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

[Linus Torvalds <torvalds@linux-foundation.org>]

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?

                                Linus