Re: [PATCH 00/46] gcc-LTO support for the kernel

[Richard Biener]

On Thu, 17 Nov 2022, Peter Zijlstra wrote:

> On Mon, Nov 14, 2022 at 08:40:50PM +0100, Ard Biesheuvel wrote:
> > On Mon, 14 Nov 2022 at 12:44, Jiri Slaby (SUSE) <jirislaby@xxxxxxxxxx> wrote:
> > >
> > > Hi,
> > >
> > > this is the first call for comments (and kbuild complaints) for this
> > > support of gcc (full) LTO in the kernel. Most of the patches come from
> > > Andi. Me and Martin rebased them to new kernels and fixed the to-use
> > > known issues. Also I updated most of the commit logs and reordered the
> > > patches to groups of patches with similar intent.
> > >
> > > The very first patch comes from Alexander and is pending on some x86
> > > queue already (I believe). I am attaching it only for completeness.
> > > Without that, the kernel does not boot (LTO reorders a lot).
> > >
> > > In our measurements, the performance differences are negligible.
> > >
> > > The kernel is bigger with gcc LTO due to more inlining.
> > 
> > OK, so if I understand this correctly:
> > - the performance is the same
> > - the resulting image is bigger
> > - we need a whole lot of ugly hacks to placate the linker.
> > 
> > Pardon my cynicism, but this cover letter does not mention any
> > advantages of LTO, so what is the point of all of this?
> 
> Seconded; I really hate all the ugly required for the GCC-LTO
> 'solution'. There not actually being any benefit just makes it a very
> simple decision to drop all these patches on the floor.

I'd say that instead a prerequesite for the series would be to actually
enforce hidden visibility for everything not part of the kernel module
API so the compiler can throw away unused functions.  Currently it has
to keep everything because with a shared object there might be external
references to everything exported from individual TUs.

There was a size benefit mentioned for module-less monolithic kernels
as likely used in embedded setups, not sure if that's enough motivation
to properly annotate symbols with visibility - and as far as I understand
all these 'required' are actually such fixes.

Richard.