[U-Boot] Relocation size penalty calculation

Joakim Tjernlund
2009-10-10T15:38:47Z

Graeme Russ <graeme.russ@gmail.com> wrote on 10/10/2009 13:21:10:
>
> On Sat, Oct 10, 2009 at 9:47 PM, Joakim Tjernlund
> <joakim.tjernlund@transmode.se> wrote:
> >
> >
> > Graeme Russ <graeme.russ@gmail.com> wrote on 10/10/2009 12:38:19:
> >>
> >> 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 todo 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
> >
> > not on x86, on ppc it is a big 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
> >
> > Ah, that is because you built an exe with -pie
> > Then you should be able to drop everything but the RELATIVE
> > from the linking, or almost in any case.
> >
> > Jocke
> >
> >
>
> Hmm, so its seems I may have hit the limit. I tried:
>
> PLATFORM_LDFLAGS += -r --emit-relocs
>
> but there is not enough information left to complete the relocation. It
> seems as though I need .rel.got, .got.plt, .dynsym and .rel.dyn in order
> to find the actual bytes that need modifying (it also seems to mess with
> the size of the stripped binary for some reason)
>
> Looks like I'll have to proceed with my original plan - a bit bloated,
> but it works

Relocation costs :(

I am not sure why you need .got.plt, it should be empty,
what is in it?
Same with dynsym, what is in it?

Memory fails me, but since u-boot is a freestanding app it I think
these two might not be needed. Perhaps there are weak unresolved
syms in there?

Jocke