linux-kernel - Re: BUG: GCC-4.4.x changes the function frame on some functions

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Date:	Thu, 19 Nov 2009 16:59:37 -0800 (PST)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Thomas Gleixner <tglx@...utronix.de>
cc:	Jeff Law <law@...hat.com>, rostedt@...dmis.org,
	"H. Peter Anvin" <hpa@...or.com>,
	David Daney <ddaney@...iumnetworks.com>,
	Andrew Haley <aph@...hat.com>,
	Richard Guenther <richard.guenther@...il.com>,
	Ingo Molnar <mingo@...e.hu>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	feng.tang@...el.com, Fr??d??ric Weisbecker <fweisbec@...il.com>,
	Peter Zijlstra <peterz@...radead.org>, jakub@...hat.com,
	gcc@....gnu.org
Subject: Re: BUG: GCC-4.4.x changes the function frame on some functions



On Fri, 20 Nov 2009, Thomas Gleixner wrote:
> 
> While testing various kernel configs we found out that the problem
> comes and goes. Finally I started to compare the gcc command line
> options and after some fiddling it turned out that the following
> minimal deltas change the code generator behaviour:
> 
> Bad:  -march=pentium-mmx                -Wa,-mtune=generic32
> Good: -march=i686        -mtune=generic -Wa,-mtune=generic32
> Good: -march=pentium-mmx -mtune-generic -Wa,-mtune=generic32
> 
> I'm not supposed to understand the logic behind that, right ?

Are you sure it's just the compiler flags?

There's another configuration portion: the size of the alignment itself. 
That's dependent on L1_CACHE_SHIFT, which in turn is taken from the kernel 
config CONFIG_X86_L1_CACHE_SHIFT.

Maybe that value matters too - for example maybe gcc will not try to align 
the stack if it's big?

[ Btw, looking at that, why are X86_L1_CACHE_BYTES and X86_L1_CACHE_SHIFT 
  totally unrelated numbers? Very confusing. ]

The compiler flags we use are tied to some of the same choices that choose 
the cache shift, so the correlation you found while debugging this would 
still hold.

			Linus

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