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Date:   Fri, 15 Jan 2021 20:18:33 +0100
From:   Borislav Petkov <bp@alien8.de>
To:     Arvind Sankar <nivedita@alum.mit.edu>
Cc:     Nathan Chancellor <natechancellor@gmail.com>,
        Arnd Bergmann <arnd@kernel.org>,
        Ard Biesheuvel <ardb@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, x86@kernel.org,
        Nick Desaulniers <ndesaulniers@google.com>,
        Arnd Bergmann <arnd@arndb.de>,
        Darren Hart <dvhart@infradead.org>,
        Andy Shevchenko <andy@infradead.org>,
        "H. Peter Anvin" <hpa@zytor.com>, linux-efi@vger.kernel.org,
        platform-driver-x86@vger.kernel.org, linux-kernel@vger.kernel.org,
        clang-built-linux@googlegroups.com
Subject: Re: [PATCH] x86: efi: avoid BUILD_BUG_ON() for non-constant p4d_index
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On Fri, Jan 15, 2021 at 02:11:25PM -0500, Arvind Sankar wrote:
> That's how build-time assertions work: they are _supposed_ to be
> optimized away completely when the assertion is true. If they're
> _not_ optimized away, the build will fail.

Yah, that I know, thanks.

If gcc really inlines p4d_index() and does a lot more aggressive
optimization to determine that the condition is false and thus optimize
everything away (and clang doesn't), then that would explain the
observation.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette