linux-kernel - Re: [PATCH 0/4] Compile kernel with -fwhole-program --combine

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Date:	Fri, 25 Aug 2006 09:55:16 +0100
From:	David Woodhouse <dwmw2@...radead.org>
To:	Jan Engelhardt <jengelh@...ux01.gwdg.de>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/4] Compile kernel with -fwhole-program --combine

On Fri, 2006-08-25 at 08:01 +0200, Jan Engelhardt wrote:
> >> Compiling files on their own (`make drivers/foo/bar.o`) seems to make 
> >> the optimization void. Sure, most people don't stop compiling in 
> >> between. Just a note
> >
> >Actually I'm not entirely sure what you write is true. It'll _build_
> >fs/jffs2/read.o, for example, but it still won't then use it when I make
> >the kernel -- it'll just use fs/jffs2/jffs2.o which is built from all
> >the C files with --combine. So the optimisation isn't lost.
> 
> Umm then it spends double the time in compilation, doing:
> 
>   read.o <- read.c
>   foo.o <- foo.c
>   bar.o <- bar.c
>   built-in.o <- read.c foo.c bar.c

Only if you invoke make explicitly for read.o, foo.o and bar.o. If you
just type 'make' then it won't build those.

> >So to overcome this, we use GCC's __attribute__((externally_visible))
> >which, as documented, just makes it global again -- undoing the effect
> >of -fwhole-program just for this _one_ symbol.
> 
> Interesting. __attribute__((visibility("default"))) does the same?

That much you can test for yourself without a fixed compiler. No, it
doesn't.

-- 
dwmw2

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