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

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Date:	Thu, 19 Nov 2009 18:36:43 +0000
From:	Andrew Haley <aph@...hat.com>
To:	rostedt@...dmis.org
CC:	Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...e.hu>,
	"H. Peter Anvin" <hpa@...or.com>,
	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

Steven Rostedt wrote:
> On Thu, 2009-11-19 at 18:20 +0000, Andrew Haley wrote:
> 
>> OK, I found it.  There is a struct defined as
>>
>> struct entry {
>>  ...
>> } __attribute__((__aligned__((1 << (4)))));
>>
>> and then in timer_stats_update_stats you have a local variable of type
>> struct entry:
>>
>> void timer_stats_update_stats()
>> {
>>  spinlock_t *lock;
>>  struct entry *entry, input;
>>
>> So, gcc has to 16-align the stack pointer to satisfy the alignment
>> for struct entry.
> 
> It has to align the entire stack? Why not just the variable within the
> stack?

How?. gcc has to know, at compile time, the offset from sp of each variable.
So, it of course makes sure that offset is 16-aligned, but it also has to
16-align the stack pointer.

Andrew.
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