[U-Boot] Relocation size penalty calculation

Graeme Russ
2009-10-10T10:38:19Z

On Sat, Oct 10, 2009 at 8:27 PM, Joakim Tjernlund
<joakim.tjernlund@transmode.se> wrote:
>
>
> Graeme Russ <graeme.russ@gmail.com> wrote on 10/10/2009 10:46:52:
>>
>> On Sat, Oct 10, 2009 at 7:07 PM, Joakim Tjernlund
>> <joakim.tjernlund@transmode.se> wrote:
>> > Graeme Russ <graeme.russ@gmail.com> wrote on 10/10/2009 06:43:52:
>> >>
>> >> 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
>> >
>> > 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