On Apr 11, 2015 2:20 AM, "Ingo Molnar" <mi...@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
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