linux-kernel - Re: [PATCH 1/2] x86: kvm: avoid -Wsometimes-uninitized warning

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Date:   Fri, 12 Jul 2019 15:14:08 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Arnd Bergmann <arnd@...db.de>, Roman Kagan <rkagan@...tuozzo.com>
Cc:     Radim Krčmář <rkrcmar@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "x86@...nel.org" <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Liran Alon <liran.alon@...cle.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "clang-built-linux@...glegroups.com" 
        <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH 1/2] x86: kvm: avoid -Wsometimes-uninitized warning

On 12/07/19 15:02, Arnd Bergmann wrote:
> I think what happens here is that clang does not treat the return
> code of track the return code of is_64_bit_mode() as a constant
> expression, and therefore assumes that the if() condition
> may or may not be true, for the purpose of determining whether
> the variable is used without an inialization. This would hold even
> if it later eliminates the code leading up to the if() in an optimization
> stage. IS_ENABLED(CONFIG_X86_64) however is a constant
> expression, so with the patch, it understands this.
> 
> In contrast, gcc seems to perform all the inlining first, and
> then see if some variable is used uninitialized in the final code.
> This gives additional information to the compiler, but makes the
> outcome less predictable since it depends on optimization flags
> and architecture specific behavior.
> 
> Both approaches have their own sets of false positive and false
> negative warnings.

True, on the other hand constant returns are not really rocket science. :)

Maybe change is_long_mode to a macro if !CONFIG_X86_64?  That would be
better if clang likes it.

Paolo