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

[Allen Martin <amartin@nvidia.com>]

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>


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.

-Allen
-- 
nvpublic