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Will any compiler actually ever elide these copies?


Will any compiler actually ever elide these copies?

By : Jeremy Ware
Date : November 21 2020, 07:38 AM
This might help you The compiler can - and normally does - elide the copies from the temporary to the argument. The compiler cannot elide the copy from the argument to members. While it may technically possible to elide these copies in some cases, the relevant permission isn't given. The section of the standard is 12.8 [class.copy] paragraph 31 which spells out 4 situations where a copy can be elided (the exact rules are a bit non-trivial):
When returning a named, function local variable using it name. When using a named, function local variable in a throw expression. When copying a temporary object. When catching an exception by value.
code :


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Can the compiler elide the following copy?

Can the compiler elide the following copy?


By : danibeet
Date : March 29 2020, 07:55 AM
With these it helps
the examples on wikipedia for example make it seem to me that copy elision can only take place if the object to be returned gets returned at the same time it gets completely constructed.
code :
vector<int> foo() {
    vector<int> a;
    vector<int> b;
    // … fill both.
    bool c;
    std::cin >> c;
    if (c) return a; else return b;
}
Can a compiler be instructed to elide away the copy of variant_t being returned in C++?

Can a compiler be instructed to elide away the copy of variant_t being returned in C++?


By : ali ali
Date : March 29 2020, 07:55 AM
will be helpful for those in need There's nothing you can do to guarantee it. If possible, the compiler will generally elide this copy via return value optimization, but there are several caveats.
Within GetQueryRows, you should have only one named variable that is returned, from only one point in the function; multiple returns, or returns that may return one value or another, break return value optimization. You cannot throw any exceptions.
Why are move semantics necessary to elide temporary copies?

Why are move semantics necessary to elide temporary copies?


By : Carlson
Date : March 29 2020, 07:55 AM
With these it helps Move functions do not elide temporary copies, exactly.
The same number of temporaries exists, it's just that instead of calling the copy constructor typically, the move constructor is called, which is allowed to cannibalize the original rather than make an independent copy. This may sometimes be vastly more efficient.
g++ and -fno-elide-constructors flag - compiler goof up or bug in the code?

g++ and -fno-elide-constructors flag - compiler goof up or bug in the code?


By : Bala
Date : March 29 2020, 07:55 AM
I hope this helps . You don't initialize ClassA::myVal in ClassA's copy constructor. Thus the value of a0.myVal is indeterminate since a0 is initialized via copy-initialization.
This is revealed only when compiling with "-fno-elide-constructors" because the standard allows a compiler to elide copies even when it changes the observable behavior of the program. In this case, when compiling without "-fno-elide-constructors", gcc constructs a0 directly, without the intermediate constructors, and a0.myVal ends up being 300. When you explicitly disable copy elision, gcc keeps all of the intermediate copies, and so the initial value from the temporary ClassA constructed via direct-initialization is lost by the copy constructor that doesn't copy that value.
code :
ClassA(const ClassA& other) : myVal(other.myVal) {
    std::cout << "copy constructor called" << std::endl;
}
passing rvalue to non-ref parameter, why can't the compiler elide the copy?

passing rvalue to non-ref parameter, why can't the compiler elide the copy?


By : Eric Ong
Date : March 29 2020, 07:55 AM
To fix this issue You're right; this looks like a missed-optimization by the compiler. You can report this bug (https://bugs.llvm.org/) if there isn't already a duplicate.
Contrary to popular belief, compilers often don't make optimal code. It's often good enough, and modern CPUs are quite good at plowing through excess instructions when they don't lengthen dependency chains too much, especially the critical path dependency chain if there is one.
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