link time analysis for the kernel.

[Carter Cheng]

I managed to find some information on this from Prof John Criswell who did
something similar for his dissertation but I do wonder how complicated the
make files for building the kernel are since the method he used would
require using llvm-link to stitch together all the different object
files(bitcode) into a single file and then convert them into machine code
at one go.

On Fri, Oct 12, 2018 at 10:20 AM <valdis.kletnieks at vt.edu> wrote:

> On Thu, 11 Oct 2018 21:45:16 +0800, Carter Cheng said:
>
> > There are some detaills about the current procedures for linking the
> kernel
> > that I am unfamiliar with. My understanding is that GCC and Clang both
> have
> > the ability to do link time analysis and transforms on code but is it
> > possible to write link time passes that will run on the kernel since the
> > linking phase is a bit different (i.e. doesnt produce an ELF file)?
>
> The fact that the kernel gets linked is an existence proof that it is
> possible
> to do link time processing on the kernel.
>
> There's no LTO support in the stock 4.19 tree, but Andi Kleen did a
> patchset
> for 4.15, and there's another patchset to enable LTO when using Clang
> rather
> than gcc. (I haven't tried either one, don't use on a production machine,
> as
> the resulting kernel may crash, eat filesystems, and/or turn your dog
> green...)
>
> Note that 'vmlinux' is a statically linked ELF binary. That  plus a
> bootstrap
> code gets merged to create a bzImage or similar thing that can be loaded by
> Grub2 or whatever boot loader.
>
>
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