[cfe-dev] new -Wuninitialized implementation in Clang

[Johannes Schaub (litb)]

Ted Kremenek wrote:

> My design goal for -Wuninitialized was threefold:
> 
> (a) Works under -O0.  Users care about seeing these warnings the most when
> they are doing their debug builds.
> 
> (b) Has consistent results that are invariant of the optimization flags,
> target architecture, phases of the moon, etc.
> 
> (c) Provides predictable results that are (for the most part) sound and
> complete.
> 
> (d) Has marginal impact on compile time.
> 
> The last three goals mean that the analysis can only do limited reasoning
> about control-dependencies, e.g.:
> 
>   int x;
>   ...
>   if (flag)
>     x = ...
>   ...
>   if (flag)
>     use(x);
> 
> Inherently analyzing this code correctly requires path-sensitive analysis,
> which inherently has exponential cost in the general case.  There are
> tricks where we can mitigate such algorithmic complexity for some common
> cases, but handling these control-dependencies in general is something
> that really is in the purview of the static analyzer.  Amazingly, GCC
> often doesn't flag warnings in such cases, but I suspect that it is
> because GCC is silently dropping warnings in some cases where it deems it
> can't accurately reason precisely about a given variable.
> 

Hmm, I would personally like to not initialize variables that I know are not 
read from, as in your above code example. I personally regard it as bad 
coding style to initialize variables with values not really used by the 
code. 

Would it be reasonable to have a flag that enables clang to try harder to 
not warn in such cases? I haven't any clue on how much this slows down 
clang's performance, but I think I'm willing to take a compile-time speed 
drop equivalent to a -O2.