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Peter Anvin" , Andrew Morton , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Michal Marek , Linux Kbuild mailing list , LKML , Kees Cook , Tony Luck , Dmitry Vyukov , Michael Ellerman , Joe Perches , Joel Fernandes , Daniel Axtens , Andy Shevchenko , Alexandru Ardelean , Yury Norov , "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" , Ard Biesheuvel , "Paul E . McKenney" , Daniel Kiper , Bruce Ashfield , Marco Elver , Vamshi K Sthambamkadi Subject: Re: [PATCH 0/4] -ffreestanding/-fno-builtin-* patches Message-ID: <20200824173450.GA4157679@rani.riverdale.lan> References: <20200817220212.338670-1-ndesaulniers@google.com> <76071c24-ec6f-7f7a-4172-082bd574d581@zytor.com> <20200818202407.GA3143683@rani.riverdale.lan> <20200818214146.GA3196105@rani.riverdale.lan> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: Sender: linux-kbuild-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kbuild@vger.kernel.org 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.