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

Linus Torvalds
2015-04-11T17:41:48Z

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