arm thumb veneer question
wd at denx.de
Tue Sep 21 21:29:16 BST 2010
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
> 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,
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
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
DENX Software Engineering GmbH, MD: Wolfgang Denk & Detlev Zundel
HRB 165235 Munich, Office: Kirchenstr.5, D-82194 Groebenzell, Germany
Phone: (+49)-8142-66989-10 Fax: (+49)-8142-66989-80 Email: wd at denx.de
Old programmers never die, they just branch to a new address.
More information about the linaro-toolchain