linux-kernel - Re: [PATCH] x86: efi: avoid BUILD_BUG_ON() for non-constant p4d

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Date:   Fri, 15 Jan 2021 20:18:33 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Arvind Sankar <nivedita@...m.mit.edu>
Cc:     Nathan Chancellor <natechancellor@...il.com>,
        Arnd Bergmann <arnd@...nel.org>,
        Ard Biesheuvel <ardb@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, x86@...nel.org,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Arnd Bergmann <arnd@...db.de>,
        Darren Hart <dvhart@...radead.org>,
        Andy Shevchenko <andy@...radead.org>,
        "H. Peter Anvin" <hpa@...or.com>, linux-efi@...r.kernel.org,
        platform-driver-x86@...r.kernel.org, linux-kernel@...r.kernel.org,
        clang-built-linux@...glegroups.com
Subject: Re: [PATCH] x86: efi: avoid BUILD_BUG_ON() for non-constant p4d_index

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