C++ Compiler Directives – How to Ensure Copy Elision

c++compiler-directivescopy-elisiondiagnostics

Copy elision is a neat optimization technique and in some cases relying on copy elision can actually be faster than passing around references "by hand".

So, let's assume you have identified a critical code path where you rely on the fact that the copy elision is performed by your compiler for the code path for maximum performance.

But now you are relying on a compiler optimization.

Is there any (compiler specific, obviously) way to ensure that the copy elision is actually performed and have the compiler (or another tool) generate a warning/error if the copy elision cannot be performed?

(I'm thinking of something remotely similar to Visual C++'s __forceinline than will generate a warning if the function marked thus isn't inlined by the compiler.)

Best Answer

No.

But you can write an equivalent, although completely unreadable, code:

BigObj f()
{
    BigObj x(g());
    x.someMethod();
    return x;
}

//...
BigObj z = f();
//...

is translated (with copy elision) to:

void f(BigObj* obj)
{
    new(obj) BigObj(g());
    obj->someMethod();
}

//...
char z[sizeof(BigObj)];
f((BigObj*)&z[0]);
//...
((BigObj*)&z[0])->~BigObj();

But seriously, just write your code in such a way that the compiler can elide the copy. I.e. return only one object without branching:

BigObj f()
{
    BigObj x, y;
    // use x and y
    if(condition)
        return x;
    else
        return y;
    // cannot be elided
}


BigObj f()
{
    if(condition)
    {
        BigObj x;
        return x;
    }
    else
    {
        BigObj y;
        return y;
    }
    // can be elided
}
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