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Date: Sat, 11 Apr 2015 10:41:46 -0700
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Subject: Re: [PATCH] x86: Turn off GCC branch probability heuristics
From: Linus Torvalds <torvalds@linux-foundation.org>
To: Ingo Molnar <mingo@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>, Denys Vlasenko <dvlasenk@redhat.com>,
        Borislav Petkov <bp@alien8.de>, Tim Chen <tim.c.chen@linux.intel.com>,
        Andy Lutomirski <luto@amacapital.net>, Jason Low <jason.low2@hp.com>,
        Brian Gerst <brgerst@gmail.com>,
        Aswin Chandramouleeswaran <aswin@hp.com>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Davidlohr Bueso <dave@stgolabs.net>,
        Peter Zijlstra <a.p.zijlstra@chello.nl>,
        "H. Peter Anvin" <hpa@zytor.com>, LKML <linux-kernel@vger.kernel.org>,
        Peter Zijlstra <peterz@infradead.org>
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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