Re: [PATCH] Change the default for Mixed declarations.

[Daniel P . Berrangé]

On Fri, Mar 24, 2023 at 05:56:46PM +0000, Alex Bennée wrote:
> 
> Juan Quintela <quintela@redhat.com> writes:
> 
> > Daniel P. Berrangé <berrange@redhat.com> wrote:
> >> On Tue, Feb 14, 2023 at 05:07:38PM +0100, Juan Quintela wrote:
> >>> Hi
> >>> 
> >>> I want to enter a discussion about changing the default of the style
> >>> guide.
> >>> 
> >>> There are several reasons for that:
> >>> - they exist since C99 (i.e. all supported compilers support them)
> >>> - they eliminate the posibility of an unitialized variable.
> >>
> >> Actually they don't do that reliably. In fact, when combined
> >> with usage of 'goto', they introduce uninitialized variables,
> >> despite the declaration having an initialization present, and
> >> thus actively mislead reviewers into thinking their code is
> >> safe.
> >
> > Wait a minute.
> > If you use goto, you are already in special rules.
> >
> > And don't get confused, I fully agree when using goto for two reasons:
> > - performance
> >   if you show that the code is x% faster when using goto, it is
> >   justified.  It is even better if you send a bug report to gcc/clang,
> >   but I will not opose that use.
> 
> I await a clear example in the context of QEMU - there is almost always
> a better way to structure things.
> 
> > - code clearity
> >   Some code (basically error paths) are clearer with goto that without
> >   them.
> 
> Now we have g_auto* and lock guards we should encourage their use. goto
> error_path is a relic of a simpler time ;-)
> 
> <snip>
> >> IMHO if we are concerned about uninitialized variables then I think
> >> a better approach is to add -ftrivial-auto-var-init=zero, which will
> >> make the compiler initialize all variables to 0 if they lack an
> >> explicit initializer. 
> >
> > I think this is a bad idea.
> > If we want to "catch" unitialized variables, using something like:
> >
> > -ftrivial-auto-var-init=pattern sounds much saner.
> >
> > Obviously gcc is missing
> >
> > -ftrivial-auto-var-init=42
> 
> I think we could at least eat the runtime cost of
> -ftrvial-auto-var-init=0xDEADBEEF for our --enable-debug builds.

If there is ever a case where an uninitialized var gets used as a
loop counter, that's 3,735,928,559 iterations. A small value pattern
would avoid such CPU burn.


With regards,
Daniel
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