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

From: Arnd Bergmann
Date: Fri Jul 12 2019 - 09:32:39 EST

On Fri, Jul 12, 2019 at 3:14 PM Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote:
> 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.

I had to also get rid of the temporary variable to make it work.
Sending v2 now.