Comment by jcranmer
4 days ago
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.
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