Date: Tue, 24 Nov 2009 10:36:34 -0500
From: Jakub Jelinek <jakub@redhat.com>
To: Andrew Haley <aph@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>, "H.J. Lu" <hjl.tools@gmail.com>,
       rostedt@goodmis.org, Ingo Molnar <mingo@elte.hu>,
       "H. Peter Anvin" <hpa@zytor.com>, LKML <linux-kernel@vger.kernel.org>,
       Andrew Morton <akpm@linux-foundation.org>,
       Heiko Carstens <heiko.carstens@de.ibm.com>, feng.tang@intel.com,
       Peter Zijlstra <peterz@infradead.org>,
       Frederic Weisbecker <fweisbec@gmail.com>,
       David Daney <ddaney@caviumnetworks.com>,
       Richard Guenther <richard.guenther@gmail.com>, gcc <gcc@gcc.gnu.org>,
       Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [PATCH][GIT PULL][v2.6.32] tracing/x86: Add check to detect GCC messing with mcount prologue
Message-ID: <20091124153634.GK22813@hs20-bc2-1.build.redhat.com>
Reply-To: Jakub Jelinek <jakub@redhat.com>
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On Tue, Nov 24, 2009 at 03:32:20PM +0000, Andrew Haley wrote:
> Jakub Jelinek wrote:
> > On Tue, Nov 24, 2009 at 03:55:49PM +0100, Thomas Gleixner wrote:
> >>> you should compile your code with -maccumulate-outgoing-args, and there's
> >>> no need to use -mtune=generic.  Is that right?
> >> Seems to work. What other side effects has that ?
> > 
> > Faster code, significant increase in code size though.
> 
> Does it affect code size when we don't have to realign the stack pointer?

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.

	Jakub
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