10-Jul-2014 13:36, bearophile пишет:
(This is a duplicated post of D.learn.)Is it possible and a good idea to change the D ABI to make code like this avoid an array copy in 100% of the cases (without inlining, and regardless the optimization levels, and in all D compilers)? ubyte[1000] foo() nothrow @safe { typeof(return) data; // Some code here. return data; } void main() nothrow { immutable data = foo(); } That means that code is equivalent to (also note the need for the explicit cast): void foo(ref ubyte[1000] __data) nothrow @safe { __data[] = 0; // Some code here. } void main() nothrow { ubyte[1000] __data = void; foo(__data); immutable data = cast(immutable ubyte[1000])__data; } If the returned fixed-size array is very small (like one or two CPU words, the new ABI can specify it's returned by value). In my @nogc code I'd like to use fixed-size arrays, so it's nice to be sure they are _always_ returned efficiently, and at the same time keep a nice syntax that allows me to tag the result as immutable.
IMO this is a good idea and it pretty much NRVO/RVO for structs extended to fixed-size arrays (which more or less a special kind of struct). Since C/C++ do not have fixed-size arrays passed by value I see no damage to ABI compatibility.
-- Dmitry Olshansky
