Bug 13093 – D ABI change for guaranteed efficient return of fixed size array

I suggest 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[10000] foo() nothrow @safe { typeof(return) data; // Some code here. return data; } void main() nothrow { immutable data = foo(); } That means that code is equivalent to (note the need for the explicit cast): void foo(ref ubyte[10000] __data) nothrow @safe { __data[] = 0; // Some code here. } void main() nothrow { ubyte[10000] __data = void; foo(__data); immutable data = cast(immutable ubyte[10000])__data; } If the returned fixed-size array is very small (like one or two CPU words, or perhaps even three), the new ABI can specify it's returned by value in registers. 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 (and more DRY code that avoids to repeat ubyte[10000] more than once as it happens in the second program). Dmitry Olshansky has commented: http://forum.dlang.org/thread/[email protected] > 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.

It's already supported, but not explicitly described in ABI page. From http://dlang.org/abi Return Value ... * 1, 2 and 4 byte structs are returned in EAX. * 8 byte structs are returned in EDX,EAX, where EDX gets the most significant half. * For other struct sizes, the return value is stored through a hidden pointer passed as an argument to the function. ... Normally static arrays are treated as the same size structs, so ubyte[1000] is always returned through hidden pointer. https://github.com/D-Programming-Language/dlang.org/pull/611

You can test it by the code. void* p; ubyte[10000] foo() nothrow { typeof(return) data; p = &data; return data; } void main() nothrow { immutable data = foo(); assert(p == &data); }

Commits pushed to master at https://github.com/D-Programming-Language/dlang.org https://github.com/D-Programming-Language/dlang.org/commit/70813e9b9fe6152c83a6d24ea44080a3697a5b00 fix Issue 13093 - D ABI change for guaranteed efficient return of fixed size array https://github.com/D-Programming-Language/dlang.org/commit/22697d464ece7d64ba5cc2792a0863a5ffbaa88d Merge pull request #611 from 9rnsr/fix13093 Issue 13093 - D ABI change for guaranteed efficient return of fixed size array

Commits pushed to 2.067 at https://github.com/D-Programming-Language/dlang.org https://github.com/D-Programming-Language/dlang.org/commit/70813e9b9fe6152c83a6d24ea44080a3697a5b00 fix Issue 13093 - D ABI change for guaranteed efficient return of fixed size array https://github.com/D-Programming-Language/dlang.org/commit/22697d464ece7d64ba5cc2792a0863a5ffbaa88d Merge pull request #611 from 9rnsr/fix13093