried to enable it was because 
the attribute was in the original code from Samsung, but was commented 
out, so, I wanted to find out what happens if I re-enable it. Besides, the 
packed attribute serves as a hint in the source, that this struct has 
special requirements for its in-memory representation.

Thanks
Guennadi
---
Guennadi Liakhovetski, Ph.D.

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