Messages in this thread | | | Date | Sun, 12 Apr 2015 12:15:51 +0200 | From | Ingo Molnar <> | Subject | Re: [PATCH] x86: Turn off GCC branch probability heuristics |
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* Markus Trippelsdorf <markus@trippelsdorf.de> wrote:
> On 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 > > If you like to play with more knobs you could explore the various > --param options that are listed in the gcc man page...
Well, I had a look, they are rather incomplete (at least as far as branch optimizations go), and I wouldn't want to rely on them for production kernel patches in any case, only on the more well-known compiler options.
Thanks,
Ingo
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