[U-Boot] Relocation size penalty calculation

[J. William Campbell]

Joakim Tjernlund 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
>>     
>
>   
It is true that --emit-relocs is not required when -pic and -pie are 
used instead. However, pic and pie are designed to allow shared code 
(libraries)  to appear at different logical addresses in several 
programs without altering the text. This is grand overkill for what we 
need, which is the ability to relocate the code. The -pic and -pie code 
will be larger than the code without pic and pie. How much larger is a 
good question. On the PPC, it is larger but not much larger, because 
there are lots of registers available and one is almost for sure got (no 
pun intended) the magic relocation constant(s) in it. On the 386 with 
many fewer registers, pic and pie will cause the code to be 
percentage-wise larger than on the PPC. Thus avoiding pic and pie is a 
Good Thing in most cases.
> Try -fvisibility=hidden
>   
I assume the -fvisibility=hidden is suggested in order to reduce 
(eliminate) the symbol table from the output, which we don't need 
because there are assumed to be no undefined symbols in our final ld. If 
that works, great! I was assuming we might need a custom "strip" program 
to delete any sections that we don't need, but this sounds easier if it 
gets them all.

Best Regards,
Bill Campbell
>  Jocke
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