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

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Date:	Sat, 11 Apr 2015 10:41:46 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Borislav Petkov <bp@...en8.de>,
	Tim Chen <tim.c.chen@...ux.intel.com>,
	Andy Lutomirski <luto@...capital.net>,
	Jason Low <jason.low2@...com>, Brian Gerst <brgerst@...il.com>,
	Aswin Chandramouleeswaran <aswin@...com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Davidlohr Bueso <dave@...olabs.net>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	"H. Peter Anvin" <hpa@...or.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH] x86: Turn off GCC branch probability heuristics

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