Performance improvements for D / DMD compiler.

[Bill Baxter]

Walter Bright wrote:
> Andrei Alexandrescu (See Website For Email) wrote:
>> No, and for a good reason:
>>
>> foo(LargeStruct s1, LargeStruct s2)
>> {
>>   ...
>> }
>> ...
>> LargeStruct s;
>> foo(s, s);
>>
>> Nobody would enjoy hidden aliasing of s1 and s2.
> 
> There's another aliasing case:
> 
> LargeStruct s;
> foo(s);
> ... here some other thread modifies s ...
> 
> And foo() suddenly has its argument values change unexpectedly.
> 
> Value and reference type usages mostly overlap, but they are 
> fundamentally different, and having the compiler switch between the two 
> in some implementation-defined manner (as suggested by others more than 
> once) is not going to work.
> 
> If one is needing reference behavior from a struct, seriously consider 
> making it a class instead. Or even just take the address of it and pass 
> the pointer.

If we're going to pass a pointer to it, then we might as well just make 
it inout and avoid having to explicitly use '&' to take the address.

The question is how can we write functions in D that take big structs as 
parameters in such a way that it is:
A) efficient and
B) guaranteed not to change the caller's value

In C++ the answer is const LargeStruct&.  In D the answer is...

--bb