My little trick is to think of them as "oh, I accidentally made an lvalue from an actual rvalue here because a name was introduced, so I need to cast (i.e move() or forward()) back to an rvalue again", that's why i have them as macros: MOVE_CAST and FORWARD_CAST defined as static_cast (also avoids blowing up compile times).
I never think in terms of "moving this object".
And the most important idea: destructive moves.
Since C++ doesn't track lifetimes it has to leave the object in a "valid state" after a move and the destructor still runs which has to have a check if it should do something or not.
BTW, Rust's lifetime annotations for borrowed references are a mostly orthogonal feature.
Liveness of objects for move/drop semantics is tracked differently, without any syntax and with implicit runtime drop flags where necessary.
C++ could probably add the same deinitialized/moved-from state tracking (with an opt-in for back compat sake) purely to avoid dtor bloat, without having to add safety of borrow checking.
> C++ could probably add the same deinitialized/moved-from state (with an opt-in for back compat sake) purely to avoid dtor bloat, without having to add safety of borrow checking.
There is a lot of talk in the C++ committee about this. The details are complex in some obscure cases.
While not the common case, C++ move semantics match the situation in systems code where correctness requires decoupling logical object lifetimes and destructors. The object is logically dead but the destructor may be deferred indefinitely for safety reasons.
Most code doesn't have the shared memory setups where deferred destruction is necessary.
What is that makes NVRO so much more difficult to implement? Why couldn't they mandate that just like RVO?
Do compilers literally just special case a simple return statement of a direct construction or something?
Is going to be much harder because you don't know are compile time which is returned and so cannot construct the one you return in the correct place. That is just off the top of my head, I'm not a compiler writer, I'm sure they have figured out the simple versions of the above, but you can start to see the complex versions that they can't.
That one is not too hard either. `one` simply can't be NRVO'd as there is a runtime path in scope of it that doesn't return it. `two` can be (unless you hide some other return within ...).
The point of (N)RVO is to directly construct the return value in-place at the calling frame. Which requires knowing what object will land there.
In RVO there is no problem because you know what object is the one you need to put there.
In NRVO there is a problem because you might have one of multiple objects being returned and you need to know which one to construct at the call site; it can't be all of them on top of each other. But you don't necessarily know at the time of construction whether that object will be the one that is actually returned. Doing so requires imperfect code analysis so the standard would need to define the complicated analyses to perform.
RVO is easy to detect since it happens only in expressions in return-statements.
NRVO requires the compiler to analyze the flow, like if 2 different variables/constructions can lead to the return (what one do we take, or can we do either later?).
Also, with RVO it's easy to detect and elide destruction calling for things going out of scope whilst NRVO would require more careful management of destruction order,etc.
Basically, NRVO touches a lot of things in "inconventient" places that can easily require reworking internal compiler structures to track destinations whilst RVO was probably far easier to just "hack in".
I figured any half decent compiler already do plenty of flow and liveness analysis on everything for register allocation, dead code elimination and what not.
Maybe it's the guaranteed elision that makes it a problem, like you can't fail the analysis, but then maybe you go the rust route - fail to compile and urge the programmer to rewrite their code so it accepts it.
Make it opt in with [[must_elide]] so old code still works I guess.
Register allocation is usually on a far "lower" codegen level as is often DCE, they should be possible to compute/run on a SSA node level or similar long after destruction sequences are applied.
Now, there is far more "language level" flow analysis today apart from this as required by allowing auto type inference in more places (and things relaxed in relation to that). Reading up it seems to be suitably done in Clang on the ClangIR(MLIR extension) level, something that sits between AST and the LLVM IR.
Regardless of how it's implemented, I'm pretty sure that NRVO carried a fair bit more complexity requirements compared to RVO depending on how prepared the corebases for different compilers were to handle it.
Despite its name, NRVO isn't an optimization performed by the optimizer, it's something done by the frontend of the compiler before it generates the code for the optimizer to run on.
The frontend is extremely reluctant to do anything like flow analysis, in large part because the frontend doesn't even really have any code to do the analysis on, just the AST. More people (including far too many on the committee itself) need to understand the separation between the different parts of the compiler, and what each part can and cannot do effectively.
They probably _were_, since lower level code representations often has little notion of complex types and their semantics they could be kept clean and focused on machine code, however type inference in a language like C++ complicates such matters immensly since an assignment can be both a register move and a function call.
Template resolution solved that in the past, but C++ today allows auto in so many places that I'm uncertain that it can be done without some flow based support (if constexpr comes to mind).
I mentioned ClangIR(MLIR) in the sibling comment here, feels like it was built for stuff like this.
> Template resolution solved that in the past, but C++ today allows auto in so many places that I'm uncertain that it can be done without some flow based support (if constexpr comes to mind).
C++ requires determining the type of every expression immediately. Even auto doesn't change that: it determines the type of the variable based on the initializer, literally following the same rules as template argument deduction (there's a little bit of patching to tweak the exact expression being used for deduction, but https://eel.is/c++draft/dcl.type.auto.deduct#3 is the core rule here).
ClangIR doesn't necessarily help here, because while it does give a more abstract C abstract machine IR semantics, it's still downstream of things like NRVO decision points--it's still fundamentally past the codegen-the-AST barrier.
> Template resolution solved that in the past, but C++ today allows auto in so many places that I'm uncertain that it can be done without some flow based support (if constexpr comes to mind).
This concern is unfounded. The auto keyword in C++ acts as mere syntactic sugar. It works only when the compiler is able to tell exactly what's the type by evaluating the expression.
The auto keyword is also considered a code smell for the same reason: just because the compiler can tell exactly what the type is expected to be, that does not mean the developer can. Therefore it makes the code harder to reason about.
> The auto keyword is also considered a code smell for the same reason: just because the compiler can tell exactly what the type is expected to be, that does not mean the developer can. Therefore it makes the code harder to reason about.
This really depends though. In many cases even the programmer can tell the type of auto because its on the very same line and not using auto would mean needlessly repeating it.
In other cases (e.g. iterators) the programmer also doesn't need to care about the concrete type.
GP was possibly arguing against Herb Sutter's "almost always auto". C++ wasn't designed for auto and it shows. You can't rely on it always doing the right thing or at least a safe thing in C++, unlike in Rust - I've seen bugs due to auto. It is also often helpful to spell out the concrete type in important places such as (most/many) variable definitions.
I'm also fine with auto if it just repeats information, especially so if the concrete type takes half of your line length budget (yes it's an iterator over that container containing...).
Yeah, when auto first came out a few people started abusing it everywhere making for unreadable code. Most developers settled quickly on much better rules for using auto that do not destroy readability, and never abused it in the first place. Auto is intended for and works very well to avoid typing very long types where you often never care.
Iterators is the common example: the type name is always long, and nearly always used in a context where it is obvious. Even if you do care about the type, looking up the iterator is wrong - you always first look up the base type (ie std:vector) first to understand the iterator, only rarely do you need to dig into the iterator once you understand the base type.
Generic code (template and non-template) are the other - they type could be anything, so trying to specify more detail isn't going to gain you anything.
The problem is that "predictable reliable NRVO" is still a research problem. Real-world compilers do NRVO a lot but not in a way that is perfectly predictable — that is to say, not in a way that could be standardized across all compilers (or even between different releases of the same compiler).
A "perfectly predictable" algorithm was proposed in Anton Zhilin's P2025, back in the year 2021:
https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2021/p20...
but unfortunately it had some subtle corner-case problems (which I do not remember), so it was sent back for revision, and never returned with a fix. (Maybe because a fix wasn't possible; again I don't remember what the deal was exactly.)
The Right Path Forward would be for MSVC, GCC, and Clang all to try implementing P2025's algorithm in their front ends. Either something concrete breaks (reminding me what the problem was), or else all three mainstream compilers gain predictable NRVO and then we can "standardize existing practice." But the Right Path Forward requires tedious work by at least three people, which is hard.
N/RVO works by (at the machine language level, of course) rewriting the function signature to return void and take an extra pointer parameter, which is written to before returning. If you're returning a newly-constructed object, the compiler can rewrite that into calling the constructor on the pointer, but if you're returning a named object, the class may have a non-trivial destructor that needs to run after the move, such that it's not possible to rewrite uses of the local object into uses of the pointer.
I'm not too confident on that last part, because such an implementation would mess with semantics in case of an exception, so anyone feel free to correct me on that.
NRVO does not affect the ABI of the function. It cannot affect the ABI, for whether or not it kicks in depends on the body of the function, and affecting the ABI would make it impossible to use it if only the declaration appears in a header.
The correct explanation is this:
In C++, classes with nontrivial destructors or copy/move constructors are considered nontrivial for the purposes of calls and are passed via pointers rather than via value. By passing via pointer, the class has a stable address and thus 'this' pointer. Returning such a class means the caller allocates the storage for the class on the stack before calling the function, and passes the pointer to that storage to the function as an extra parameter. This is based solely on the definition of the class itself; this happens whether or not NRVO kicks in.
Usually, when you declare a variable, the abstract machine of C++ requires you to construct a new object and call the copy/move constructors or assignment operators and the destructors at various times as appropriate. With nontrivial versions of these special functions, it is possible to observe whether or not they were called (these things still happen with trivial classes, but it's not so easy to observe). Returning a value requires constructing the storage space for that object--with all the attendant abstract machinery that involves.
What NRVO does is to say that, under certain conditions, rather than constructing storage space for a given variable that is normally required, the storage space that is allocated for the return value by the ABI is used instead. In essence, you are promoting a given variable to the return value hence the name 'Named Return Value Optimization'. What makes this annoying to implement is that you have to track at the AST level, before doing any code generation at all, whether or not a given variable is eligible for NRVO, and then use that information to control the code generation for allocating storage space.
Despite its name, NRVO is not actually an 'optimization' in the compiler. The optimizer plays no role in it, since the optimizer is fulfilling the requirements of the abstract machine. Instead, it is a set of conditions that allows the frontend to omit calls to copy constructors, etc. under specific circumstances.
> Despite its name, NRVO is not actually an 'optimization' in the compiler. The optimizer plays no role in it, since the optimizer is fulfilling the requirements of the abstract machine. Instead, it is a set of conditions that allows the frontend to omit calls to copy constructors, etc. under specific circumstances.
That's semantics. The way the compiler is structured and in which component the transformation is implemented has no bearing on whether something is an optimization.
> N/RVO works by (at the machine language level, of course) rewriting the function signature to return void and take an extra pointer parameter
This sounds wrong, are you sure? Would you mind demonstrating with an example on godbolt? Whether NRVO applies or not, the ABI should be the same, AFAIK.
Yes, it works exactly like this, this is a demo on godbolt [0]. rdi stores the pointer in both cases, makeS1() uses RVO, makeS2() takes it explicitly and constructs with placement new.
I will say before testing this i didn't realize the RVO calling convention was to return the pointer you pass in, but apparently so. If makeS2() returned void, it's just a tail call to the constructor, but makeS1() has to spill rbx and use it to save the pointer.
No, all you're showing in that example is that a pointer is passed as part of the ABI. You're not showing that RVO relates to that in any way whatsoever. If you write the same function in a manner that (N)RVO can't kick in, does the pointer no longer get passed?
The reason this should sound dubious is that you're suggesting the caller needs to know the callee's body in order to know how to call it, but it should be possible for the two to be compiled entirely independently, and in fact mutual recursions should be fine too. After all, the callee knows where the return value has to land either way, and the caller similarly knows where to expect it, regardless of when/how the object is constructed or destroyed.
Yes, of that I'm sure. This optimization is only possible if the compiler has control of both sides of a call. If the function may be callable from other translation units or modules I imagine it generates a thin wrapper that's externally callable.
The optimization is often possible even if the computer does not see the call, because most (all?) ABIs have always required hidden pointer parameters for class types with non-trivial destructors.
https://godbolt.org/z/9WvnEvEYh
Note how `std::unique_ptr<int>` effectively passed as a `int**`; and that the by-value unique_ptr is not destroyed at the end of the function -- destroying parameters is instead the caller's job (and commonly only happens at the end of the full expression containing the call -- though this choice is implementation-defined).
But that can only work if the caller can see the updated value of the parameter (to avoid double-free for `clear`) -> thus the need to pass the parameter by hidden pointer.
I don't know why I wrote the comment about parameters earlier -- (N)RVO is about return values.
Those have a different reason for being passed behind a hidden pointer: the class type might have self-referencing pointers, so there must be an explicit move/copy constructor call whenever it changes address, to give the class an opportunity to update those pointers. This cannot work when returning in a register: the callee doesn't know the target address, and the caller doesn't the source address, so neither can call the move constructor.
Thus, all ABIs must pass a pointer (or let caller+callee agree on a memory location in some other way) for types that aren't trivially copyable.
> What is that makes NVRO so much more difficult to implement?
I recall reading that at a high level RVO is implemented by treating the return value as an external object. In simple terms (simplistic terms) RVO then works by
- first instantiating the return variable,
- passing the var by reference to the function,
- and then use return value to actually initialize the variable passed by reference.
The moment there's some funny logic on what to write to that output value, the problem gets far more complex.
