[cfe-dev] Undiagnosed "reference to stack memory [...] returned"

[Argyrios Kyrtzidis]

On Nov 30, 2010, at 9:56 AM, Matthieu Monrocq wrote:

> Hi Argyrios,
> 
> Thank you very much!
> 
> It's great that clang will diagnose this UB, it's a really nasty bug to track down :)

I reverted the previous commit, much better fix in http://lists.cs.uiuc.edu/pipermail/cfe-commits/Week-of-Mon-20101129/036875.html.
e.g. for

struct S {
  int x;
};

int &get_ref() {
  S s;
  S &s2 = s;
  int &x2 = s2.x;
  return x2;
}

we get

t3.cpp:9:10: warning: reference to stack memory associated with local variable 's' returned
  return x2;
         ^~
t3.cpp:8:8: note: binding reference variable 'x2' here
  int &x2 = s2.x;
       ^    ~~
t3.cpp:7:6: note: binding reference variable 's2' here
  S &s2 = s;
     ^    ~
1 warning generated.

> 
> Matthieu.
> 
> 2010/11/29 Argyrios Kyrtzidis <akyrtzi at gmail.com>
> Hi Matthieu,
> 
> Sorry for the (very..) late response. We now warn for all cases since http://lists.cs.uiuc.edu/pipermail/cfe-commits/Week-of-Mon-20101129/036838.html
> 
> Thanks for reporting the issue!
> 
> -Argiris
> 
> On Wed, Sep 15, 2010 at 11:04 AM, Matthieu Monrocq <matthieu.monrocq at gmail.com> wrote:
> Hi guys,
> 
> here is a very simple test case:
> 
>     int const& get_reference_warning()
>     {
>       int w = 0;
>       return w; // expected-warning{{reference to stack memory associated with local variable 'w' returned}}
>     }
> 
>     int get_value();
> 
>     int const& get_reference_no_warning() { return get_value(); } // Diagnosed by VC++ 2010 (haven't tested on gcc), but unnoticed by Clang
> 
>     int const& get_reference_no_warning_2()
>     {
>       int const& w2 = get_value();         // Correct, lifetime of the value is extended to the lifetime of the const reference
>       return w2;                                   // Undefined behavior: goes unnoticed by Clang, VC++ 2010 and gcc 3.4.2
>     }
> 
> I compiled it with:   clang.exe -fsyntax-only -Wall -pedantic test.cpp   (on Windows, using PowerShell)
> 
>     test.cpp(9) :  warning: reference to stack memory associated with local variable 'w' returned
>       return w; // expected-warning{{reference to stack memory associated with local variable 'w' returned}}
>          ^
>     1 warning generated.
> 
> As expected, the first return provoked a warning.
> 
> However in the two other cases, no warning is generated.
> 
> I was wondering if it would be possible for clang to diagnose those cases as well. I guess the third case is the trickier, though since the lifetime of the value is correctly extended, therre should be a way to detect that `w2` is not just an ordinary const reference. I must admit I haven't come over the Clang code base yet so it's still a bit blurry to me (by the way if someone could kindly point me to some doc for developers...)
> 
> Should I file a bug ? (or perhaps one already exists...)
> 
> Matthieu.
> 
> _______________________________________________
> cfe-dev mailing list
> cfe-dev at cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/cfe-dev
> 
> 
> 

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/cfe-dev/attachments/20101130/b3c2b684/attachment.html>