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Re: BUG: GCC-4.4.x changes the function frame on some functions
- From: Linus Torvalds <torvalds at linux-foundation dot org>
- To: Thomas Gleixner <tglx at linutronix dot de>
- Cc: Jeff Law <law at redhat dot com>, rostedt at goodmis dot org, "H. Peter Anvin" <hpa at zytor dot com>, David Daney <ddaney at caviumnetworks dot com>, Andrew Haley <aph at redhat dot com>, Richard Guenther <richard dot guenther at gmail dot com>, Ingo Molnar <mingo at elte dot hu>, LKML <linux-kernel at vger dot kernel dot org>, Andrew Morton <akpm at linux-foundation dot org>, Heiko Carstens <heiko dot carstens at de dot ibm dot com>, feng dot tang at intel dot com, Fr??d??ric Weisbecker <fweisbec at gmail dot com>, Peter Zijlstra <peterz at infradead dot org>, jakub at redhat dot com, gcc at gcc dot gnu dot org
- Date: Thu, 19 Nov 2009 16:59:37 -0800 (PST)
- Subject: Re: BUG: GCC-4.4.x changes the function frame on some functions
- References: <viy9pk4xossnb6m2v041qu00.1258665258503@email.android.com> <4B05B7AD.20500@redhat.com> <1258670580.22249.1002.camel@gandalf.stny.rr.com> <4B05DBBF.5000804@redhat.com> <alpine.LFD.2.00.0911200122190.24119@localhost.localdomain>
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