[U-Boot] Relocation size penalty calculation

Graeme Russ
2009-10-10T04:43:52Z

On Fri, Oct 9, 2009 at 10:12 AM, Joakim Tjernlund
<joakim.tjernlund@transmode.se> wrote:
>>
>> 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
>
> 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