Debugging and preprocessor directive

Question

For debugging I have many calls to a debug log function in my application. Of course in the production version these debugging calls need to be skipped. Instead of writing:

#if DEVEL == 1
    Log::debug(...);
#endif

around all calls to the debug function I decided the write the following in the debug function itself:

#if DEVEL != 1
    return;
#endif

Will the overhead of the useless function call be avoided by the compiler or am I better off by using (many ugly) #if #endif construction for performance reasons?

iammilind · Accepted Answer · 2011-06-12 07:53:55Z

7

Instead of worrying about optimizer, you can do a simple trick:

#if DEVEL == 1
#define LOG_DEBUG(...) Log::Debug(__VA_ARGS__)  // variadic macro
#else
#define LOG_DEBUG
#endif

Now use LOG_DEBUG everywhere to keep it simple.

answered Jun 12, 2011 at 7:53

iammilind

70.5k39 gold badges188 silver badges364 bronze badges

Sign up to request clarification or add additional context in comments.

4 Comments

Kerrek SB Over a year ago

Nice. Why doesn't the #else version take parameters, though?

iammilind Over a year ago

@Kerrek, because there are only 2 conditions. If it's not DEVEL == 1 then it will be surely DEVEL != 1.

Kerrek SB Over a year ago

No sorry, what I meant is why does the first version of LOG_DEBUG have arguments (parentheses), but the second doesn't? If you write LOG_DEBUG("hello"), then the second macro would expand to "hello", not nothing, wouldn't it?

c-urchin Over a year ago

No, in the second case LOG_DEBUG will be replaced by nothing, regardless of arguments.

Motti · Accepted Answer · 2011-06-12 07:25:56Z

5

If the function is available for inlining (e.g. it is implemented in the header) then the optimizer will have no trouble getting rid of the function call and thus leave you with no overhead.

answered Jun 12, 2011 at 7:25

Motti

116k56 gold badges195 silver badges277 bronze badges

2 Comments

Timo Over a year ago

Even if it's not defined in the header file the call will most likely be removed by link-time optimizations.

Paul Beusterien Over a year ago

Better to be removed at compile time, so the call doesn't impact register allocation or prolog/epilog for a leaf versus non-leaf caller function

Gilad Naor · Accepted Answer · 2011-06-12 07:28:03Z

2

Why don't you check?

With gcc just compile with -S, and look at the output.

answered Jun 12, 2011 at 7:28

Gilad Naor

21.7k15 gold badges49 silver badges54 bronze badges

1 Comment

Paul Beusterien Over a year ago

and turn on the optimizer: -O or -O3

Collectives™ on Stack Overflow

Debugging and preprocessor directive

3 Answers 3

4 Comments

2 Comments

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

4 Comments

2 Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related