From: Arvind Sankar <nivedita@alum.mit.edu>
To: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Cc: "Arvind Sankar" <nivedita@alum.mit.edu>,
"Nick Desaulniers" <ndesaulniers@google.com>,
"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: Thu, 20 Aug 2020 13:56:17 -0400 [thread overview]
Message-ID: <20200820175617.GA604994@rani.riverdale.lan> (raw)
In-Reply-To: <df6c1da4-b910-ecb8-0de2-6156dd651be6@rasmusvillemoes.dk>
On Thu, Aug 20, 2020 at 04:56:02PM +0200, Rasmus Villemoes wrote:
> On 18/08/2020 23.41, Arvind Sankar wrote:
> >
> > 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.
> >
>
> I think it's more than that. I've always read gcc's documentation
>
> '-fno-builtin'
> '-fno-builtin-FUNCTION'
> Don't recognize built-in functions that do not begin with
> '__builtin_' as prefix. ...
>
> GCC normally generates special code to handle certain built-in
> functions more efficiently; for instance, calls to 'alloca' may
> become single instructions which adjust the stack directly, and
> calls to 'memcpy' may become inline copy loops.
> ...
>
> to mean exactly that observed above and nothing more, i.e. that
> -fno-builtin-foo merely means that gcc stops treating a call of a
> function named foo to mean a call to a function implementing the
> standard function by that name (and hence allows it to e.g. replace a
> memcpy(d, s, 1) by byte load+store). It does not mean to prevent
> emitting calls to foo, and I don't think it ever will - it's a bit sad
> that clang has chosen to interpret these options differently.
That documentation is misleading, as it also goes on to say:
"...nor can you change the behavior of the functions by linking with a
different library"
which implies that you _can_ change the behavior if you use the option,
and which is what your "i.e." is saying as well.
My point is that this is not completely true: in gcc, foo by default is
defined to be __builtin_foo, and -fno-builtin-foo simply removes this
definition. So the effect is just that calls to foo in the original
source will be left alone.
But in order for an optimization that introduces a new call to foo to be
valid, foo _must_ have standard semantics: strchr(s,'\0') is not s +
strlen(s) unless strlen has standard semantics. This is an oversight in
gcc's optimizations: it converts to s + __builtin_strlen(s), which then
(normally) becomes s + strlen(s).
Check out this horror: https://godbolt.org/z/a1r9fK
Clang will disable this optimization if -fno-builtin-strlen is
specified.
Clang's interpretation is more useful for embedded, since you can use
-fno-builtin-foo and avoid calling __builtin_foo directly, and be
guaranteed that there will be no calls to foo that you didn't write
explicitly (outside of memcpy/memset/memcmp). In this case you are free
to implement foo with non-standard semantics, or avoid implementing it
altogether, and be reasonably confident that it will all work.
>
> Thinking out load, it would be useful if both compilers grew
>
> -fassume-provided-std-foo
>
> and
>
> -fno-assume-provided-std-foo
>
> options to tell the compiler that a function named foo with standard
> semantics can be assumed (or not) to be provided by the execution
> environment; i.e. one half of what -f(no-)builtin-foo apparently does
> for clang currently.
Not following: -fno-assume-provided-std-foo sounds like it would have
exactly the same semantics as Clang's -fno-builtin-foo, except maybe in
addition it should cause the compiler to error on seeing __builtin_foo
if it can't implement that without calling foo.
>
> And yes, the positive -fbuiltin-foo would also be quite useful in order
> to get the compiler to recognize a few important functions (memcpy,
> memcmp) while using -ffreestanding (or just plain -fno-builtin) to tell
> it to avoid assuming anything about most std functions - I've worked on
> a VxWorks target where snprintf() didn't have the correct "return what
> would be written" semantics but rather behaved like the kernel's
> non-standard scnprintf(), and who knows what other odd quirks that libc had.
>
> Rasmus
next prev parent reply other threads:[~2020-08-20 17:56 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
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 [this message]
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=20200820175617.GA604994@rani.riverdale.lan \
--to=nivedita@alum.mit.edu \
--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=linux@rasmusvillemoes.dk \
--cc=masahiroy@kernel.org \
--cc=michal.lkml@markovi.net \
--cc=mingo@redhat.com \
--cc=mpe@ellerman.id.au \
--cc=ndesaulniers@google.com \
--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).