[fedora-arm] [Fwd: Activity Day June 10th - ARMv7 F15 hardfp bringup]

[Jon Masters]

On Sun, 2011-06-05 at 22:35 -0400, Jon Masters wrote:

> The only item up for (some) discussion seems to be use of Thumb2. Builds
> so far have been having problems when turning on T2. For various
> reasons, I'm not particularly desperate to see us build with T2. A todo
> of mine is to confirm that the GCC interworking trampolines mean it
> doesn't matter and we can make changes to have some T2 bits later (which
> is how I understand that this should be working, but I want to verify by
> spending some quality time with objdump and a few compiled libraries).

For the record, after re-learning way too much about how the dynamic
linker, GOT, PLT, etc. work, and going through a bunch of example
objdumps so far...and then also discovering section 3.1.3.2 of the
AAELF, I think I can answer the question with the following quotes:

1). "A PLT entry must be able to branch any distance to either
instruction-set state. The span and state are fixed when the executable
is linked dynamically. A PLT entry must therefore end with code similar
to the following." (snip example using ip for linkage)

2). " ...it is generally pointless trying to construct a PLT entry
entirely in 16-bit Thumb instructions. Even with the overhead of an
inline Thumb-to-ARM state change, an ARM-state entry is usually smaller
and always faster".

Which gels with the ARM-state PLTs I've seen generated from builds (even
with Thumb2 enabled during build time). I still want to actually finish
testing Thumb2 build libraries used with a non-Thumb application, but I
think it's now reasonable to say we don't need to make the call on 
Thumb2, should just complete the v7 bringup without Thumb2 and then turn
on Thumb2 later/fix up bugs we find it doing that at that point.

Please tell me if I am wildly off the mark on this technically.

Jon.

P.S. It seems that interworking is a requirement for Linux AAPCS built
toolchains and will necessarily enable this as part of the build - am I
wrong or are the GCC docs are out of date in this respect?