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

[Albert ARIBAUD]

Hi Allen,

On Fri, 6 Jul 2012 16:17:19 -0700, Allen Martin <amartin at nvidia.com> wrote:
> On Fri, Jul 06, 2012 at 01:44:32PM -0700, Stephen Warren wrote:
> > 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?
> 
> And I forgot to mention, the code bloat from disabling the
> optimization is about 400 bytes (185136 -> 185540), so it's not bad,
> but it still seems a shame to disable all short branches because of
> one misoptimized one.

Can this not be limited to compiling the object files which are known to be
sensitive to the problem?

> -Allen

Amicalement,
-- 
Albert.