[U-Boot] Relocation size penalty calculation

[Graeme Russ]

On Sat, Oct 10, 2009 at 8:27 PM, Joakim Tjernlund
<joakim.tjernlund at transmode.se> wrote:
>
>
> Graeme Russ <graeme.russ at gmail.com> wrote on 10/10/2009 10:46:52:
>>
>> On Sat, Oct 10, 2009 at 7:07 PM, Joakim Tjernlund
>> <joakim.tjernlund at transmode.se> wrote:
>> > Graeme Russ <graeme.russ at gmail.com> wrote on 10/10/2009 06:43:52:
>> >>
>> >> 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
>> >
>> > Great, so now you are back at just a few percent added I guess?
>> >
>> >
>>
>> Not really - The .eh_frame saving applies to both relocated and non
>> relocated builds
>
> OK, so you didn't use PIC before at all?
>
> Anyway I think you can do more. Using -Bsymbolic you should get
> away with RELATIVE relocs only and be able to skip a lot of segments above.
> Have a look at uClibc ldso/ldso/dl-startup.c
>
>

My build options thus far are:

PLATFORM_RELFLAGS += -fpie -fvisibility=hidden
PLATFORM_CPPFLAGS += -fno-dwarf2-cfi-asm
PLATFORM_LDFLAGS += -pie

-fpic / -pic make no difference

Interestingly, -Bsymbolic adds exactly 8 bytes to .dynamic, but doesn't
change the size of any other section

Pulling apart the relocation sections, it seems that all relocations are
already RELATIVE even without -Bsymbolic