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

[Harald Seiler]

On Fri, 2021-03-12 at 15:26 +0100, Marek Behun wrote:
> 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?

I'm not entirely sure but I think it acts as a unique identifier for
a certain binary.  So you can match up a core-dump with its debug info for
example.

But I am unsure if anyone in the firmware space is actively using this
feature... At least U-Boot does not actually include the build-id on the
target - it is not generated for SPL at all and U-Boot proper only
contains it in the ELF file, it is not exported into the raw binary.

-- 
Harald