linux-kernel - Re: [patch] measurements, numbers about CONFIG_OPTIMIZE

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Date:	Fri, 9 Jan 2009 12:13:25 -0500
From:	Christoph Hellwig <hch@...radead.org>
To:	jim owens <jowens@...com>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Chris Mason <chris.mason@...cle.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	paulmck@...ux.vnet.ibm.com, Gregory Haskins <ghaskins@...ell.com>,
	Matthew Wilcox <matthew@....cx>,
	Andi Kleen <andi@...stfloor.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	linux-btrfs <linux-btrfs@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Nick Piggin <npiggin@...e.de>,
	Peter Morreale <pmorreale@...ell.com>,
	Sven Dietrich <SDietrich@...ell.com>
Subject: Re: [patch] measurements, numbers about CONFIG_OPTIMIZE_INLINING=y
	impact

On Fri, Jan 09, 2009 at 09:03:12AM -0500, jim owens wrote:
> They also know inlining may increase program object size.
> That inlining will reduce object size on many architectures
> if the function is small is just a happy side effect to them.

The problem is that the threshold for that is architecture specific.
While e.g. x86 has relatively low overhead of prologue/epilogue other
architectures like s390 have enormous overhead.  So handling this in
the compiler would be optimal, but it would need at least whole-program
optimization and a compiler aware of the inline assembly to get it
half-way right.

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