[U-Boot] [RFC] [PATCH] arm: arm926ejs: use ELF relocations

Albert ARIBAUD
2010-10-05T00:16:59Z

Le 05/10/2010 01:21, Graeme Russ a ?crit :
> On Tue, Oct 5, 2010 at 9:57 AM, Albert ARIBAUD<albert.aribaud@free.fr> wrote:
>> Le 05/10/2010 00:22, Graeme Russ a ?crit :
>>>
>>> On Tue, Oct 5, 2010 at 9:01 AM, Albert Aribaud<albert.aribaud@free.fr>
>>> wrote:
>>>>
>
>> The output from MAKEALL is curiously calculated... If I look at objdumps of
>> the GOT and ELF binaries, I find that:
>>
>> - the GOT .text section is 118960 bytes and the ELF .text section only
>> 108112. This is due to the fact that GOT relocation requires additional
>> instruction for GOT indirection whereas ELF relocations work by patching the
>> code.
>
> It would be interesting to compare against the basline non-relocatable
> version

I #defined CONFIG_RELOC_FIXUP_WORKS and removed -pie from the ARM
config.mk. This puts the edminiv2 code in the non-reloc build case, and
produces identical .text and .data, and almost identical .rodata, as the
ELF case.

>> - the .rodata section is 22416 for GOT, 22698 for ELF, whereas the .data
>> section is 2908 for GOT, 2627 for ELF. Some initialized data apparently
>> moved from non-const ton const for some reason, but basically, initialized
>> data remains constant.
>>
>> - the .bss section remains constant too, 16640 for GOT vs. 16636 for ELF.
>> I'm not going to track what causes the 4 byte difference. :)
>>
>> Many sections are output in the ELF file which do not appear in the GOT
>> file, such as .interp, .dynamic, .dynstr etc. They probably pollute
>> MAKEALL's figures.
>
> I now discard a few sections:
>
> /DISCARD/ : { *(.dynstr*) }
> /DISCARD/ : { *(.dynamic*) }
> /DISCARD/ : { *(.plt*) }
> /DISCARD/ : { *(.interp*) }
> /DISCARD/ : { *(.gnu*) }
>
> Not that it makes a huge difference - most of these are trivially small

Thanks. I'll add this to the .lds as a measure of clarity.

>> That's roughly consistent with the numbers I get: about 19 KB of .rel.dyn
>> plus .dynsym, which we will be able to cut by half if we preprocess it.
>
> Which is not copied to RAM, so not as nasty as the .got related increase

True also. Note that we could probably shrink the table to 1/4 of its
current size by taking advantage from the fact that the few
non-program-base-relative relocations it has can easily be converted to
program-base-relative, and that two consecutive relocations are always
less than 64 KB away from each other. Of course that moves away from
using the ELF structures as-is, and requires additional build steps, but
people with small FLASH devices may want it.

> I'm also looking at moving the low-level intialisation and relocation code
> into a seperate section (outside .text) so I even less to relocate to RAM

As Wolfgang pointed out, there might be issues in that all the code that
runs in FLASH should be truly PI, which might not be a piece of cake.
ARM C code, for instance, tends to generate literals which need to be
relocated if you don't run the code where it was linked for.

> Then I could even compress the relocatable section, but that is just being
> silly ;)

:)

> Regards,
>
> Graeme

Amicalement,
--
Albert.