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

From: Nick Desaulniers <ndesaulniers@google.com>
To: Linus Torvalds <torvalds@linux-foundation.org>,
	Clement Courbet <courbet@google.com>
Cc: "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>,
	"Arvind Sankar" <nivedita@alum.mit.edu>,
	"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 12:25:30 -0700	[thread overview]
Message-ID: <CAKwvOdkut+GTLxX9U=hxDC8SaugW487XD_98d9yFU2VzShyz0A@mail.gmail.com> (raw)
In-Reply-To: <CAHk-=wiPeRQU_5JXCN0TLoW-xHZHp7dmrhx0wyXUSKxiCxE02Q@mail.gmail.com>

On Tue, Aug 18, 2020 at 12:19 PM Linus Torvalds
<torvalds@linux-foundation.org> 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.
>
> 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