Re: [edk2-devel] [PATCH] BaseTools/GenFw: Change opcode when converting ADR to ADRP

["Ard Biesheuvel" <ardb@kernel.org>]

On Fri, 27 Oct 2023 at 16:26, Pedro Falcato <pedro.falcato@gmail.com> wrote:
>
> On Fri, Oct 27, 2023 at 3:13 PM Ard Biesheuvel <ardb@kernel.org> wrote:
> >
> > On Fri, 27 Oct 2023 at 16:09, Jake Garver via groups.io
> > <jake=nvidia.com@groups.io> wrote:
> > >
> > > Hi Ard:
> > >
> > > > Can you double check the object file? I suspect this is a linker relaxation not a compiler issue.
> > >
> > > With LTO in play, is there a way to check the object file?  It's not in aarch64 assembly.
> > >
> >
> > Perhaps not.
> >
> > Could you try adding -Wl,--no-relax to the DLINK_FLAGS for your build
> > and see if it makes a difference?
> >
> > We should be adding that in any case, but it would be good to know if
> > it helps here too.
>
> I've never checked on aarch64, but don't you get solid space savings
> with linker relaxation? Or is it mostly meaningless?
>

The most important use case for linker relaxation is turning GOT
lookups into relative references, given that PIC object files are
typically constructed without prior knowledge of whether a given
external reference will be satisfied by the same dynamic object or a
different one. So each relaxation removes a load from the program and
may reduce the size of the GOT (if no other non-relaxable references
to it exist) and this might matter on a hot path.

None of this makes sense for UEFI, though, given that all executables
are fully linked and performance doesn't usually matter to the same
extent.

On AArch64, there is a specific advantage to being able to relax ADRP
to ADR (and this is why GenFw implements this relaxation itself too):
ADRP instructions need to appear at the same offset modulo 4k as they
were linked at, which means 4k alignment for the entire code section.
EDK2 builds are made up of lots of tiny SEC and PEI drivers and
rounding up each of those to 4k would result in a substantial increase
in footprint of the firmware image.


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