[cfe-dev] False positive for -Wunreachable-code

[Tom Care]

-Wunreachable-code works by looking for CFG blocks without predecessors. If we compare the two attached CFGs, we can see that when the destructor is present there is an extra 'unreachable' block that starts with the throw statement. I'm guessing this has to do with the implementation of C++ destructors and exceptions (implicit destructor call?), but I'm not an expert in this area. I'll let someone with more C++ CFG knowledge confirm.


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On Aug 3, 2010, at 8:22 AM, Abramo Bagnara wrote:

> 
> The following typescript shows that clang incorrectly seems to mark as
> unreachable a throw of a temporary.
> 
> Please note that if I remove the destructor of struct s, no warning is
> generated.
> 
> $ cat w.cc
> struct s {
>  explicit s() { }
>  ~s() { }
> };
> 
> int f_bad(int x) {
>  if (x)
>    throw s();
>  return 0;
> }
> 
> int f_good(int x) {
>  if (x) {
>    s e;
>    throw e;
>  }
>  return 0;
> }
> $ llvm/Debug+Asserts/bin/clang++ -Wunreachable-code -c w.cc
> w.cc:8:5: warning: will never be executed [-Wunreachable-code]
>    throw s();
>    ^~~~~~~~~
> 1 warning generated.
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