linux-kernel - Re: [RFC PATCH] x86: enable dead code and data elimination (LTO)

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Message-ID: <alpine.LFD.2.20.1707121331060.1959@knanqh.ubzr>
Date:   Wed, 12 Jul 2017 13:34:46 -0400 (EDT)
From:   Nicolas Pitre <nico@...xnic.net>
To:     Andi Kleen <andi@...stfloor.org>
cc:     npiggin@...il.com, linux-kernel@...r.kernel.org,
        linux-kbuild@...r.kernel.org
Subject: Re: [RFC PATCH] x86: enable dead code and data elimination (LTO)

On Wed, 12 Jul 2017, Andi Kleen wrote:

> Nicholas Piggin <npiggin@...il.com> writes:
> >> 
> >> I think we should aim for gc-sections to be used by default and have LTO 
> >> as a possible option only.
> >
> > I agree after it starts getting implemented and debugged by small
> > system users, we could make it default in the interest of sharing
> > testing and reducing combinations.
> 
> From what i understand the main drawback in the past was
> is that various linker versions become very slow with thousands of
> sections.
> 
> So it may cost you built time. For a special small build it's probably
> ok, but you wouldn't want to make it default.
> 
> Also usually it's only useful without modules because if you
> use modules EXPORT_SYMBOL pulls in a lot of unused functions.

I created CONFIG_TRIM_UNUSED_KSYMS mainly to avoid that issue. It is 
highly effective with either gc-sections and LTO.


Nicolas