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

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Message-ID: <d865ceb8ef534c21b82f205635426101@AcuMS.aculab.com>
Date:   Wed, 19 Aug 2020 08:26:51 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Arvind Sankar' <nivedita@...m.mit.edu>,
        Nick Desaulniers <ndesaulniers@...gle.com>
CC:     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@...r.kernel.org" <linux-kbuild@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <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>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Alexandru Ardelean <alexandru.ardelean@...log.com>,
        Yury Norov <yury.norov@...il.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "H . Peter Anvin" <hpa@...or.com>,
        "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>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Dávid Bolvanský <david.bolvansky@...il.com>,
        "Eli Friedman" <efriedma@...cinc.com>
Subject: RE: [PATCH 0/4] -ffreestanding/-fno-builtin-* patches

From: Arvind Sankar
> Sent: 18 August 2020 23:26
...
> gcc-10 optimizes the generic memset implementation in there into a call
> to memset. Now that's on x86 which doesn't use the generic
> implementation, but this is just waiting to bite us.
> 
> https://godbolt.org/z/6EhG15

Gah, if I want a call to memset() I'll write a call to memset().
If I'm trying to zero a small number of bytes I'll write a loop
because it'll be faster.

I wish compilers wouldn't even consider these sorts of 'optimisations'.

	David

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