On 07/06/2012 02:33 PM, Allen Martin wrote:
> On Fri, Jul 06, 2012 at 12:09:43PM -0700, Stephen Warren wrote:
>> On 07/06/2012 12:08 PM, Allen Martin wrote:
>>> Rearrange the link order of libraries to avoid out of bound
>>> relocations in thumb mode. I have no idea how to fix this for real.
>>
>> Are the relocations branches or something else? It looks like
>> unconditional jump range is +/-4MB for Thumb1 and +/-16MB for Thumb2, so
>> I'm surprised we'd be exceeding that, considering the U-boot binary is
>> on the order of 256KB on Tegra right now.
>
>
> This is the relcation type:
>
> arch/arm/lib/libarm.o: In function `__flush_dcache_all':
> /home/arm/u-boot/arch/arm/lib/cache.c:52: relocation truncated to fit:
> R_ARM_THM_JUMP11 against symbol `flush_cache' defined in .text section in
> arch/arm/cpu/armv7/libarmv7.o
>
> The instruction is a "b.n" not a "b", which is what is causing the problem.
>
> I think because of the weak alias the compiler used a short jump to
> the local function, but when it got linked it resolved to a function
> that was too far away for the short jump:
>
>
> void flush_cache(unsigned long start, unsigned long size)
> __attribute__((weak, alias("__flush_cache")));
>
> 00000002 <__flush_dcache_all>:
> 2: 2000 movs r0, #0
> 4: f04f 31ff mov.w r1, #4294967295 ; 0xffffffff
> 8: e7fe b.n 0 <__flush_cache>
Ah, that explanation makes sense.
> It looks like there's a "-fno-optimize-sibling-calls" option to gcc to
> avoid this problem. Seems a shame to disable all short jumps for this
> one case though.
It seems like a bug that the b-vs-b.n optimization is applied to a weak
symbol, since the compiler can't possibly know the range of the jump.
Also, I've seen ld for some architectures rewrite the equivalent of b.n
to plain b when needing to expand the branch target range; IIRC a
process known as "relaxing"? Perhaps gcc is expecting ld to do that, but
ld isn't?
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