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

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Date:	Tue, 14 Apr 2015 07:38:21 +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:

> On 2015.04.13 at 11:31 -0700, Linus Torvalds wrote:
> > On Mon, Apr 13, 2015 at 10:26 AM, Markus Trippelsdorf
> > <markus@...ppelsdorf.de> wrote:
> > >
> > > I must have made a measurement mistake above, because the actual code
> > > size savings are roughly 5%:
> > 
> > Can you check against the -fno-guess-branch-probability output?
> 
>    text    data     bss     dec     filename
>    8746230  970072  802816 10519118 ./vmlinux gcc-5 (lto) 
>    9202488  978512  811008 10992008 ./vmlinux gcc-5
>    8036915  970296  802816 9810027  ./vmlinux gcc-5 (lto -fno-guess-branch-probability)
>    8593615  978512  811008 10383135 ./vmlinux gcc-5 (-fno-guess-branch-probability)

Just to make sure, could you please also apply the 3 alignment patches 
attached below? There's a lot of noise from extra alignment.

Having said that, LTO should have three main effects:

 1) better cross-unit inlining decisions

 2) better register allocation and clobbering knowledge (if a small 
    function is known not to clobber caller-saved registers, then the 
    saving can be skipped)

 3) better dead code elimination

1)-2) is probably worth the price, 3) in isolation isn't. So we'd have 
to estimate which one is how significant, to judge the value of LTO - 
but I haven't seen any effort so far to disambiguate it.

_Possibly_ if you build kernel/built-in.o only, and compared its 
sizes, that would help a bit, because the core kernel has very little 
dead code, giving a fairer estimation of 'true' optimizations.

Thanks,

	Ingo

======

 arch/x86/Makefile | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/arch/x86/Makefile b/arch/x86/Makefile
index 5ba2d9ce82dc..10989a73b986 100644
--- a/arch/x86/Makefile
+++ b/arch/x86/Makefile
@@ -77,6 +77,15 @@ else
         KBUILD_AFLAGS += -m64
         KBUILD_CFLAGS += -m64
 
+        # Pack jump targets tightly, don't align them to the default 16 bytes:
+        KBUILD_CFLAGS += -falign-jumps=1
+
+        # Pack functions tightly as well:
+        KBUILD_CFLAGS += -falign-functions=1
+
+        # Pack loops tightly as well:
+        KBUILD_CFLAGS += -falign-loops=1
+
         # Don't autogenerate traditional x87 instructions
         KBUILD_CFLAGS += $(call cc-option,-mno-80387)
         KBUILD_CFLAGS += $(call cc-option,-mno-fp-ret-in-387)
--
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