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

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Message-ID: <84fc9c000911190749y294381a2q16f4547991525cee@mail.gmail.com>
Date:	Thu, 19 Nov 2009 16:49:57 +0100
From:	Richard Guenther <richard.guenther@...il.com>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>, 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>,
	Steven Rostedt <rostedt@...dmis.org>,
	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 Thu, Nov 19, 2009 at 4:45 PM, H. Peter Anvin <hpa@...or.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.
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