So why doesn't popFront return an element?

[Jonathan M Davis]

> Oh, I thought the compiler could optimize calls to popFront if I'm not
> assigning its value.
> 
> Doesn't a technique exist where if a function is called and I'm not
> assigning its result anywhere it should simply exit the function
> without returning anything? It seems like a valuable compiler
> optimization, if that is even possible.

That would depend on the type. For anything other than structs, if the 
function is inlined, it should be able to do that (if it's not inlined, it 
can't, because the returning is done inside of the function call). For 
structs, it can probably do that if there's no postblit constructor and none 
of its member variables have postblit constructors (or have member variables 
of their own which have postblit constructors, etc.). If there's a postblit 
constructor, however, it would have to be pure, otherwise it could have side 
effects, and not doing the return could then alter the behavior of the 
program. However, if it's pure, it probably can. I don't know what the 
compiler does exactly.

Regardless, in the general case, the return value can't be optimized out, 
because that's done in the function call, not at the call site. Inlined 
functions should be able to get the optimization as long as it doesn't change 
the behavior of the program to do so, which could require that the postblit 
constructor is pure if a struct is being returned, and if you're returning an 
expression, then that expression is still going to have to be evaluated unless 
the compiler can determine that it has no side effects, which again, could 
require any functions involved to be pure.

So, don't bet on such an optimization happening. dmd might manage it in some 
cases, but in the most obvious cases, the cost of the return is pretty low 
anyway (since it's either a primitive type or a reference).

- Jonathan M Davis