Re: [PATCH] x86: Turn off GCC branch probability heuristics

Ingo Molnar
2015-04-12T05:47:55Z

* 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)