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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>
Subject: Re: [PATCH] x86: Turn off GCC branch probability heuristics
Date: Sat, 11 Apr 2015 10:41:46 -0700	[thread overview]
Message-ID: <CA+55aFygq02Y9FATg9JOrLkM97a__DNi_LoMd+FmX9n3a-JoBA@mail.gmail.com> (raw)
In-Reply-To: <20150411092021.GA9478@gmail.com>

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

  reply	other threads:[~2015-04-11 17:41 UTC|newest]

Thread overview: 108+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2015-04-08 19:39 [PATCH 0/2] locking: Simplify mutex and rwsem spinning code Jason Low
2015-04-08 19:39 ` [PATCH 1/2] locking/mutex: Further refactor mutex_spin_on_owner() Jason Low
2015-04-09  9:00   ` [tip:locking/core] locking/mutex: Further simplify mutex_spin_on_owner() tip-bot for Jason Low
2015-04-08 19:39 ` [PATCH 2/2] locking/rwsem: Use a return variable in rwsem_spin_on_owner() Jason Low
2015-04-09  5:37   ` Ingo Molnar
2015-04-09  6:40     ` Jason Low
2015-04-09  7:53       ` Ingo Molnar
2015-04-09 16:47         ` Linus Torvalds
2015-04-09 17:56           ` Paul E. McKenney
2015-04-09 18:08             ` Linus Torvalds
2015-04-09 18:16               ` Linus Torvalds
2015-04-09 18:39                 ` Paul E. McKenney
2015-04-10  9:00                   ` [PATCH] mutex: Speed up mutex_spin_on_owner() by not taking the RCU lock Ingo Molnar
2015-04-10  9:12                     ` Ingo Molnar
2015-04-10  9:21                       ` [PATCH] uaccess: Add __copy_from_kernel_inatomic() primitive Ingo Molnar
2015-04-10 11:14                         ` [PATCH] x86/uaccess: Implement get_kernel() Ingo Molnar
2015-04-10 11:27                           ` [PATCH] mutex: Improve mutex_spin_on_owner() code generation Ingo Molnar
2015-04-10 12:08                             ` [PATCH] x86: Align jump targets to 1 byte boundaries Ingo Molnar
2015-04-10 12:18                               ` [PATCH] x86: Pack function addresses tightly as well Ingo Molnar
2015-04-10 12:30                                 ` [PATCH] x86: Pack loops " Ingo Molnar
2015-04-10 13:46                                   ` Borislav Petkov
2015-05-15  9:40                                   ` [tip:x86/asm] " tip-bot for Ingo Molnar
2015-05-17  6:03                                   ` [tip:x86/apic] " tip-bot for Ingo Molnar
2015-05-15  9:39                                 ` [tip:x86/asm] x86: Pack function addresses " tip-bot for Ingo Molnar
2015-05-15 18:36                                   ` Linus Torvalds
2015-05-15 20:52                                     ` Denys Vlasenko
2015-05-17  5:58                                     ` Ingo Molnar
2015-05-17  7:09                                       ` Ingo Molnar
2015-05-17  7:30                                         ` Ingo Molnar
2015-05-18  9:28                                       ` Denys Vlasenko
2015-05-19 21:38                                       ` [RFC PATCH] x86/64: Optimize the effective instruction cache footprint of kernel functions Ingo Molnar
2015-05-20  0:47                                         ` Linus Torvalds
2015-05-20 12:21                                           ` Denys Vlasenko
2015-05-21 11:36                                             ` Ingo Molnar
2015-05-21 11:38                                             ` Denys Vlasenko
2016-04-16 21:08                                               ` Denys Vlasenko
2015-05-20 13:09                                           ` Ingo Molnar
2015-05-20 11:29                                         ` Denys Vlasenko
2015-05-21 13:28                                           ` Ingo Molnar
2015-05-21 14:03                                           ` Ingo Molnar
2015-04-10 12:50                               ` [PATCH] x86: Align jump targets to 1 byte boundaries Denys Vlasenko
2015-04-10 13:18                                 ` H. Peter Anvin
2015-04-10 17:54                                   ` Ingo Molnar
2015-04-10 18:32                                     ` H. Peter Anvin
2015-04-11 14:41                                   ` Markus Trippelsdorf
2015-04-12 10:14                                     ` Ingo Molnar
2015-04-13 16:23                                       ` Markus Trippelsdorf
2015-04-13 17:26                                         ` Markus Trippelsdorf
2015-04-13 18:31                                           ` Linus Torvalds
2015-04-13 19:09                                             ` Markus Trippelsdorf
2015-04-14  5:38                                               ` Ingo Molnar
2015-04-14  8:23                                                 ` Markus Trippelsdorf
2015-04-14  9:16                                                   ` Ingo Molnar
2015-04-14 11:17                                                     ` Markus Trippelsdorf
2015-04-14 12:09                                                       ` Ingo Molnar
2015-04-10 18:48                                 ` Linus Torvalds
2015-04-12 23:44                                   ` Maciej W. Rozycki
2015-04-10 19:23                                 ` Daniel Borkmann
2015-04-11 13:48                                 ` Markus Trippelsdorf
2015-04-10 13:19                               ` Borislav Petkov
2015-04-10 13:54                                 ` Denys Vlasenko
2015-04-10 14:01                                   ` Borislav Petkov
2015-04-10 14:53                                     ` Denys Vlasenko
2015-04-10 15:25                                       ` Borislav Petkov
2015-04-10 15:48                                         ` Denys Vlasenko
2015-04-10 15:54                                           ` Borislav Petkov
2015-04-10 21:44                                             ` Borislav Petkov
2015-04-10 18:54                                       ` Linus Torvalds
2015-04-10 14:10                               ` Paul E. McKenney
2015-04-11 14:28                                 ` Josh Triplett
2015-04-11  9:20                               ` [PATCH] x86: Turn off GCC branch probability heuristics Ingo Molnar
2015-04-11 17:41                                 ` Linus Torvalds [this message]
2015-04-11 18:57                                   ` Thomas Gleixner
2015-04-11 19:35                                     ` Linus Torvalds
2015-04-12  5:47                                       ` Ingo Molnar
2015-04-12  6:20                                         ` Markus Trippelsdorf
2015-04-12 10:15                                           ` Ingo Molnar
2015-04-12  7:56                                         ` Mike Galbraith
2015-04-12  7:41                                       ` Ingo Molnar
2015-04-12  8:07                                     ` Ingo Molnar
2015-04-12 21:11                                     ` Jan Hubicka
2015-05-14 11:59                               ` [PATCH] x86: Align jump targets to 1 byte boundaries Denys Vlasenko
2015-05-14 18:17                                 ` Ingo Molnar
2015-05-14 19:04                                   ` Denys Vlasenko
2015-05-14 19:44                                     ` Ingo Molnar
2015-05-15 15:45                                   ` Josh Triplett
2015-05-17  5:34                                     ` Ingo Molnar
2015-05-17 19:18                                       ` Josh Triplett
2015-05-18  6:48                                         ` Ingo Molnar
2015-05-15  9:39                               ` [tip:x86/asm] x86: Align jump targets to 1-byte boundaries tip-bot for Ingo Molnar
2015-04-10 11:34                           ` [PATCH] x86/uaccess: Implement get_kernel() Peter Zijlstra
2015-04-10 18:04                             ` Ingo Molnar
2015-04-10 17:49                           ` Linus Torvalds
2015-04-10 18:04                             ` Ingo Molnar
2015-04-10 18:09                               ` Linus Torvalds
2015-04-10 14:20                     ` [PATCH] mutex: Speed up mutex_spin_on_owner() by not taking the RCU lock Paul E. McKenney
2015-04-10 17:44                       ` Ingo Molnar
2015-04-10 18:05                         ` Paul E. McKenney
2015-04-09 19:43                 ` [PATCH 2/2] locking/rwsem: Use a return variable in rwsem_spin_on_owner() Jason Low
2015-04-09 19:58                   ` Paul E. McKenney
2015-04-09 20:58                     ` Jason Low
2015-04-09 21:07                       ` Paul E. McKenney
2015-04-09 19:59                   ` Davidlohr Bueso
2015-04-09 20:36                 ` Jason Low
2015-04-10  2:43                   ` Andev
2015-04-10  9:04                   ` Ingo Molnar
2015-04-08 19:49 ` [PATCH 0/2] locking: Simplify mutex and rwsem spinning code Davidlohr Bueso
2015-04-08 20:10   ` Jason Low

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