arm thumb veneer question

[Wolfgang Denk]

Dear Dave Martin,

In message <AANLkTikHv1SpjcyRbRxGK2QeCoq96tsTken3LmP5bsse at mail.gmail.com> you wrote:
> 
> I believe such calls are getting resolved via a veneer because of a
> combination the thumb2-ness of libgcc and the toolchain being used.
>
> In principle, the linker can know that it is linking for >= ARMv5T due
> to the way it was configured and the way the input objects were built,
> but GNU ld is conservative and doesn't do this automatically.  As a
> result, it has to generate a veneer, reached via a normal
> non-interworking branch.  ld has no way the veneer needs to be PIC and
> use the GOT, so it isn't and doesn't.

Stupid question: why not?


>    The `--use-blx' switch enables the linker to use ARM/Thumb BLX
> instructions (available on ARMv5t and above) in various situations.
> Currently it is used to perform calls via the PLT from Thumb code using
> BLX rather than using BX and a mode-switching stub before each PLT
> entry. This should lead to such calls executing slightly faster.
>
> ...so you might explicitly want to enable this whenever building for
> ARMv5 or later.
>
> Wolfgang, can you foresee any reason not to do that?  As far as I can
> see it will be safe so long as we don't use it when building for
> architectured <ARMv5 (where the BLX instruction isn't supported).

If care is taken that it causes no conflicts with older tool chains
I'm happy with  that. I guess that can be added to
arch/arm/cpu/armv7/config.mk

> Alternatively, using ld --emit-relocs and then embedding the
> relocation information in the image so that U-Boot can use it could
> help to solve the problem.  I'm guessing that isn't set up at present,
> though.

Is there any information available about relative code sizes /
performance numbers of "--emit-relocs" versus "--use-blx"?


> > 2) is there an option for the toolchain to use an arm libgcc instead of thumb?
>
> You'd need to rebuild the toolchain (or at least libgcc).  I believe
> that no ARM libgcc is built at present for the linaro/Ubuntu tools. I
> don't think the GCC packages currently support this kind of thing
> well.

I think that should be fixed. I guess you will run intot hat again
sooner or later.



BTW: why does nobody answer my questions?

        Can anybody shed some light on 1) when these routines have
        been introduced ... ?

        Is the specific tool chain in question available somewhere
        for testing?

Best regards,

Wolfgang Denk

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