[PATCH] x86: Pack function addresses tightly as well

Ingo Molnar
2015-04-10T12:18:15Z

* Ingo Molnar <mingo@kernel.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@kernel.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@kernel.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)