>
> On 10/12/2010 11:30 PM, Albert ARIBAUD wrote:
> > Le 12/10/2010 23:00, Joakim Tjernlund a écrit :
> >
> >> Yes, but the difference isn't really the arch. It is the
-mrelocatable
> >> flag that is the big difference.
> > Not only: obviously, implementing GOT relocation is not done the same
on
> > both archs, and it simply is not beneficial on ARM wrt PPC in terms of
> > instructions. I did a pretty extensive run of tests with and without
> > -fPIC and -fPIE on ARM, and GOT relocation clearly makes code bigger,
> > whereas it does not PPC.
> >
> > This simply implies that -fPIC is a better choice for PPC (and hence
> > -mrelocatable) while -fpie is a better one for ARM.
> Hi All,
> In particular, the PPC takes two 32 bit instructions to load the
> known address of a variable into a register. If the GOT is used, a
> single 32 bit instruction can load the address of a variable from the
> GOT table (pointed to by a "fixed" register) into a register. In both
> cases, there are two memory cycles, but in the GOT case, only one
> instruction is required. This is why the GOT based code is smaller.
> However, the GOT cannot be used to address constants and some other
> items that are not "variables". I do think that -fPIC and -fpie are not
> mutually incompatible. On the PPC, the GOT references would be relocated
> in the loop that updates the GOT and the references to constants would
> be relocated by the ELF relocation code. That is how shared libraries
> are relocated.
hmm, what constants and why would you relocate these?
Curious, what other data that are not "variables" are
you thinking about? Could such data be present in
u-boot too? Possibly the fixup's(initialized static ptrs)?
Relocs for these are emitted with -mrelocatable for ppc and
u-boot has a small routine to relocate these too.
Jocke
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