[PATCH] arm: boot/compressed: fix decompressor header layout for v7-M

[Dave Martin]

On Fri, May 19, 2017 at 06:16:36PM +0100, Robin Murphy wrote:
> On 19/05/17 17:59, Ard Biesheuvel wrote:
> > On 19 May 2017 at 17:46, Dave Martin <Dave.Martin at arm.com> wrote:

[...]

> >> Spelling it out might be more readable:
> >>
> >>    ARM(                 mov     r0, r0          )
> >>    ARM(                 mov     r0, r0          )
> >>
> >>  THUMB( AR_CLASS(       sub     pc, pc, #3      ))
> >>  THUMB( M_CLASS(        nop.w                   ))
> >>  THUMB(                 .thumb                  )
> >>  THUMB(                 b.w     1f              )
> >>
> >> But I guess it works either way.
> >>
> > 
> > Indeed. Apart from the error in the second line, this sequence should
> > be equivalent, but the nested macro invocations don't make it clearer
> > imo.
> > 
> > I could add some more comments as well if that helps?
> 
> Hmm, couldn't it be ISA-agnostic?
> 
> 	ldr pc, 1f
> 	.ltorg
> THUMB(	.thumb	)
> 	...
> 
> given that that should be interworking on anything where THUMB() is
> valid in the first place.

Did you mean

	ldr	pc, =BSYM(1f)
?

Otherwise this is quite neat: providing there are no pending literals
when the assembler reaches the ldr pc, and providing that the ldr pc is
word-aligned (it certainly should be) then that sequence should always
expand to two words.

Cheers
---Dave