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

[Zack Weinberg <zackw at panix dot com>]

On 8/21/19 12:10 PM, Florian Weimer wrote:
> * 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.

That is the behavior I was seeing when the wrapper was an extern inline,
but not when it is a macro.  I'm going to do more thorough testing.

Also, `__builtin_constant_p (ptr != NULL) && ptr != NULL` would warn
only for compile-time non-NULL, if I'm reading it right.  But in this
case we also want to issue a warning for any argument that isn't a
compile-time constant.  In other words, we want to warn for everything
_except_ compile-time NULL.  Hence `! (__builtin_constant_p (ptr) && ptr
== NULL)`.

zw