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

Thomas Gleixner
2015-04-11T18:57:57Z

On Sat, 11 Apr 2015, Linus Torvalds wrote:
> On Apr 11, 2015 2:20 AM, "Ingo Molnar" <mingo@kernel.org> wrote:
> >
> > Another thing caught my attention (and I'm hijacking the RCU thread
> > again): GCC's notion of how to place branches seems somewhat random,
> > and rather bloaty.
> >
> > So I tried the experiment below on an x86 defconfig, turning off GCC's
> > branch heuristics, and it's rather surprising:
> >
> > text data bss dec filename
> > 12566447 1617840 1089536 15273823 vmlinux.fguess-branch-probability
> > 11923593 1617840 1089536 14630969 vmlinux.fno-guess-branch-probability
> >
> > That's an 5.4% code size improvement!
>
> Ugh. That's much larger than I would have expected. Is it just because
> gcc ends up turning
>
> if (a)
> b
> c
>
> into
>
> if (a) goto out-of-line
> return:
> c
> ..
> out-of-line:
> c
> goto return;
>
> a lot? Still, 5% sounds insanely big.
>
> How much of that 5% comes from code alignment? Or was this on *top* of
> the 1-byte alignment testt?

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%

So with the no-guess applied the align-1 does not matter anymore.

Thanks,

tglx