Messages in this thread Patch in this message | | | Date | Sun, 12 Apr 2015 07:47:47 +0200 | From | Ingo Molnar <> | Subject | Re: [PATCH] x86: Turn off GCC branch probability heuristics |
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* Linus Torvalds <torvalds@linux-foundation.org> wrote:
> On Sat, Apr 11, 2015 at 11:57 AM, Thomas Gleixner <tglx@linutronix.de> wrote: > > > > I thinks its just the no-guess one: > > > > text data dec patch reduction > > 7563475 1781048 10302987 > > 7192973 1780024 9931461 no-guess -4.8% > > 7354819 1781048 958464 align-1 -2.7% > > 7192973 1780024 9931461 no-guess + align-1 -4.8% > > Yeah, a 5% code expansion is a big deal. Sadly, it looks like > 'no-guess' also disables our explicit likely/unlikely handling.
So I spent some time trying to get as much code size reduction as possible via GCC optimization options, and the total savings possible are 10.1%:
text data bss dec filename 12566391 1617840 1089536 15273767 vmlinux.vanilla 11416805 1617840 1089536 14124181 vmlinux.combo 10532552 1596080 1089536 13218168 vmlinux.Os
(combo patch attached below.)
The -Os savings are 19% total - but as you mentioned before, it's sometimes achieved through unacceptable techniques.
Unfortunately I found no other GCC options to achieve what -Os does - the missing 9% can purely be achieved via -Os, with no cherry-picking possible.
The other, smaller savings are:
+ # 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
(each of them has to be double checked to make sure it leads to nothing silly and unacceptable - I just blindly tried to find GCC options that impacted kernel code size.)
Thanks,
Ingo
--- arch/x86/Makefile | 24 ++++++++++++++++++++++++ 1 file changed, 24 insertions(+)
diff --git a/arch/x86/Makefile b/arch/x86/Makefile index 5ba2d9ce82dc..999e94685d12 100644 --- a/arch/x86/Makefile +++ b/arch/x86/Makefile @@ -77,10 +77,34 @@ 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) + # + # Don't guess branch probabilities, follow the code and unlikely()/likely() hints, + # which reduces vmlinux size by about 5.4%: + # + KBUILD_CFLAGS += -fno-guess-branch-probability + + # 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 + # Use -mpreferred-stack-boundary=3 if supported. KBUILD_CFLAGS += $(call cc-option,-mpreferred-stack-boundary=3)
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