linux-kernel - Re: [PATCH] x86: unify x86 Makefile(s)

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Date:	Sat, 29 Dec 2007 19:22:41 +0100
From:	Sam Ravnborg <sam@...nborg.org>
To:	Adrian Bunk <bunk@...nel.org>
Cc:	Andi Kleen <ak@...e.de>, Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86: unify x86 Makefile(s)

On Sat, Dec 29, 2007 at 02:54:13PM +0200, Adrian Bunk wrote:
> On Sat, Dec 29, 2007 at 01:16:07PM +0100, Andi Kleen wrote:
> > 
> > If -funit-at-a-time really increases stack size too much on some compiler
> > version the right fix would be to check where it does that using make checkstack
> > and then add "noinline" attributes there to prevent the compiler from inlining.
> > That would prevent them.
> > 
> > Globally disabling it is too big a hammer.
> > 
> > e.g. I know XFS did it in a similar way to prevent this problem.
> > 
> > So I would reenable it for now and if you know it causes problems on specific
> > compiler versions, Adrian, you could watch make checkstack there and submit
> > noinline patches as needed.
> 
> The main point is that we are _only_ talking about gcc 3.4 on i386 - for 
> more recent compilers we do not disable unit-at-a-time.

I took a second look and realized that Adrian is right about the disabling.
I had overlooked the 'no' part in -fno-unit-at-a-time.

So to try it out I build a gcc with crosstool:
gcc --version:
i686-unknown-linux-gnu-gcc (GCC) 3.4.5

This is the version that should exhibit the behaviour with increased text size.
Build two kernels - one without and one with -funit-at-a-time:

size output:
   text    data     bss     dec     hex filename
4878494  471580  610304 5960378  5af2ba o-i386/vmlinux
4955008  472934  614400 6042342  5c32e6 o-i386-no-funit-at-a-time/vmlinux

[Last line is with unit-at-a-time disabled as per the filename.]

With a i386 defconfig we see an *increase* in text size if we disable -funit-at-a-time.
That is we see the opposite of the expected.

Now the bug in gcc may have been fixed between 3.4 and 3.4.5 - I dunno.

So this simple test favour to always enable unit-at-a-time.

	Sam
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