arm thumb veneer question

[Dave Martin]

Hi,

On Wed, Sep 22, 2010 at 2:04 PM, Loïc Minier <loic.minier at linaro.org> wrote:
> On Wed, Sep 22, 2010, Dave Martin wrote:
>> I'm not a toolchain expert, so I'm happy to be overridden... but my
>> _guess_ is:
>>
>> I think that in practice (at least on arm) cc -fPIC = cc -fPIE, and ld
>> -pie just forces ld to generate PIC veneers (as for -shared).  Beyond
>> this, I think ld -shared / -pie / (nothing) probably just changes
>> which linker script is used by default.  U-Boot overrides the default
>> with its own linker script anyway, so it may make no difference.
>
>  Catching up on email, I just came across:
>    http://article.gmane.org/gmane.comp.boot-loaders.u-boot/84789
>  so it seems to be different, but not significantly

Hmmm, interesting discussion: looks like my guess was naive ;)

It looks like -fPIE provides some savings after all, since it enables
the compiler to make some additional assumptions.

>  I don't care too strongly, but it might make sense to try to use the
>  flag which means exactly what we want to allow for future
>  optimizations?

Since U-Boot is now trying to build PIC anyway, I suggest that for
things to work robustly one of the following is needed anyway:

  * -fPIE + -pie
  * -fPIC + -shared (less optimal?)

Logically, pie seems to be the closest match to what is actually
happening in U-Boot.

Then we can optionally add:

  * --use-blx (to get rid of ARM<->Thumb interworking veneers)

--pic-veneers could be used as a temporary fix, but this feels fragile
to me: it only controls a speicific aspect of linker behaviour, so we
could hit other problems in the future.  We could do this as a
short-term workaround to allow the linaro toolchain to be used in the
meantime, though.

Cheers
---Dave