Is using alloca() for variable length arrays better than using a vector on the heap?

Question

I have some code using a variable length array (VLA), which compiles fine in gcc and clang, but does not work with MSVC 2015.

class Test {
public:
    Test() {
        P = 5;
    }
    void somemethod() {
        int array[P];
        // do something with the array
    }
private:
    int P;
}

There seem to be two solutions in the code:

• using alloca(), taking the risks of alloca in account by making absolutely sure not to access elements outside of the array.
• using a vector member variable (assuming that the overhead between vector and c array is not the limiting factor as long as P is constant after construction of the object)

The ector would be more portable (less #ifdef testing which compiler is used), but I suspect alloca() to be faster.

The vector implementation would look like this:

class Test {
public:
    Test() {
        P = 5;
        init();
    }
    void init() {
        array.resize(P);
    }
    void somemethod() {
        // do something with the array
    }
private:
    int P;
    vector<int> array;
}

Another consideration: when I only change P outside of the function, is having a array on the heap which isn't reallocated even faster than having a VLA on the stack?

Maximum P will be about 400.

Answer 1

You could and probably should use some dynamically allocated heap memory, such as managed by a std::vector (as answered by Peter). You could use smart pointers, or plain raw pointers (new, malloc,....) that you should not forget to release (delete,free,....). Notice that heap allocation is probably faster than what you believe (practically, much less than a microsecond on current laptops most of the time).

Sometimes you can move the allocation out of some inner loop, or grow it only occasionally (so for a realloc-like thing, better use unsigned newsize=5*oldsize/4+10; than unsigned newsize=oldsize+1; i.e. have some geometrical growth). If you can't use vectors, be sure to keep separate allocated size and used lengths (as std::vector does internally).

Another strategy would be to special case small sizes vs bigger ones. e.g. for an array less than 30 elements, use the call stack; for bigger ones, use the heap.

If you insist on allocating (using VLAs -they are a commonly available extension of standard C++11- or alloca) on the call stack, be wise to limit your call frame to a few kilobytes. The total call stack is limited (e.g. often to about a megabyte or a few of them on many laptops) to some implementation specific limit. In some OSes you can raise that limit (see also setrlimit(2) on Linux)

Be sure to benchmark before hand-tuning your code. Don't forget to enable compiler optimization (e.g. g++ -O2 -Wall with GCC) before benchmarking. Remember that caches misses are generally much more expensive than heap allocation. Don't forget that developer's time also has some cost (which often is comparable to cumulated hardware costs).

Notice that using static variable or data has also issues (it is not reentrant, not thread safe, not async-signal-safe -see signal-safety(7) ....) and is less readable and less robust.

Answer 2

First of all, you're getting lucky if your code compiles with ANY C++ compiler as is. VLAs are not standard C++. Some compilers support them as an extension.

Using alloca() is also not standard, so is not guaranteed to work reliably (or even at all) when using different compilers.

Using a static vector is inadvisable in many cases. In your case, it gives behaviour that is potentially not equivalent to the original code.

A third option you may wish to consider is

 // in definition of class Test
void somemethod()
{
    std::vector<int> array(P);      // assume preceding #include <vector>
    // do something with array
}

A vector is essentially a dynamically allocated array, but will be cleaned up properly in the above when the function returns.

The above is standard C++. Unless you perform rigorous testing and profiling that provides evidence of a performance concern this should be sufficient.

Answer 3

Why don't you make the array a private member?

#include <vector>

class Test
{
public:
    Test()
    {
        data_.resize(5);
    }
    void somemethod()
    {
        // do something with data_
    }
private:
    std::vector<int> data_;
}

As you've specified a likely maximum size of the array, you could also look at something like boost::small_vector, which could be used like:

#include <boost/container/small_vector.hpp>

class Test
{
public:
    Test()
    {
        data_.resize(5);
    }
    void somemethod()
    {
        // do something with data_
    }
private:
    using boc = boost::container;

    constexpr std::size_t preset_capacity_ = 400;
    boc::small_vector<int, preset_capacity_> data_;
}

You should profile to see if this is actually better, and be aware this will likely use more memory, which could be an issue if there are many Test instances.