Avoid initializing a struct field.

[jerro]

> What code have you used to test it, and what is the result you 
> have seen?

I was using this code:

extern(C) void foo(Foo* r)
{
     Foo tmp;
     *r = tmp;
}

And here is the generated assembly (when using "= void"):

push   %rbp
mov    %rsp,%rbp
sub    $0xc0,%rsp
mov    %rdi,-0x10(%rbp)
movabs $0x65a360,%rsi

lea    -0xc0(%rbp),%rdi
mov    $0x15,%ecx
rep    movsq %ds:(%rsi),%es:(%rdi)
movsb  %ds:(%rsi),%es:(%rdi)
movsb  %ds:(%rsi),%es:(%rdi)
movsb  %ds:(%rsi),%es:(%rdi)
movsb  %ds:(%rsi),%es:(%rdi)
lea    -0xc0(%rbp),%rsi
mov    -0x10(%rbp),%rdi
mov    $0x15,%cl
rep    movsq %ds:(%rsi),%es:(%rdi)
movsb  %ds:(%rsi),%es:(%rdi)
movsb  %ds:(%rsi),%es:(%rdi)
movsb  %ds:(%rsi),%es:(%rdi)
movsb  %ds:(%rsi),%es:(%rdi)
leaveq
retq

The problem here is that the code copies the entire object from 
some static address instead of just the fields that need to be 
initialized, and that can be a performance problem. The same is 
true for your void version.

I tried using float[42] instead of int[42] and found out that 
buffer actually isn't initialized to its default initializer if I 
use "= void" (the numbers were all 0 instead of NaN), but the 
performance cost is still there.