linux-kernel - [PATCH] x86: Pack function addresses tightly as well

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Message-ID: <20150410121808.GA19918@gmail.com>
Date:	Fri, 10 Apr 2015 14:18:08 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	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>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Brian Gerst <brgerst@...il.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: [PATCH] x86: Pack function addresses tightly as well


* Ingo Molnar <mingo@...nel.org> wrote:

> I realize that x86 CPU manufacturers recommend 16-byte jump target 
> alignments (it's in the Intel optimization manual), but the cost of 
> that is very significant:
> 
>         text           data       bss         dec      filename
>     12566391        1617840   1089536    15273767      vmlinux.align.16-byte
>     12224951        1617840   1089536    14932327      vmlinux.align.1-byte
> 
> By using 1 byte jump target alignment (i.e. no alignment at all) we 
> get an almost 3% reduction in kernel size (!) - and a probably 
> similar reduction in I$ footprint.

Likewise we could pack functions tightly as well via the patch below:

     text	   data	    bss	     dec	 filename
 12566391	1617840	1089536	15273767	 vmlinux.align.16-byte
 12224951	1617840	1089536	14932327	 vmlinux.align.1-byte
 11976567	1617840	1089536	14683943	 vmlinux.align.1-byte.funcs-1-byte

Which brings another 2% reduction in the kernel's code size.

It would be interesting to see some benchmarks with these two patches 
applied. Only lightly tested.

Thanks,

	Ingo

============================>
>From 773dbbfbf37df3520dd98e91972fbfdef5fe91ad Mon Sep 17 00:00:00 2001
From: Ingo Molnar <mingo@...nel.org>
Date: Fri, 10 Apr 2015 14:14:29 +0200
Subject: [PATCH] x86: Pack function addresses tightly as well

Not-Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 arch/x86/Makefile | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/arch/x86/Makefile b/arch/x86/Makefile
index 0366d6b44a14..573d0c459f99 100644
--- a/arch/x86/Makefile
+++ b/arch/x86/Makefile
@@ -77,9 +77,12 @@ else
         KBUILD_AFLAGS += -m64
         KBUILD_CFLAGS += -m64
 
-	# Align jump targets to 1 byte, not the default 16 bytes:
+        # 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
+
         # 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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