Performance issue in struct initialization

[Basile B via Digitalmars-d]

On Saturday, 23 April 2016 at 13:37:31 UTC, Andrei Alexandrescu 
wrote:
> https://issues.dlang.org/show_bug.cgi?id=15951. I showed a few 
> obvious cases, but finding the best code in general is tricky. 
> Ideas? -- Andrei

In the first example you've forget to assign void:

http://d.godbolt.org/#compilers:!((compiler:gdc46,options:'-O3',source:'struct+RCStr(C)%0A%7B%0A++++enum+uint+maxSmall+%3D+64+/+C.sizeof+-+1%3B%0A++++C%5BmaxSmall%5D+small+%3D+void%3B%0A++++ubyte+smallLen+%3D+void%3B%0A%7D%0A%0Aauto+fun()+%7B%0A++RCStr!!char+result+%3D+void%3B%0A++return+result%3B%0A%7D%0A')),filterAsm:(commentOnly:!t,directives:!t,labels:!t),version:3

to get the real equivalent to the C++ version.

The problem in the second example is that the full initializer is 
always copied from the static layout 
typeid(Stuff).initializer[0..$], despite of the fact that some of 
the members are not iitialized, while in C++ only members that 
are init are copied.

The DMD code is shorter than the GDC one because it copies the 
initializer with `rep movsq` while GDC unrolls the `rep` with a 
bunch of `movq`, but in both cases the init is always fully 
copied.

So the problem is the suboptimal copy of the initializer.