import std;

​

version( X86_64) {} else { static assert( !"need x86_64 CPU" ); }

version( linux) {} else { static assert( !"need Linux" ); }

​

struct r128 { ulong r1, r2; }

​

// Linux: RDI, RSI, RDX, RCX, R8, R9. returns RDX:RAX

extern (C)

r128 checkRegsC( ulong a, ulong b, ref ulong c ) pure nothrow @nogc @trusted {

    asm pure @nogc nothrow {

        naked;

        mov RAX, RDI;

        ret;

    }

}

​

// D: RDX, RSI, RDI.. why backward?

extern (D) // D here

r128 checkRegsD( ulong a, ulong b, ref ulong c ) pure nothrow @nogc @trusted {

    asm pure @nogc nothrow {

        naked;

        mov RAX, RDI;

        ret;

    }

}

​

void main() {

    ulong a = 0x0102030405060708, b = 0x8000700060005000;

    ulong c;

    writefln( "%016x \t=addr(c), 3rd arg", &c);

    auto r = checkRegsC( a, b, c );

    "%016x %016x".writefln( r.r1, r.r2 );

    r = checkRegsD( a, b, c );

    "%016x %016x".writefln( r.r1, r.r2 );

}

​