linux-kernel - Re: [PATCH][GIT PULL][v2.6.32] tracing/x86: Add check to detect GCC messing with mcount prologue

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Date:	Tue, 24 Nov 2009 08:38:42 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Andrew Haley <aph@...hat.com>
CC:	Jakub Jelinek <jakub@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H.J. Lu" <hjl.tools@...il.com>, rostedt@...dmis.org,
	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, Peter Zijlstra <peterz@...radead.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	David Daney <ddaney@...iumnetworks.com>,
	Richard Guenther <richard.guenther@...il.com>,
	gcc <gcc@....gnu.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH][GIT PULL][v2.6.32] tracing/x86: Add check to detect GCC
 messing with mcount prologue

On 11/24/2009 07:46 AM, Andrew Haley wrote:
>>
>> Yes, a lot.  The difference is that -maccumulate-outgoing-args allocates
>> space for arguments of the callee with most arguments in the prologue, using
>> subtraction from sp, then to pass arguments uses movl XXX, 4(%esp) etc.
>> and the stack pointer doesn't usually change within the function (except for
>> alloca/VLAs).
>> With -mno-accumulate-outgoing-args args are pushed using push instructions
>> and stack pointer is constantly changing.
> 
> Alright.  So, it is possible in theory for gcc to generate code that
> only uses -maccumulate-outgoing-args when it needs to realign SP.
> And, therefore, we could have a nice option for the kernel: one with
> (mostly) good code density and never generates the bizarre code
> sequence in the prologue.
> 

If we're changing gcc anyway, then let's add the option of intercepting
the function at the point where the machine state is well-defined by
ABI, which is before the function stack frame is set up.

-maccumulate-outgoing-args sounds like it would be painful on x86 (not
using its cheap push/pop instructions), but I guess since it's only when
tracing it's less of an issue.

	-hpa

-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.

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