Re: [PATCH 10/12] Warn when gettimeofday is called with non-null tzp argument.

[Florian Weimer <fweimer at redhat dot com>]

* Zack Weinberg:

> I didn't look into it in any detail, but the uses of __warndecl +
> __builtin_constant_p in bits/string2.h inlines all follow the pattern
>
>     __extern_always_inline T func (ptr)
>     {
>       if (__builtin_constant_p (ptr) && something_else_p (ptr))
> __issue_warning ();
>       ...
>     }
>
> that is, issue a warning if ptr is a compile-time constant with some
> property.  What we want for gettimeofday is just the opposite,
>
>     if (! (__builtin_constant_p (ptr) && ptr == NULL)) __issue_warning ();
>
> that is, warn whenever ptr is _not_ a compile-time NULL.  I wouldn't
> be surprised if GCC's earliest unreachable-code removal passes, the
> ones that happen early enough to prevent __attribute__((warning))
> diagnostics from triggering, were tuned precisely for what
> bits/string2.h does, and don't handle this inverse case properly.

I don't think this will work.  You would have to use something like
this:

  __builtin_constant_p (ptr != NULL) && ptr != NULL

Otherwise you will produce a warning every time someone uses the
gettimeofday wrapper in a function for which optimization has been
disabled.

Thanks,
Florian