[U-Boot] [PATCH 2/7] HACK: rearrange link order for thumb

[Stephen Warren <swarren@wwwdotorg.org>]

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?