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Re: BUG: GCC-4.4.x changes the function frame on some functions
- From: Richard Guenther <richard dot guenther at gmail dot com>
- To: "H. Peter Anvin" <hpa at zytor dot com>
- Cc: Thomas Gleixner <tglx at linutronix dot de>, 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>, Steven Rostedt <rostedt at goodmis dot org>, Peter Zijlstra <peterz at infradead dot org>, jakub at redhat dot com, gcc at gcc dot gnu dot org
- Date: Thu, 19 Nov 2009 16:49:57 +0100
- Subject: Re: BUG: GCC-4.4.x changes the function frame on some functions
- References: <alpine.LFD.2.00.0911181933540.24119@localhost.localdomain> <tip-887a29f59b93cf54e21814869a4ab6e80b6fa623@git.kernel.org> <20091119072040.GA23579@elte.hu> <alpine.LFD.2.00.0911191053390.24119@localhost.localdomain> <alpine.LFD.2.00.0911191423190.24119@localhost.localdomain> <alpine.LFD.2.00.0911191607440.24119@localhost.localdomain> <4B056820.1050104@zytor.com>
On Thu, Nov 19, 2009 at 4:45 PM, H. Peter Anvin <hpa@zytor.com> wrote:
> On 11/19/2009 07:37 AM, Thomas Gleixner wrote:
>>
>> modified function start on a handful of functions only seen with gcc
>> 4.4.x on x86 32 bit:
>>
>> ? ? ? push ? %edi
>> ? ? ? lea ? ?0x8(%esp),%edi
>> ? ? ? and ? ?$0xfffffff0,%esp
>> ? ? ? pushl ?-0x4(%edi)
>> ? ? ? push ? %ebp
>> ? ? ? mov ? ?%esp,%ebp
>> ? ? ? ...
>> ? ? ? call ? mcount
>>
>
> The real questions is why we're aligning the stack in the kernel. ?It is
> probably not what we want -- we don't use SSE for anything but a handful
> of special cases in the kernel, and we don't want the overhead.
It's likely because you have long long vars on the stack which is
faster when they are aligned. -mno-stackrealign may do what you
want (or may not, I have not checked). I assume you already
use -mpreferred-stack-boundary=2.
Richard.