Does compiler remove conditionals on types in templates

Question

Consider below code:

template<typename T> void f(T a[]) { if(sizeof(T) > 1) for(...)a[i] = j else memset(a, j, ... }

I think compiler should remove one of the if-branches in compile-time. Is it true?

Why don't you compile it with optimizations with your compiler an see what happens? — Mat
– Mat, Commented Feb 23, 2012 at 6:56
If you're trying to optimize copies by using memset() instead of for loops for some type T, sizeof(T) is not an appropriate way to determine if it is eligible for such an optimization. You want to use is_pod<T>::value instead of sizeof(T). — In silico
– In silico, Commented Feb 23, 2012 at 6:56
@Volkan Sirin: Depending on your setup, the compiler is probably smarter than you think it is. :-) The only way to be sure is to test and profile. — In silico
– In silico, Commented Feb 23, 2012 at 7:16
@VolkanSirin: You think the compiler doesn't implement inlining or something? Usually the person worried most about performance needs it the least. You might want a working program first. — GManNickG
– GManNickG, Commented Feb 23, 2012 at 7:34

Sarfaraz Nawaz · Accepted Answer · 2012-02-23 10:52:47Z

Since the compiler can decide if (sizeof(T) > 1) will return true or not, at compile time itself, the compiler can emit machine code without any branch if compiled with optimization flag on. So the resultant code will have either for loop (or an equivalent code generated out of it), or memset. In other words, the resultant code will not have if-else block (provided you're using a smart compiler).

Matthieu M. · Accepted Answer · 2012-02-23 08:04:44Z

1

What is wrong with std::fill ?

std::fill(begin(a), end(a), j);

or are you worried that your Standard Library is not optimized enough ?

answered Feb 23, 2012 at 8:04

Matthieu M.

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3 Comments

Volkan Over a year ago

we measured that std::fill is significantly slower than above approach

MSalters Over a year ago

@Volkan: You probably want to check your compiler settings then. It should be almost as fast, if not faster. (For the latest version of all major compilers)

Matthieu M. Over a year ago

@VolkanSirin: I hate to ask but... did you actually profile optimized code ? FIY I am not sure that the compiler will detect the memset switch, that would perhaps be a good optimization to suggest though.

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