linux-kernel - Re: "KVM: x86: generalize guest_cpuid_has

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Date:   Tue, 12 Sep 2017 18:03:47 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Dmitry Vyukov <dvyukov@...gle.com>,
        Radim Krčmář <rkrcmar@...hat.com>
Cc:     David Hildenbrand <david@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        KVM list <kvm@...r.kernel.org>,
        llvmlinux@...ts.linuxfoundation.org,
        Alexander Potapenko <glider@...gle.com>,
        andreyknvl <andreyknvl@...gle.com>,
        Michael Davidson <md@...gle.com>,
        Greg Hackmann <ghackmann@...gle.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>
Subject: Re: "KVM: x86: generalize guest_cpuid_has_ helpers" breaks clang

On 12/09/2017 17:54, Dmitry Vyukov wrote:
>> I guess clang still eliminates dead branches. Clang optimizer does
>> know that these are constant, it just does not allow build
>> success/failure nor runtime behavior depend on optimization level and
>> compiler version. I.e. with gcc you can get build failure with only
>> some compiler flags and/or compiler versions. Clang gives stable
>> result. But the optimizer does use constant propagation, etc during
>> optimization.

I can reproduce it:

$ cat f.c
int bad_code();

static inline void __attribute__((always_inline)) f(int x)
{
        if (!__builtin_constant_p(x))
                bad_code();
}

int main()
{
        f(0);
        f(1);
        f(100);
}

$ clang --version
clang version 4.0.0 (tags/RELEASE_400/final)
$ clang f.c -O2 -c -o f.o
$ nm f.o
                 U bad_code
0000000000000000 T main

$ gcc f.c -O2 -c -o f.o
$ nm f.o
0000000000000000 T main

... but I don't know, it seems very weird.  The purpose of
__builtin_constant_p is to be resolved only relatively late in the
optimization pipeline, and it has been like this for at least 15 years
in GCC.

The docs say what to expect:

  You may use this built-in function in either a macro or an inline
  function. However, if you use it in an inlined function and pass an
  argument of the function as the argument to the built-in, GCC never
  returns 1 when you call the inline function with a string constant or
  compound literal (see Compound Literals) and does not return 1 when
  you pass a constant numeric value to the inline function **unless you
  specify the -O option**.

(emphasis mine).

> I've installed clang-3.9 (the closest version to yours my distribution
> gives me) and still got the same error with it. I would expect that
> 4.0 should give the same result as well... Are you sure you enabled
> KVM/intel/amd? (yes, I know you are maintaining KVM code :))

Heh, I don't know about Radim but "^Rmake" goes straight to "make
arch/x86/kvm/" for me. :)

Paolo