From: Nick Desaulniers <ndesaulniers@google.com>
To: Arvind Sankar <nivedita@alum.mit.edu>
Cc: "Dávid Bolvanský" <david.bolvansky@gmail.com>,
"Eli Friedman" <efriedma@quicinc.com>,
"Linus Torvalds" <torvalds@linux-foundation.org>,
"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>,
"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>
Subject: Re: [PATCH 0/4] -ffreestanding/-fno-builtin-* patches
Date: Tue, 18 Aug 2020 14:59:56 -0700 [thread overview]
Message-ID: <CAKwvOdnW8zjcxmHwu5PhHa1hMFu=S=qPh5gfC6tN7FrSE+3kKg@mail.gmail.com> (raw)
In-Reply-To: <20200818214146.GA3196105@rani.riverdale.lan>
On Tue, Aug 18, 2020 at 2:41 PM Arvind Sankar <nivedita@alum.mit.edu> wrote:
>
> On Tue, Aug 18, 2020 at 01:58:51PM -0700, Nick Desaulniers wrote:
> > On Tue, Aug 18, 2020 at 1:27 PM Nick Desaulniers
> > <ndesaulniers@google.com> wrote:
> > >
> > > On Tue, Aug 18, 2020 at 1:24 PM Arvind Sankar <nivedita@alum.mit.edu> wrote:
> > > >
> > > > On Tue, Aug 18, 2020 at 12:13:22PM -0700, Linus Torvalds 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.
> > > > >
> > > >
> > > > -ffreestanding as it stands today does have __builtin_memcpy and
> > > > friends. But you need to then use #define memcpy __builtin_memcpy etc,
> > > > which is messy and also doesn't fully express what you want. #pragma, or
> > > > even just allowing -fbuiltin-foo options would be useful.
> >
> > I do really like the idea of -fbuiltin-foo. For example, you'd specify:
> >
> > -ffreestanding -fbuiltin-bcmp
> >
> > as an example. `-ffreestanding` would opt you out of ALL libcall
> > optimizations, `-fbuiltin-bcmp` would then opt you back in to
> > transforms that produce bcmp. That way you're informing the compiler
> > more precisely about the environment you'd be targeting. It feels
> > symmetric to existing `-fno-` flags (clang makes -f vs -fno- pretty
> > easy when there is such symmetry). And it's already convention that
> > if you specify multiple conflicting compiler flags, then the latter
> > one specified "wins." In that sense, turning back on specific
> > libcalls after disabling the rest looks more ergonomic to me.
> >
> > Maybe Eli or David have thoughts on why that may or may not be as
> > ergonomic or possible to implement as I imagine?
> >
>
> Note that -fno-builtin-foo seems to mean slightly different things in
> clang and gcc. From experimentation, clang will neither optimize a call
> to foo, nor perform an optimization that introduces a call to foo. gcc
> will avoid optimizing calls to foo, but it can still generate new calls
> to foo while optimizing something else. Which means that
> -fno-builtin-{bcmp,stpcpy} only solves things for clang, not gcc. It's
> just that gcc doesn't seem to have implemented those optimizations.
Can you please share some godbolt links that demonstrate these observations?
--
Thanks,
~Nick Desaulniers
next prev parent reply other threads:[~2020-08-18 22:00 UTC|newest]
Thread overview: 65+ 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
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
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 [this message]
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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