忽略C / C ++中的死代码警告

Question

Having the following code (greatly simplified):

#define DO_SOME_STUFF(val,x,y) \
  if (x>y) \
  { \
    val = val >> (x-y); \
  } else { \
    val = val >> (y-x); \
  }

An example of calling the macro with expansion will be:

 int val = 5;
 DO_SOME_STUFF(val,5,10);
 => 
 if (5>10)
 {
   // dead code
   val = val >> (5-10); // negative shift warning here
 } else {
   // live code
   val = val >> (10-5); // valid code
 }

Now, since there is dead code, it will be removed by the compiler and everyone will be happy since I will never negative shift - so I'm doing legal stuff here.

Unfortunately, I receive the warnings before the code gets removed (or so it seems).

In the project I'm working the function will be called 100+ times, and the only solution I came up so far is to do the following:

#pragma GCC diagnostic ignored "-Wshift-count-negative"
DO_SOME_STUFF(val,300,20);
#pragma GCC diagnostic pop

This isn't very nice, since i will add a ton of these which will result in hard-to-read blue code, and I would rather avoid it if possible.

Is there an elegant way to remove the warning just for my macro expansion or to ask the compiler to ignore dead code ? (unfortunately i cannot add the #pragma options to my macro definition).

Answer 1

In c++11 you could use a template constexpr function instead of the macro. It can then be executed during the compilation and will not produce a warning.

template <typename T, typename T2>
constexpr T do_some_stuff(const T& val, const T2& x, const T2& y)
{  
  return x > y ? val >> (x - y) : val >> (y - x);
}

template <typename T, typename T2>
constexpr T do_some_stuff_wrong(const T& val, const T2& x, const T2& y)
{  
  return x < y ? val >> (x - y) : val >> (y - x);
}

int main() 
{
    constexpr int val = do_some_stuff(5, 5, 10); // no warning
    constexpr int val2 = do_some_stuff_wrong(5, 5, 10); // warning
}

Live example