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

Mike Galbraith
2015-04-12T07:56:19Z

On Sun, 2015-04-12 at 07:47 +0200, Ingo Molnar wrote:
> * 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.)

Ew, my i4790 really hated this patch.

taskset 0xc pipe-test 1
avg
733.2 727.1 743.1 746.6 737.1 737 KHz 1.000 -gcc_twiddle
713.7 717.9 715.5 718.0 708.7 714 KHz .968 +gcc_twiddle

tbench.sh 8 30

3566.14 3560.91 3566.35 3556.57 3549.69 3559.93 MB/S 1.000 -gcc_twiddle
2862.18 2899.51 2888.74 2897.18 2878.63 2885.25 MB/S .810 +gcc_twiddle

-Mike