From: Nick Desaulniers <ndesaulniers@google.com>
To: Linus Torvalds <torvalds@linux-foundation.org>,
Clement Courbet <courbet@google.com>
Cc: "H. Peter Anvin" <hpa@zytor.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 12:58:27 -0700 [thread overview]
Message-ID: <CAKwvOdkJxmSXg+v1pG4+DkhoJzEE47smce6pB=Zhy6viY_++xw@mail.gmail.com> (raw)
In-Reply-To: <CAKwvOdkut+GTLxX9U=hxDC8SaugW487XD_98d9yFU2VzShyz0A@mail.gmail.com>
On Tue, Aug 18, 2020 at 12:25 PM Nick Desaulniers
<ndesaulniers@google.com> wrote:
>
> On Tue, Aug 18, 2020 at 12:19 PM Linus Torvalds
> <torvalds@linux-foundation.org> wrote:
> >
> > 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.
That's asking for trouble; please don't implement routines with
identifiers from libc but with differing function signatures, and then
proceed to *not* use -ffreestanding. You can't have it both ways
(optimizations from *not* using -ffreestanding, then breaking all
kinds of assumptions based on conventions used across userspace), at
least not with the tools you currently have.
> >
> > 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,
I wrote a paper in college on the philosophy and symbolism in "Gran
Torino." Would recommend (the movie, not the paper).
> > and leave them alone. But never ever should "use bcmp" be
> > any kind of default behavior. That's some batshit crazy stuff.
> >
> > Linus
>
> You'll have to ask Clement about that. I'm not sure I ever saw the
> "faster bcmp than memcmp" implementation, but I was told "it exists"
> when I asked for a revert when all of our kernel builds went red.
Also, to Clement's credit, every patch I've ever seen from Clement is
backed up by data; typically fleetwide profiles at Google. "we spend
a lot of time in memcmp, particularly comparing the result against
zero and no other value; hmm...how do we spend less time in
memcmp...oh, well there's another library function with slightly
different semantics we can call instead." I don't think anyone would
consider the optimization batshit crazy given the number of cycles
saved across the fleet. That an embedded project didn't provide an
implementation, is a footnote that can be fixed in the embedded
project, either by using -ffreestanding or -fno-builtin-bcmp, which is
what this series proposes to do.
--
Thanks,
~Nick Desaulniers
next prev parent reply other threads:[~2020-08-18 19:58 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
2020-08-18 19:25 ` Nick Desaulniers
2020-08-18 19:58 ` Nick Desaulniers [this message]
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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