linux-kernel - Re: [PATCH 30/46] Kbuild, lto: Add Link Time Optimization support

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Date:   Mon, 14 Nov 2022 10:55:45 -0800
From:   Josh Poimboeuf <jpoimboe@...nel.org>
To:     "Jiri Slaby (SUSE)" <jirislaby@...nel.org>
Cc:     linux-kernel@...r.kernel.org, Andi Kleen <ak@...ux.intel.com>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        linux-kbuild@...r.kernel.org, Richard Biener <RGuenther@...e.com>,
        Jan Hubicka <jh@...e.de>, "H . J . Lu" <hjl.tools@...il.com>,
        Don Zickus <dzickus@...hat.com>, Martin Liska <mliska@...e.cz>,
        Bagas Sanjaya <bagasdotme@...il.com>,
        Jiri Slaby <jslaby@...e.cz>
Subject: Re: [PATCH 30/46] Kbuild, lto: Add Link Time Optimization support

On Mon, Nov 14, 2022 at 12:43:28PM +0100, Jiri Slaby (SUSE) wrote:
> +++ b/Documentation/kbuild/lto-build.rst
> @@ -0,0 +1,76 @@
> +=====================================================
> +gcc link time optimization (LTO) for the Linux kernel
> +=====================================================
> +
> +Link Time Optimization allows the compiler to optimize the complete program
> +instead of just each file.
> +
> +The compiler can inline functions between files and do various other global
> +optimizations, like specializing functions for common parameters,
> +determing when global variables are clobbered, making functions pure/const,
> +propagating constants globally, removing unneeded data and others.
> +
> +It will also drop unused functions which can make the kernel
> +image smaller in some circumstances, in particular for small kernel
> +configurations.
> +
> +For small monolithic kernels it can throw away unused code very effectively
> +(especially when modules are disabled) and usually shrinks
> +the code size.
> +
> +Build time and memory consumption at build time will increase, depending
> +on the size of the largest binary. Modular kernels are less affected.
> +With LTO incremental builds are less incremental, as always the whole
> +binary needs to be re-optimized (but not re-parsed)
> +
> +Oopses can be somewhat more difficult to read, due to the more aggressive
> +inlining: it helps to use scripts/faddr2line.
> +
> +It is currently incompatible with live patching.

... because ?

-- 
Josh