linux-kernel - Re: [PATCH 0/4] -ffreestanding/-fno-builtin-* patches

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Date:   Tue, 18 Aug 2020 12:25:30 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Clement Courbet <courbet@...gle.com>
Cc:     "H. Peter Anvin" <hpa@...or.com>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Michal Marek <michal.lkml@...kovi.net>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Kees Cook <keescook@...omium.org>,
        Tony Luck <tony.luck@...el.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Joe Perches <joe@...ches.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Daniel Axtens <dja@...ens.net>,
        Arvind Sankar <nivedita@...m.mit.edu>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Alexandru Ardelean <alexandru.ardelean@...log.com>,
        Yury Norov <yury.norov@...il.com>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        Ard Biesheuvel <ardb@...nel.org>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Daniel Kiper <daniel.kiper@...cle.com>,
        Bruce Ashfield <bruce.ashfield@...il.com>,
        Marco Elver <elver@...gle.com>,
        Vamshi K Sthambamkadi <vamshi.k.sthambamkadi@...il.com>,
        Andi Kleen <ak@...e.de>,
        Dávid Bolvanský <david.bolvansky@...il.com>,
        Eli Friedman <efriedma@...cinc.com>
Subject: Re: [PATCH 0/4] -ffreestanding/-fno-builtin-* patches

On Tue, Aug 18, 2020 at 12:19 PM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> On Tue, Aug 18, 2020 at 12:03 PM H. Peter Anvin <hpa@...or.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.
>
> 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, 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.
-- 
Thanks,
~Nick Desaulniers