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

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Message-ID: <20150514194415.GA27518@gmail.com>
Date:	Thu, 14 May 2015 21:44:15 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Denys Vlasenko <dvlasenk@...hat.com>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	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>,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH] x86: Align jump targets to 1 byte boundaries


* Denys Vlasenko <dvlasenk@...hat.com> wrote:

> On 05/14/2015 08:17 PM, Ingo Molnar wrote:
> > There were also these other changes I tested:
> > 
> > +        # 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
> > 
> > We could maybe consider -fno-caller-saves. What do you think about 
> > that option?
> 
> Quick googling din't even find what it does.
> It would need a comment :)

The GCC manpage says:

       -fcaller-saves
           Enable allocation of values to registers that are clobbered 
           by function calls, by emitting extra instructions to save 
           and restore the registers around such calls.  Such 
           allocation is done only when it seems to result in better 
           code.

           This option is always enabled by default on certain 
           machines, usually those which have no call-preserved 
           registers to use instead.

           Enabled at levels -O2, -O3, -Os.

but I have not checked the disassembly.

0.25% defconfig reduction still seems significant.

Thanks,

	Ingo
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