Re: PR80806

[Martin Sebor <msebor at gmail dot com>]

On 06/29/2017 12:05 PM, Jeff Law wrote:
> On 06/29/2017 11:57 AM, Jeff Law wrote:
> > On 05/23/2017 09:58 AM, Martin Sebor wrote:
> > > On 05/18/2017 12:55 PM, Prathamesh Kulkarni wrote:
> > > > Hi,
> > > > The attached patch tries to fix PR80806 by warning when a variable is
> > > > set using memset (and friends) but not used. I chose to warn in dse
> > > > pass since dse would detect if the variable passed as 1st argument is
> > > > a dead store. Does this approach look OK ?
> > >
> > > Detecting -Wunused-but-set-variable in the optimizer means that
> > > the warning will not be issued without optimization.  It also
> > > means that the warning will trigger in cases where the variable
> > > is used conditionally and the condition is subject to constant
> > > propagation.  For instance:
> > Yea.  There's definitely tradeoffs for implementing warnings early vs
> > late.  There's little doubt we could construct testcases where an early
> > warning would miss cases that could be caught by a late warning.
> >
> >
> > >   void sink (void*);
> > >
> > >   void test (int i)
> > >   {
> > >       char buf[10];   // -Wunused-but-set-variable
> > >       memset (buf, 0, sizeof(buf));
> > >
> > >       if (i)
> > >         sink (buf);
> > >   }
> > >
> > >   void f (void)
> > >   {
> > >       test (0);
> > >   }
> > >
> > > I suspect this would be considered a false positive by most users.
> > > In my view, it would be more in line with the design of the warning
> > > to enhance the front end to detect this case, and it would avoid
> > > these issues.
> > Given no knowledge of sink() here, don't we have to assume that buf is
> > used?  So, yea, I'd probably consider that a false positive.
> Oh, wait, I missed the constant propagation.  That makes this one less
> clear cut in my mind -- it means its context sensitive.  I could easily
> argue either way on this one.

Suppose buf were the small buffer in std::string and sink() some
conditional use of the class, like in this more complicated code:

  void foo (std::string&);
  void bar (char*);

  void test (int i)
  {
    std::string s;   // calls memset (s.buf, 0, sizeof s.buf)

    char array[100] = "";

    if (i > 100)
      foo (s);       // needs a large buffer
    else
      bar (array);   // works with a small buffer
  }

Variations on this idiom aren't uncommon.

Martin