[U-Boot] [PATCH] arm: lds: Remove libgcc eabi exception handling tables

[Albert ARIBAUD <albert.u.boot@aribaud.net>]

Hi Michal,

On Thu, 25 Jul 2013 16:16:46 +0200, Michal Simek
<michal.simek@xilinx.com> wrote:

> Remove ARM eabi exception handling tables (for frame unwinding).
> AFAICT, u-boot stubs away the frame unwiding routines, so the tables will
> more or less just consume space. It should be OK to remove them.
> 
> Signed-off-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
> Signed-off-by: Michal Simek <michal.simek@xilinx.com>
> ---
> This patch was sent to ML as RFC (May 9)
> 
> Here is the origin response in connection to this patch.
> 
> Ok, so Michal and I just did some fiddling with zynq builds and
> *exidx* sections.
> 
> By default the *exidx* sections are between rodata and data, so
> removing them causes many apparent changes at the binary level.
> However, builds of zynq based on ARM master with the patch above vs
> master with a patch mapping *exidx* sections after BSS gives identical
> binaries. Thus the RFC has no functional effect.
> 
> Also, ARM EHABI states that [exception] Tables are not required for ABI
> compliance at the C/Assembler level but are required for C++.
> 
> http://infocenter.arm.com/help/topic/com.arm.doc.ihi0038a/IHI0038A_ehabi.pdf
> 
> So as long as we don't put any C++ code in U-Boot (a prospect that I
> don't see happening any time soon), this RFC is safe and either is a
> no-op or removes useless bytes from the binary.
> 
> ---
>  arch/arm/cpu/u-boot.lds | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/arch/arm/cpu/u-boot.lds b/arch/arm/cpu/u-boot.lds
> index 3037885..8894c8a 100644
> --- a/arch/arm/cpu/u-boot.lds
> +++ b/arch/arm/cpu/u-boot.lds
> @@ -113,4 +113,6 @@ SECTIONS
>  	/DISCARD/ : { *(.plt*) }
>  	/DISCARD/ : { *(.interp*) }
>  	/DISCARD/ : { *(.gnu*) }
> +	/DISCARD/ : { *(.ARM.exidx*) }
> +	/DISCARD/ : { *(.gnu.linkonce.armexidx.*) }
>  }
> --
> 1.8.2.3
> 

The good news is, it applies and does not cause any build issues.

The less good news is, arch/arm/cpu/u-boot.lds is not the only linker
script file to fix if we want to fix it this way.

The fact is, I am not sure removing sections from the ELF filein order
to avoid them being in the binary is actually the right way to go. I
have recently had to 'un-discard' some sections in order for
'offline' debugging tools to properly understand the ELF file.

So I am wondering if we should not stop using /DISCARD/ and start using
$(CROSS_COMPILE)objcopy options (-j or --strip-* and --remove-section).

This way, the ELF file would be as complete as any debug tool might
expect it (e.g. objdump, especially when debugging relocation issues)
yet the binary file would remain unchanged.

But past rc2 is not a time to start such a change.

Still, I'd like your fix to be consistent across all of ARM. Can you
change all the linker scripts used for build ARM targets? These are,
according to an ugly mix of cat LOG/*.MAKELOG, sed, grep and sort -u:

arch/arm/cpu/arm1136/u-boot-spl.lds
arch/arm/cpu/arm926ejs/mxs/u-boot-spl.lds
arch/arm/cpu/arm926ejs/spear/u-boot-spl.lds
arch/arm/cpu/armv7/am33xx/u-boot-spl.lds
arch/arm/cpu/armv7/omap-common/u-boot-spl.lds
arch/arm/cpu/armv7/socfpga/u-boot-spl.lds
arch/arm/cpu/ixp/u-boot.lds
arch/arm/cpu/u-boot.lds
arch/arm/cpu/u-boot-spl.lds
board/actux1/u-boot.lds
board/actux2/u-boot.lds
board/actux3/u-boot.lds
board/ait/cam_enc_4xx/u-boot-spl.lds
board/davinci/da8xxevm/u-boot-spl-da850evm.lds
board/davinci/da8xxevm/u-boot-spl-hawk.lds
board/dvlhost/u-boot.lds
board/freescale/mx31ads/u-boot.lds
board/samsung/common/exynos-uboot-spl.lds
board/ti/am335x/u-boot.lds
board/vpac270/u-boot-spl.lds

Amicalement,
-- 
Albert.