arm questions

[Scott Bambrough]

Ian Lance Taylor wrote:
> 
>    Date: Thu, 15 Apr 1999 10:38:25 -0700
>    From: Doug Evans <devans@cygnus.com>
> 
>    Two random questions.
> 
>    1)
>    elfarm-oabi.c uses USE_RELA
>    elfarm-nabi.c uses USE_REL
>    That seems rather backwards, doesn't it?
>    I'd expect a new version of an abi to switch from USE_REL to USE_RELA,
>    but not the other way around.
>    [not that it can be changed, but rather ... why the switch?]
> 
> This is my understanding of the situation.  If anybody knows that I am
> wrong, please correct me.
> 
> The ARM ELF situation is a mess because there were two separate
> implementations.
> 
> Some folks in England put together ARM GNU/Linux, but they took a very
> long time to get the copyright stuff done.  For some reason that I do
> not know, that implementation used REL rather than RELA relocs.

Actually Phil is in England, Pat Beirne and I are in Canada.  And that very long
copyright time was due entirely to Corel's legal department.  They were a pain
in the ass.  We chose REL because we did not need the addend.  In hindsight I
would choose RELA.  We did this work over a period of about 6 months when we
brought up gcc, binutils, gdb and glibc on the NetWinder.

> In the meantime, Cygnus implemented ARM ELF, using RELA relocs.

Something of which we were entirely unaware of at the time, otherwise we would
have attempted to collaborate in a closer fashion.

> It's obviously desirable to have a single ARM ELF ABI, and the best
> choice is probably the one currently being used for GNU/Linux.
> Therefore, when we straightened out the copyright situation, we dumped
> the Cygnus RELA implementation in favour of the GNU/Linux REL
> implementation.

I don't know about better.  Cygnus does pretty good work.  It probably has a
larger installed base ATM, as the GNU/Linux implementation is used by HJ Lu's
2.9.1.0.xx releases of binutils.  I think most of the ARM community uses this
rather than the GAS2 snapshots.

>    Whats the R_ARM_THM_XPC22 reloc for?

The docs say it is a Thumb BLX pair.  It is a two byte Thumb instruction.  It is
a relocateable branch instruction, where bits 0-10 encode the 11 most
significant bits of the branch offset, and bits 0-10 of the next instruction
word encode the least significant bits of the branch offset.  There is also a
note about it being provisional.  The least significant bit of the branch offset
must be zero.

Scott Bambrough