[U-Boot] Relocation size penalty calculation

[Graeme Russ]

On Fri, Oct 9, 2009 at 10:12 AM, Joakim Tjernlund
<joakim.tjernlund at transmode.se> wrote:
>>
>> On Fri, Oct 9, 2009 at 9:27 AM, J. William Campbell
>> <jwilliamcampbell at comcast.net> wrote:
>> > Graeme Russ wrote:
>> >>
>> >> On Fri, Oct 9, 2009 at 2:58 AM, J. William Campbell
>> >> <jwilliamcampbell at 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
>
> Try -fvisibility=hidden

Thanks - Shaved another 2539 bytes off the binary

Also found out how to get rid of .eh_frame (crept in when I upgraded to
gcc 4.4.1) with -fno-dwarf2-cfi-asm, so that shaves another 13452 bytes

Total saving of 15.6k

>
>  Jocke
>
>

Regards,

Graeme