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

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Date:   Mon, 24 Aug 2020 13:34:50 -0400
From:   Arvind Sankar <nivedita@...m.mit.edu>
To:     Masahiro Yamada <masahiroy@...nel.org>
Cc:     Arvind Sankar <nivedita@...m.mit.edu>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Dávid Bolvanský <david.bolvansky@...il.com>,
        Eli Friedman <efriedma@...cinc.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        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>,
        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>
Subject: Re: [PATCH 0/4] -ffreestanding/-fno-builtin-* patches

On Tue, Aug 25, 2020 at 12:57:22AM +0900, Masahiro Yamada wrote:
> 
> 
> To prevent transformation from foo() into bar(),
> there are two ways in Clang to do that;
> -fno-builtin-foo, and -fno-builtin-bar.
> There is only one in GCC; -fno-buitin-foo.
> 
> Is this correct?
> 

It looked that way from previous experimentation, but...

> 
> 
> I just played the optimization
> from printf("helloworld\n") to puts("helloworld").
> 
> https://godbolt.org/z/5s4ded
> 
> 
> -fno-builtin-puts cannot prevent clang
> from emitting puts.
> Is it because clang does not support
> -fno-builtin-puts?

Ugh. clang doesn't have __builtin_puts() but it optimizes printf() into
puts(). It doesn't have __builtin_putchar() but will optimize
printf("c") into putchar('c').

And it has .. half of __builtin_fwrite()? fwrite() of a single byte gets
optimized into fputc(), fprintf() of a string literal gets optimized
into fwrite(), -fno-builtin-fwrite prevents both optimizations, but
__builtin_fwrite() gives a compile error.

I give up.