Messages in this thread Patch in this message | | | Date | Fri, 10 Apr 2015 14:18:08 +0200 | From | Ingo Molnar <> | Subject | [PATCH] x86: Pack function addresses tightly as well |
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* 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)
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