[PATCH u-boot v2 00/38] U-Boot LTO (Sandbox + Some ARM boards)

[Marek Behun]

On Fri, 12 Mar 2021 15:21:05 +0100
Harald Seiler <hws at denx.de> wrote:

> Hi Marek,
> 
> On Fri, 2021-03-12 at 11:33 +0100, Marek Behún wrote:
> > Hello,
> > 
> > I am sending version 2 of patches adding support for LTO to U-Boot.
> > 
> > This series was tested by Github/Azure CI at
> >   https://github.com/u-boot/u-boot/pull/57
> > 
> > Code reduction is on average 4.23% for u-boot.bin and 13.58% for
> > u-boot-spl.bin.
> > 
> > I am currently running a build test for all 1077 ARM defconfigs.
> > Of the first 232 defconfigs, 2 are failing when LTO is enabled
> > (chromebook_jerry and chromebook_speedy). Note that this series
> > only enables LTO for tested boards.
> > 
> > Changes since v1:
> > - remove patches applied into u-boot-marvell
> > - added Reviewed-by tags
> > - addressed some issues discovered by Bin Meng, Marek Vasut,
> >   Heinrich Schuchardt
> > - added more ARM boards (thanks to Adam Ford, Tim Harvey and Bin Meng)
> > - removed --gc-sections for ARM if internal libgcc is used
> > - remove -fwhole-program in final LTO LDFLAGS
> > - declared all 4 functions (memcpy, memset, memcmp, memmove) __used,
> >   (these are mentioned in GCC man page for option -nodefaultlibs that
> >    the compiler may generate; this seems to be a bug in GCC that linking
> >    fails with LTO even if these functions are present, because the
> >    symbols can be renamed on some targets by optimization)  
> 
> I'm hitting a compiler error when building with imx6q_logic_defconfig:
> 
>   real-ld: error: no memory region specified for loadable section `.note.gnu.build-id'
> 
> It seems this is caused by calling the linker through a gcc invocation
> which adds a `--build-id` commandline flag.  I think the linker script
> which is used for SPL in this case (arch/arm/mach-omap2/u-boot-spl.lds)
> isn't properly set up to deal with a build-id.
> 
> I'm not sure how to deal with this.  One could either add
> `--build-id=none` to the GCC commandline to suppress generation of this
> section entirely (it is not emitted in non-LTO builds right now anyway) or
> include it in .text in said linker script so it is visible on the target.
> What do you think?
> 
> I should note that I am using a Yocto-generated toolchain.  I suppose most
> standard toolchains' behavior regarding the `--build-id` flag probably
> differs.
> 
> Regards,

I encountered this with Debian's cross toolchain, but since this did
not happen on my station with Gentoo crossdev toolchain, nor on Azure
CI, I ignored it.

What is the purpose of --build-id? Why do people use it?

Marek