C++ difference between structs and classes what about initialization in VS2015

Question

I had problem with performance of "stack initialization". I am compiling my code with Visual Studio 2015 with optimization /O2 (speed).

See following structure:

template<typename T, size_t SIZE = 0>
TYPE SimpleArray;

template<typename T>
TYPE SimpleArray<T, 0> {
public:
    size_t _size;
    T * _ptr;
    SimpleArray(T * ptr, size_t size) : _size(size) , _ptr(ptr) {}
    SimpleArray() : _ptr(0), _size(0) {}
};

template<typename T, size_t SIZE>
TYPE SimpleArray : public SimpleArray<T> {
public:
    T _array[SIZE];
    SimpleArray() : SimpleArray<T>(_array, SIZE) {}
};

I tried to measure performance in following way:

template<size_t SIZE>
void test() {
    Stopwatch stackWatch;
    stackWatch.Start();
    for (volatile size_t i = 0; i < 100000000; ++i) {
        SimpleArray<char, SIZE> list;
    }
    stackWatch.Stop();

    std::cout << "size\t" << SIZE << "\ttime\t" << stackWatch.ElapsedMilliseconds()  << std::endl;
}

And I profiled it for sizes smaller than 1MB:

template<size_t SIZE = 1>
void invokeTests() { test<SIZE>(); invokeTests<SIZE * 2>(); }
template<>
void invokeTests<(1 << 20)>() {}

int main() {
    invokeTests();
    return 0;
}

if I use class:

#define TYPE class

I obtain results which I am not expecting:

size    1       time    204
size    2       time    187
size    4       time    203
size    8       time    219
size    16      time    187
size    32      time    250
size    64      time    688
size    128     time    1422
size    256     time    1719
size    512     time    1859
size    1024    time    2265
size    2048    time    3422
size    4096    time    6063
size    8192    time    10812
size    16384   time    20203
size    32768   time    40953
size    65536   time    141813
...

if I use struct:

#define TYPE struct

I obtain following results, which is what I am expecting:

size    1       time    203
size    2       time    187
size    4       time    188
size    8       time    187
size    16      time    203
size    32      time    203
size    64      time    188
size    128     time    187
size    256     time    188
size    512     time    187
size    1024    time    188
size    2048    time    187
size    4096    time    203
size    8192    time    188
size    16384   time    187
size    32768   time    188
size    65536   time    187
...

I know what causes performance drop. The performance drop what about class is caused by "zeroing" all the _array element. But why is that? Is it bug or expected behavior?

Edit: I found out that this happens if the compiler option /sdl /GS is set. However there is still same question: why VS does difference between structs and classes?

There should be no difference between a class and a struct. Not sure why the compiler optimizes out the initialization in the struct case but not the class case. — NathanOliver, Mar 03 '17 at 21:06
http://stackoverflow.com/a/1191880/1491895 has a good explanation of the differences between `struct` and `class`. Maybe your issue is related to how constructors work. — Barmar, Mar 03 '17 at 21:12
Using `chrono::system_clock::now()` instead of `Stopwatch` I get no difference in timing for the two cases (VS2015 latest update). — Bo Persson, Mar 04 '17 at 00:41
@BoPersson thank you for giving me that feedback. Have you set /GS compiler option? — user4663214, Mar 04 '17 at 09:39

score 0 · Answer 1 · answered Mar 06 '17 at 06:38

0

Answer from MSDN - It is a bug.

https://social.msdn.microsoft.com/Forums/en-US/a65ebdf2-78ed-48ed-a353-864c8593e695/c-difference-between-structs-and-classes-what-about-initialization-in-vs2015?forum=vclanguage

answered Mar 06 '17 at 06:38

user4663214

147
11

C++ difference between structs and classes what about initialization in VS2015

1 Answers1