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From: Linus Torvalds <torvalds@linux-foundation.org>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: Kees Cook <keescook@chromium.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Josh Poimboeuf <jpoimboe@redhat.com>,
	Rasmus Villemoes <linux@rasmusvillemoes.dk>,
	"Gustavo A. R. Silva" <gustavo@embeddedor.com>,
	"Tobin C. Harding" <me@tobin.cc>,
	Steven Rostedt <rostedt@goodmis.org>,
	Jonathan Corbet <corbet@lwn.net>, Chris Mason <clm@fb.com>,
	Josef Bacik <jbacik@fb.com>, David Sterba <dsterba@suse.com>,
	"David S. Miller" <davem@davemloft.net>,
	Alexey Kuznetsov <kuznet@ms2.inr.ac.ru>,
	Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>,
	Ingo Molnar <mingo@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Masahiro Yamada <yamada.masahiro@socionext.com>,
	Borislav Petkov <bp@suse.de>,
	Randy Dunlap <rdunlap@infradead.org>,
	Ian Abbott <abbotti@mev.co.uk>,
	Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>,
	Petr Mladek <pmladek@suse.com>,
	Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
	Pantelis Antoniou <pantelis.antoniou@konsulko.com>,
	Linux Btrfs <linux-btrfs@vger.kernel.org>,
	Network Development <netdev@vger.kernel.org>,
	Kernel Hardening <kernel-hardening@lists.openwall.com>
Subject: Re: [PATCH v3] kernel.h: Skip single-eval logic on literals in min()/max()
Date: Mon, 12 Mar 2018 16:57:59 -0700	[thread overview]
Message-ID: <CA+55aFwK7W0SsaP5eR+2TxOj-j_Mu_E11fRw2Gk8ptV71ebvww@mail.gmail.com> (raw)
In-Reply-To: <20180312155524.b421f07d7f08f24c57bd1887@linux-foundation.org>

On Mon, Mar 12, 2018 at 3:55 PM, Andrew Morton
<akpm@linux-foundation.org> wrote:
>
> Replacing the __builtin_choose_expr() with ?: works of course.

Hmm. That sounds like the right thing to do. We were so myopically
staring at the __builtin_choose_expr() problem that we overlooked the
obvious solution.

Using __builtin_constant_p() together with a ?: is in fact our common
pattern, so that should be fine. The only real reason to use
__builtin_choose_expr() is if you want to get the *type* to vary
depending on which side you choose, but that's not an issue for
min/max.

> What will be the runtime effects?

There should be none. Gcc will turn the conditional for the ?: into a
constant, and DTRT.

              Linus

WARNING: multiple messages have this Message-ID (diff)
From: Linus Torvalds <torvalds@linux-foundation.org>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: Kees Cook <keescook@chromium.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Josh Poimboeuf <jpoimboe@redhat.com>,
	Rasmus Villemoes <linux@rasmusvillemoes.dk>,
	"Gustavo A. R. Silva" <gustavo@embeddedor.com>,
	"Tobin C. Harding" <me@tobin.cc>,
	Steven Rostedt <rostedt@goodmis.org>,
	Jonathan Corbet <corbet@lwn.net>, Chris Mason <clm@fb.com>,
	Josef Bacik <jbacik@fb.com>, David Sterba <dsterba@suse.com>,
	"David S. Miller" <davem@davemloft.net>,
	Alexey Kuznetsov <kuznet@ms2.inr.ac.ru>,
	Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>,
	Ingo Molnar <mingo@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Masahiro Yamada <yamada.masahiro@socionext.com>,
	Borislav Petkov <bp@suse.de>,
	Randy Dunlap <rdunlap@infra
Subject: Re: [PATCH v3] kernel.h: Skip single-eval logic on literals in min()/max()
Date: Mon, 12 Mar 2018 16:57:59 -0700	[thread overview]
Message-ID: <CA+55aFwK7W0SsaP5eR+2TxOj-j_Mu_E11fRw2Gk8ptV71ebvww@mail.gmail.com> (raw)
In-Reply-To: <20180312155524.b421f07d7f08f24c57bd1887@linux-foundation.org>

On Mon, Mar 12, 2018 at 3:55 PM, Andrew Morton
<akpm@linux-foundation.org> wrote:
>
> Replacing the __builtin_choose_expr() with ?: works of course.

Hmm. That sounds like the right thing to do. We were so myopically
staring at the __builtin_choose_expr() problem that we overlooked the
obvious solution.

Using __builtin_constant_p() together with a ?: is in fact our common
pattern, so that should be fine. The only real reason to use
__builtin_choose_expr() is if you want to get the *type* to vary
depending on which side you choose, but that's not an issue for
min/max.

> What will be the runtime effects?

There should be none. Gcc will turn the conditional for the ?: into a
constant, and DTRT.

              Linus

  reply	other threads:[~2018-03-12 23:57 UTC|newest]

Thread overview: 85+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-03-09 20:05 [PATCH v3] kernel.h: Skip single-eval logic on literals in min()/max() Kees Cook
2018-03-09 20:05 ` Kees Cook
2018-03-09 21:10 ` Linus Torvalds
2018-03-09 21:10   ` Linus Torvalds
2018-03-09 21:47   ` Kees Cook
2018-03-09 21:47     ` Kees Cook
2018-03-11 22:46   ` Tobin C. Harding
2018-03-11 22:46     ` Tobin C. Harding
2018-03-11 22:46     ` Tobin C. Harding
2018-03-13 13:31   ` David Laight
2018-03-13 13:31     ` David Laight
2018-03-10  0:07 ` Andrew Morton
2018-03-10  0:07   ` Andrew Morton
2018-03-10  0:28   ` Linus Torvalds
2018-03-10  0:28     ` Linus Torvalds
2018-03-10  0:32     ` Andrew Morton
2018-03-10  0:32       ` Andrew Morton
2018-03-10  0:38       ` Linus Torvalds
2018-03-10  0:38         ` Linus Torvalds
2018-03-10  1:30         ` Kees Cook
2018-03-10  1:30           ` Kees Cook
2018-03-10  1:31           ` Kees Cook
2018-03-10  1:31             ` Kees Cook
2018-03-10  2:37             ` Linus Torvalds
2018-03-10  2:37               ` Linus Torvalds
2018-03-12 22:55           ` Andrew Morton
2018-03-12 22:55             ` Andrew Morton
2018-03-12 23:57             ` Linus Torvalds [this message]
2018-03-12 23:57               ` Linus Torvalds
2018-03-13  4:28               ` Kees Cook
2018-03-13  4:28                 ` Kees Cook
2018-03-13 21:02                 ` Andrew Morton
2018-03-13 21:02                   ` Andrew Morton
2018-03-13 22:14                   ` Kees Cook
2018-03-13 22:14                     ` Kees Cook
2018-03-14 11:35                     ` David Laight
2018-03-14 11:35                       ` David Laight
2018-03-10  3:11   ` Randy Dunlap
2018-03-10  3:11     ` Randy Dunlap
2018-03-10  6:10     ` Miguel Ojeda
2018-03-10  6:10       ` Miguel Ojeda
2018-03-10  7:03       ` Miguel Ojeda
2018-03-10  7:03         ` Miguel Ojeda
2018-03-10 16:04         ` Linus Torvalds
2018-03-10 16:04           ` Linus Torvalds
2018-03-10 15:33       ` Kees Cook
2018-03-10 15:33         ` Kees Cook
2018-03-10 16:11         ` Linus Torvalds
2018-03-10 16:11           ` Linus Torvalds
2018-03-10 16:30         ` Linus Torvalds
2018-03-10 16:30           ` Linus Torvalds
2018-03-10 17:34           ` Miguel Ojeda
2018-03-10 17:34             ` Miguel Ojeda
2018-03-10 17:51             ` Linus Torvalds
2018-03-10 17:51               ` Linus Torvalds
2018-03-10 19:08               ` Miguel Ojeda
2018-03-10 19:08                 ` Miguel Ojeda
2018-03-11 11:05               ` Ingo Molnar
2018-03-11 11:05                 ` Ingo Molnar
2018-03-11 18:23                 ` Linus Torvalds
2018-03-11 18:23                   ` Linus Torvalds
2018-03-10  2:34 [PATCH 0/3] tracing: Rewrite the function filter code Steven Rostedt
2018-03-10  2:34 ` [PATCH 1/3] tracing: Combine enum and arrays into single macro in " Steven Rostedt
2018-03-12 10:31   ` Masami Hiramatsu
2018-03-10  2:34 ` [PATCH 2/3] tracing: Clean up and document pred_funcs_##type creation and use Steven Rostedt
2018-03-12 13:42   ` Masami Hiramatsu
2018-03-10  2:34 ` [PATCH 3/3] tracing: Rewrite filter logic to be simpler and faster Steven Rostedt
2018-03-10  3:10   ` Steven Rostedt
2018-03-10  3:10     ` Steven Rostedt
2018-03-10  3:15     ` Steven Rostedt
2018-03-10  3:15       ` Steven Rostedt
2018-03-10  3:22       ` Steven Rostedt
2018-03-10  3:22         ` Steven Rostedt
2018-03-10  3:18   ` Steven Rostedt
2018-03-12 12:42   ` Jiri Olsa
2018-03-12 18:38     ` Steven Rostedt
2018-03-12 15:10   ` Jiri Olsa
2018-03-12 18:40     ` Steven Rostedt
2018-03-12 18:54       ` Jiri Olsa
2018-03-12 19:10         ` Steven Rostedt
2018-03-12 23:52         ` Steven Rostedt
2018-03-13 10:14           ` Jiri Olsa
2018-03-13 14:12             ` Steven Rostedt
2018-03-13 14:27               ` Jiri Olsa
2018-03-11 19:54 ` [PATCH 0/3] tracing: Rewrite the function filter code Jiri Olsa

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