[U-Boot] Relocation size penalty calculation

Joakim Tjernlund
2009-10-10T09:27:37Z

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