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

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Date:	Sat, 10 Jan 2009 02:08:51 +0100
From:	Andi Kleen <andi@...stfloor.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Andi Kleen <andi@...stfloor.org>,
	Nicholas Miell <nmiell@...cast.net>,
	Ingo Molnar <mingo@...e.hu>, jim owens <jowens@...com>,
	"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>,
	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

> I thought -Os actually disabled the basic-block reordering, doesn't it?

Not in current gcc head no (just verified by stepping through) 

> 
> And I thought it did that exactly because it generates bigger code and 
> much worse I$ patterns (ie you have a lot of "conditional branch to other 
> place and then unconditional branch back" instead of "conditional branch 
> over the non-taken code".
> 
> Also, I think we've had about as much good luck with guessing 
> "likely/unlikely" as we've had with "inline" ;)

That's true.

But if you look at the default heuristics that gcc has (gcc/predict.def
in the gcc sources) like == NULL, < 0, branch guarding etc.
I would expect a lot of them to DTRT for the kernel.

Honza at some point even fixed goto to be unlikely after I complained :)
 
> Sadly, apart from some of the "never happens" error cases, the kernel 
> doesn't tend to have lots of nice patterns. We have almost no loops (well, 
> there are loops all over, but most of them we hopefully just loop over 
> once or twice in any good situation), and few really predictable things.

That actually makes us well suited to gcc, it has a relatively poor
loop optimizer compared to other compilers ;-)

> Or rather, they can easily be very predictable under one particular load, 
> and the totally the other way around under another ..

Yes that is why we got good branch predictors in CPUs I guess.

-Andi
-- 
ak@...ux.intel.com
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