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

From: Linus Torvalds
Date: Sat Apr 11 2015 - 13:42:05 EST

On Apr 11, 2015 2:20 AM, "Ingo Molnar" <mingo@xxxxxxxxxx> 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)


if (a) goto out-of-line
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?

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