[U-Boot] Relocation size penalty calculation

J. William Campbell
2009-10-08T16:50:03Z

Peter Tyser wrote:
> On Thu, 2009-10-08 at 08:53 -0700, J. William Campbell wrote:
>
>> Peter Tyser wrote:
>>
>>> On Thu, 2009-10-08 at 22:54 +1100, 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
>>>>
>>>> Have any metrics been done for PPC?
>>>>
>>>>
>>> Things actually improve a little bit when we use -mrelocatable and get
>>> rid of all the manual "+= gd->reloc_off" fixups:
>>>
>>> 1) Top of mainline on XPedite5370:
>>> text data bss dec hex filename
>>> 308612 24488 33172 366272 596c0 u-boot
>>>
>>> 2) Top of "reloc" branch on XPedite5370 (ie -mrelocatable):
>>> text data bss dec hex filename
>>> 303704 28644 33156 365504 593c0 u-boot
>>>
>>>
>>>
>> Hi Peter,
>> Just to be clear, the total text+data length of u-boot with the
>> "manual" relocations (#1) is LARGER than the text+data length of u-boot
>> with the "manual" relocations removed and the necessary centralized
>> relocation code added, along with any additional data sections required
>> by -mrelocateable (#2), by 768 (dec) bytes?
>>
>
> Hi Bill,
> Doah, looks like I chose a bad board as an example. The XPedite5370
> already had -mrelocatable defined in its own
> board/xes/xpedite5370/config.mk in mainline, so the above comparison
> should be ignored as both builds used -mrelocatable.
>
> Here's some *real* results from the MPC8548CDS:
> 1) Top of mainline:
> text data bss dec hex filename
> 219968 17052 22992 260012 3f7ac u-boot
>
> 2) Top of "reloc" branch (ie -mrelocatable)
> text data bss dec hex filename
> 219192 20640 22980 262812 4029c u-boot
>
> So the reloc branch is 2.7K bigger for the MPC8548CDS.
>
Hi Peter,
OK, that's more like it! A 1.2 % size increase in ROM seems like a
very small price to pay for a truly relocatable u-boot image that will
run on any size memory without the programmer having to actively worry
about what may need relocating as code is written. . Also, it should be
noted that the size increase in 2) is mostly in relocation segments
that do not need to be copied into ram, so the ram footprint should be
smaller for 2) than 1). The relocation code itself could also be placed
is a segment that is not copied into ram, although that may be more
trouble than it is worth.
I am looking forward to Graeme's results with the 386. I expect
that it will not be quite so favorable, perhaps a 4 or 5% size increase
for -mrelocatable over an absolute build. However, -mrelocatable vs.
-fpic may be comparable, with -mrelocatable actually winning. But then
again, I could be totally wrong!

Best Regards,
Bill Campbell
> Best,
> Peter
>
>
>
>