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From:   =?UTF-8?B?RMOhdmlkIEJvbHZhbnNrw70=?= <david.bolvansky@gmail.com>
Date:   Wed, 19 Aug 2020 00:05:34 +0200
Message-ID: <CAOrgDVO=NBaqGP2Fs6X4FHeLfbaAA7Km8i2ttcGf0kwfojmVSA@mail.gmail.com>
Subject: Re: [PATCH 0/4] -ffreestanding/-fno-builtin-* patches
To:     Nick Desaulniers <ndesaulniers@google.com>
Cc:     Arvind Sankar <nivedita@alum.mit.edu>,
        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>
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Here:
https://godbolt.org/z/qjo5P6

st 19. 8. 2020 o 0:00 Nick Desaulniers <ndesaulniers@google.com> nap=C3=ADs=
al(a):
>
> On Tue, Aug 18, 2020 at 2:41 PM Arvind Sankar <nivedita@alum.mit.edu> wro=
te:
> >
> > 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.ed=
u> 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 p=
laying
> > > > > > > whack-a-mole *for a limited time* is unreasonable. But I woul=
d like to go back
> > > > > > > to the compiler authors and get them to implement such a #pra=
gma: "this
> > > > > > > freestanding implementation *does* support *this specific lib=
rary 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 stan=
dard
> > > > > > function").
> > > > > >
> > > > > > IOW, there's nothing wrong with -ffreestanding if you then also=
 have
> > > > > > __builtin_memcpy() etc, and they do the sane compiler optimizat=
ions
> > > > > > for memcpy().
> > > > > >
> > > > > > What we want to avoid is the compiler making *assumptions* base=
d 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. #pra=
gma, or
> > > > > even just allowing -fbuiltin-foo options would be useful.
> > >
> > > I do really like the idea of -fbuiltin-foo.  For example, you'd speci=
fy:
> > >
> > > -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 observatio=
ns?
> --
> Thanks,
> ~Nick Desaulniers