[U-Boot] Relocation size penalty calculation

Graeme Russ
2009-10-08T22:39:39Z

On Fri, Oct 9, 2009 at 9:27 AM, J. William Campbell
<jwilliamcampbell@comcast.net> wrote:
> Graeme Russ wrote:
>>
>> On Fri, Oct 9, 2009 at 2:58 AM, J. William Campbell
>> <jwilliamcampbell@comcast.net> wrote:
>>
>>>
>>> Graeme Russ wrote:
>>>
>>>>
>>>> Out of curiosity, I wanted to see just how much of a size penalty I am
>>>> incurring by using gcc -fpic / ld -pic on my x86 u-boot build. Here are
>>>> the results (fixed width font will help - its space, not tab,
>>>> formatted):
>>>>
>>>> Section non-reloc reloc
>>>> ---------------------------------------
>>>> .text 000118c4 000137fc <- 0x1f38 bytes (~8kB) bigger
>>>> .rodata 00005bad 000059d0
>>>> .interp n/a 00000013
>>>> .dynstr n/a 00000648
>>>> .hash n/a 00000428
>>>> .eh_frame 00003268 000034fc
>>>> .data 00000a6c 000001dc
>>>> .data.rel n/a 00000098
>>>> .data.rel.ro.local n/a 00000178
>>>> .data.rel.local n/a 000007e4
>>>> .got 00000000 000001f0
>>>> .got.plt n/a 0000000c
>>>> .rel.got n/a 000003e0
>>>> .rel.dyn n/a 00001228
>>>> .dynsym n/a 00000850
>>>> .dynamic n/a 00000080
>>>> .u_boot_cmd 000003c0 000003c0
>>>> .bss 00001a34 00001a34
>>>> .realmode 00000166 00000166
>>>> .bios 0000053e 0000053e
>>>> =======================================
>>>> Total 0001d5dd 00022287 <- 0x4caa bytes (~19kB) bigger
>>>>
>>>> Its more than a 16% increase in size!!!
>>>>
>>>> .text accounts for a little under half of the total bloat, and of that,
>>>> the crude dynamic loader accounts for only 341 bytes
>>>>
>>>>
>>>
>>> Hi Graeme,
>>> I would be interested in a third option (column), the x86 build with
>>> just -mrelocateable but NOT -fpic. It will not be definitive because
>>> there
>>> will be extra code that references the GOT and missing code to do some of
>>> the relocation, but it would still be interesting.
>>>
>>
>> x86 does not have -mrelocatable. This is a PPC only option :(
>>
>
> Hi Graeme,
> You are unfortunately correct. However, I wonder if we can get
> essentially the same result by executing the final ld step with the
> --emit-relocs switch included. This may also include some "extra" sections
> that we would want to strip out, but if it works, it could give all
> ELF-based systems a way to a relocatable u-boot.
>

I don't think --emit-relocs is necessary with -pic. I haven't gone through
all the permutations to see if there is a smaller option, but gcc -fpic and
ld -pie creates enough information to perform relocation on the x86
platform

Regards,

Graeme

> Best Regards,
> Bill Campbell
> **
>>
>>
>>>
>>> Best Regards,
>>> Bill Campbell
>>>
>>>>
>>>> Have any metrics been done for PPC?
>>>>
>>>> Regards,
>>>>
>>>> Graeme
>>>>
>>
>> Once the reloc branch has been merged, how many arches are left which do
>> not support relocation?
>>
>> Regards,
>>
>> Graeme
>>
>>
>>
>
>