linux-kernel - Re: gcc inlining heuristics was Re: [PATCH -v7][RFC]: mutex: implement adaptive spinning

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Message-ID: <20090112230117.GC26430@shareable.org>
Date:	Mon, 12 Jan 2009 23:01:17 +0000
From:	Jamie Lokier <jamie@...reable.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	"H. Peter Anvin" <hpa@...or.com>, Andi Kleen <andi@...stfloor.org>,
	Bernd Schmidt <bernds_cb1@...nline.de>,
	David Woodhouse <dwmw2@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Harvey Harrison <harvey.harrison@...il.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>,
	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>, jh@...e.cz
Subject: Re: gcc inlining heuristics was Re: [PATCH -v7][RFC]: mutex: implement adaptive spinning

Linus Torvalds wrote:
> > This is about storage allocation, not aliases.  Storage allocation only
> > depends on lifetime.
> 
> Well, the thing is, code motion does extend life-times, and if you think 
> you can move stores across each other (even when you can see that they 
> alias statically) due to type-based alias decisions, that does essentially 
> end up making what _used_ to be disjoint lifetimes now be potentially 
> overlapping.

Sometimes code motion makes code faster and/or smaller but use more
stack space.  If you want to keep the stack use down, it blocks some
other optimisations.

Register allocation is similar: code motion optimisations may use more
registers due to overlapping lifetimes, which causes more register
spills and changes the code.  The two interact; it's not trivial to
optimise fully.

-- Jamie
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