From mboxrd@z Thu Jan 1 00:00:00 1970 From: Stephen Warren Date: Fri, 06 Jul 2012 14:44:32 -0600 Subject: [U-Boot] [PATCH 2/7] HACK: rearrange link order for thumb In-Reply-To: <20120706203329.GA29103@nvidia.com> References: <1341598142-28873-1-git-send-email-amartin@nvidia.com> <1341598142-28873-3-git-send-email-amartin@nvidia.com> <4FF737F7.8040008@wwwdotorg.org> <20120706203329.GA29103@nvidia.com> Message-ID: <4FF74E30.3000101@wwwdotorg.org> List-Id: MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: u-boot@lists.denx.de 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?