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From: Linus Torvalds <torvalds@linux-foundation.org>
To: "H. Peter Anvin" <hpa@zytor.com>
Cc: "Nick Desaulniers" <ndesaulniers@google.com>,
	"Masahiro Yamada" <masahiroy@kernel.org>,
	"Andrew Morton" <akpm@linux-foundation.org>,
	"Thomas Gleixner" <tglx@linutronix.de>,
	"Ingo Molnar" <mingo@redhat.com>,
	"Borislav Petkov" <bp@alien8.de>,
	"Michal Marek" <michal.lkml@markovi.net>,
	"Linux Kbuild mailing list" <linux-kbuild@vger.kernel.org>,
	LKML <linux-kernel@vger.kernel.org>,
	"Kees Cook" <keescook@chromium.org>,
	"Tony Luck" <tony.luck@intel.com>,
	"Dmitry Vyukov" <dvyukov@google.com>,
	"Michael Ellerman" <mpe@ellerman.id.au>,
	"Joe Perches" <joe@perches.com>,
	"Joel Fernandes" <joel@joelfernandes.org>,
	"Daniel Axtens" <dja@axtens.net>,
	"Arvind Sankar" <nivedita@alum.mit.edu>,
	"Andy Shevchenko" <andriy.shevchenko@linux.intel.com>,
	"Alexandru Ardelean" <alexandru.ardelean@analog.com>,
	"Yury Norov" <yury.norov@gmail.com>,
	"maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)"
	<x86@kernel.org>, "Ard Biesheuvel" <ardb@kernel.org>,
	"Paul E . McKenney" <paulmck@kernel.org>,
	"Daniel Kiper" <daniel.kiper@oracle.com>,
	"Bruce Ashfield" <bruce.ashfield@gmail.com>,
	"Marco Elver" <elver@google.com>,
	"Vamshi K Sthambamkadi" <vamshi.k.sthambamkadi@gmail.com>,
	"Andi Kleen" <ak@suse.de>,
	"Dávid Bolvanský" <david.bolvansky@gmail.com>,
	"Eli Friedman" <efriedma@quicinc.com>
Subject: Re: [PATCH 0/4] -ffreestanding/-fno-builtin-* patches
Date: Tue, 18 Aug 2020 12:13:22 -0700	[thread overview]
Message-ID: <CAHk-=wiPeRQU_5JXCN0TLoW-xHZHp7dmrhx0wyXUSKxiCxE02Q@mail.gmail.com> (raw)
In-Reply-To: <76071c24-ec6f-7f7a-4172-082bd574d581@zytor.com>

On Tue, Aug 18, 2020 at 12:03 PM H. Peter Anvin <hpa@zytor.com> wrote:
>
> I'm not saying "change the semantics", nor am I saying that playing
> whack-a-mole *for a limited time* is unreasonable. But I would like to go back
> to the compiler authors and get them to implement such a #pragma: "this
> freestanding implementation *does* support *this specific library function*,
> and you are free to call it."

I'd much rather just see the library functions as builtins that always
do the right thing (with the fallback being "just call the standard
function").

IOW, there's nothing wrong with -ffreestanding if you then also have
__builtin_memcpy() etc, and they do the sane compiler optimizations
for memcpy().

What we want to avoid is the compiler making *assumptions* based on
standard names, because we may implement some of those things
differently.

And honestly, a compiler that uses 'bcmp' is just broken. WTH? It's
the year 2020, we don't use bcmp. It's that simple. Fix your damn
broken compiler and use memcmp. The argument that memcmp is more
expensive than bcmp is garbage legacy thinking from four decades ago.

It's likely the other way around, where people have actually spent
time on memcmp, but not on bcmp.

If somebody really *wants* to use bcmp, give them the "Get off my
lawn" flag, and leave them alone. But never ever should "use bcmp" be
any kind of default behavior. That's some batshit crazy stuff.

               Linus

  reply	other threads:[~2020-08-18 19:13 UTC|newest]

Thread overview: 68+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-08-17 22:02 [PATCH 0/4] -ffreestanding/-fno-builtin-* patches Nick Desaulniers
2020-08-17 22:02 ` [PATCH 1/4] Makefile: add -fno-builtin-stpcpy Nick Desaulniers
2020-08-17 22:31   ` H. Peter Anvin
2020-08-17 23:36     ` Nick Desaulniers
2020-08-18 19:21     ` Kees Cook
2020-08-18  7:10   ` Ard Biesheuvel
2020-08-18  7:25     ` Greg KH
2020-08-18  7:29       ` Ard Biesheuvel
2020-08-18  7:34         ` Greg KH
2020-08-18 19:23   ` Kees Cook
2020-08-17 22:02 ` [PATCH 2/4] Revert "lib/string.c: implement a basic bcmp" Nick Desaulniers
2020-08-18  5:44   ` Nathan Chancellor
2020-08-18 18:00     ` Nick Desaulniers
2020-08-18 19:24       ` Kees Cook
2020-08-17 22:02 ` [PATCH 3/4] x86/boot: use -fno-builtin-bcmp Nick Desaulniers
2020-08-18 19:24   ` Kees Cook
2020-08-17 22:02 ` [PATCH 4/4] x86: don't build CONFIG_X86_32 as -ffreestanding Nick Desaulniers
2020-08-18 19:24   ` Kees Cook
2021-01-07  0:27   ` Fangrui Song
2022-04-07 15:34   ` [tip: x86/build] x86/build: Don't " tip-bot2 for Nick Desaulniers
2022-04-07 17:01     ` Nick Desaulniers
2022-04-07 22:28       ` Borislav Petkov
2020-08-17 22:44 ` [PATCH 0/4] -ffreestanding/-fno-builtin-* patches H. Peter Anvin
2020-08-18 17:56   ` Nick Desaulniers
2020-08-18 19:02     ` H. Peter Anvin
2020-08-18 19:13       ` Linus Torvalds [this message]
2020-08-18 19:25         ` Nick Desaulniers
2020-08-18 19:58           ` Nick Desaulniers
2020-08-19 12:19             ` Clement Courbet
2020-08-18 20:24         ` Arvind Sankar
2020-08-18 20:27           ` Nick Desaulniers
2020-08-18 20:58             ` Nick Desaulniers
2020-08-18 21:41               ` Arvind Sankar
2020-08-18 21:51                 ` Dávid Bolvanský
2020-08-18 21:59                 ` Nick Desaulniers
2020-08-18 22:05                   ` Dávid Bolvanský
2020-08-18 23:22                     ` Nick Desaulniers
2020-08-20 14:56                 ` Rasmus Villemoes
2020-08-20 17:56                   ` Arvind Sankar
2020-08-20 18:05                     ` Dávid Bolvanský
2020-08-20 23:33                     ` Linus Torvalds
2020-08-21 17:29                       ` Arvind Sankar
2020-08-21 17:54                         ` Linus Torvalds
2020-08-21 18:02                           ` Linus Torvalds
2020-08-21 19:14                             ` Arvind Sankar
2020-08-21 19:23                               ` Linus Torvalds
2020-08-21 19:57                           ` Arvind Sankar
2020-08-21 20:03                             ` Peter Zijlstra
2020-08-21 21:39                             ` Linus Torvalds
2020-08-22  0:12                               ` Nick Desaulniers
2020-08-22 12:20                                 ` David Laight
2020-08-21  6:45                     ` Rasmus Villemoes
2020-08-24 15:57                 ` Masahiro Yamada
2020-08-24 17:34                   ` Arvind Sankar
2020-08-25  7:10                     ` Nick Desaulniers
2020-08-25  7:31                       ` Nick Desaulniers
2020-08-25 12:28                       ` Masahiro Yamada
2020-08-25 14:02                         ` Nick Desaulniers
2020-08-26 13:28                           ` Masahiro Yamada
2020-08-18 21:53               ` David Laight
2020-08-20 22:41               ` H. Peter Anvin
2020-08-20 23:17                 ` Arvind Sankar
2020-08-18 19:35       ` Nick Desaulniers
2020-08-18 22:25 ` Arvind Sankar
2020-08-18 22:59   ` Nick Desaulniers
2020-08-18 23:51     ` Arvind Sankar
2020-08-19  0:20     ` Arvind Sankar
2020-08-19  8:26   ` David Laight

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