linux-kernel - Re: [PATCH] x86: Align jump targets to 1 byte boundaries

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Message-ID: <20150414120954.GA23761@gmail.com>
Date:	Tue, 14 Apr 2015 14:09:55 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Markus Trippelsdorf <markus@...ppelsdorf.de>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Jason Low <jason.low2@...com>,
	Peter Zijlstra <peterz@...radead.org>,
	Davidlohr Bueso <dave@...olabs.net>,
	Tim Chen <tim.c.chen@...ux.intel.com>,
	Aswin Chandramouleeswaran <aswin@...com>,
	LKML <linux-kernel@...r.kernel.org>,
	Borislav Petkov <bp@...en8.de>,
	Andy Lutomirski <luto@...capital.net>,
	Brian Gerst <brgerst@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH] x86: Align jump targets to 1 byte boundaries


* Markus Trippelsdorf <markus@...ppelsdorf.de> wrote:

> > I'm not so sure about that one, our data access patterns are 
> > usually a lot more well thought out than our code alignment (which 
> > is really mostly compiler controlled). It also gives limited 
> > savings:
> > 
> >   9202488 vmlinux gcc-5
> >   9186105 vmlinux gcc-5 (-malign-data=abi)
> > 
> > Which is 0.1%. I've got a handful of options in that size range:
> > 
> > +        # Reduces vmlinux size by 0.25%:
> > +        KBUILD_CFLAGS += -fno-caller-saves
> > +
> > +        # Reduces vmlinux size by 1.10%:
> > +        KBUILD_CFLAGS += -fno-inline-small-functions
> > +
> > +        # Reduces vmlinux size by about 0.95%:
> > +        KBUILD_CFLAGS += -fno-tree-ch
> > 
> > but obviously they are more obscure and thus riskier. Find below 
> > an updated "Ingo's combo patch". It gives more than 10% savings 
> > here on x86 defconfig using gcc 4.9, without LTO.
> 
> Well obviously, if you do not care about performance you can reduce 
> the text size further and further. [...]

Yes, but I picked GCC options that I don't think impact performance 
negatively and offer a sizable debloating effect. Especially with 
inlining if code size increases it's probably a net loss.

> [...] But what is interesting is to keep the performance up (or even 
> increase it) and still reduce the text size.

By my (admittedly quick) review I think those 3 extra options I added 
still generate pretty OK code in the end. I.e. they are not like -Os 
that generates utter crap to save a byte.

Thanks,

	Ingo
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