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

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Date:	Thu, 19 Nov 2009 14:12:21 -0700
From:	Jeff Law <law@...hat.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	"H. Peter Anvin" <hpa@...or.com>, rostedt@...dmis.org,
	David Daney <ddaney@...iumnetworks.com>,
	Andrew Haley <aph@...hat.com>,
	Richard Guenther <richard.guenther@...il.com>,
	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>,
	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 11/19/09 13:06, Linus Torvalds wrote:
>
> On Thu, 19 Nov 2009, H. Peter Anvin wrote:
>    
>> Calling the profiler immediately at the entry point is clearly the more
>> sane option.  It means the ABI is well-defined, stable, and independent
>> of what the actual function contents are.  It means that ABI isn't the
>> normal C ABI (the __fentry__ function would have to preserve all
>> registers), but that's fine...
>>      
> As far as I know, that's true of _mcount already: it's not a normal ABI
> and is rather a highly architecture-specific special case to begin with.
> At least ARM has some (several?) special mcount calling conventions,
> afaik.
>    
Correct.  _mcount's ABI typically has been defined by the implementation 
of the vendor's C library mcount.

GCC has options to emit the profiling code prior to or after the 
prologue controllable through the usual variety of target macros & 
hooks.  I can't imagine anyone would object to a clean, tested patch to 
change how x86-linux's profiling code was implemented.

jeff



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