From: Nick Desaulniers <ndesaulniers@google.com>
To: Arvind Sankar <nivedita@alum.mit.edu>
Cc: "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>,
"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 13:27:54 -0700 [thread overview]
Message-ID: <CAKwvOdnfh9nWwu1xV=WDbETGiabwDxXxQDRCAfpa-+kSZijb9w@mail.gmail.com> (raw)
In-Reply-To: <20200818202407.GA3143683@rani.riverdale.lan>
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.
>
> The two compilers have some peculiarities, which means you really can't
> have functions with the same name that do something else if you want to
> use builtins at all, and can also lead to missed optimizations.
>
> For eg, __builtin_strchr(s,'\0') can be optimized to strlen. gcc will
> optimize it that way even if -ffreestanding is used (so strlen has to
> mean strlen), while clang won't, so it misses a potential optimization.
> This is admittedly a silly example, but you could imagine something like
> strncpy being optimized to memcpy+memset if the source length was
> previously computed.
>
> PS: clang optimizes sprintf, but doesn't provide __builtin_sprintf?
https://bugs.llvm.org/show_bug.cgi?id=47224
--
Thanks,
~Nick Desaulniers
next prev parent reply other threads:[~2020-08-18 20:28 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
2020-08-19 12:19 ` Clement Courbet
2020-08-18 20:24 ` Arvind Sankar
2020-08-18 20:27 ` Nick Desaulniers [this message]
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
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to='CAKwvOdnfh9nWwu1xV=WDbETGiabwDxXxQDRCAfpa-+kSZijb9w@mail.gmail.com' \
--to=ndesaulniers@google.com \
--cc=ak@suse.de \
--cc=akpm@linux-foundation.org \
--cc=alexandru.ardelean@analog.com \
--cc=andriy.shevchenko@linux.intel.com \
--cc=ardb@kernel.org \
--cc=bp@alien8.de \
--cc=bruce.ashfield@gmail.com \
--cc=daniel.kiper@oracle.com \
--cc=david.bolvansky@gmail.com \
--cc=dja@axtens.net \
--cc=dvyukov@google.com \
--cc=efriedma@quicinc.com \
--cc=elver@google.com \
--cc=hpa@zytor.com \
--cc=joe@perches.com \
--cc=joel@joelfernandes.org \
--cc=keescook@chromium.org \
--cc=linux-kbuild@vger.kernel.org \
--cc=linux-kernel@vger.kernel.org \
--cc=masahiroy@kernel.org \
--cc=michal.lkml@markovi.net \
--cc=mingo@redhat.com \
--cc=mpe@ellerman.id.au \
--cc=nivedita@alum.mit.edu \
--cc=paulmck@kernel.org \
--cc=tglx@linutronix.de \
--cc=tony.luck@intel.com \
--cc=torvalds@linux-foundation.org \
--cc=vamshi.k.sthambamkadi@gmail.com \
--cc=x86@kernel.org \
--cc=yury.norov@gmail.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).