Performance improvements for D / DMD compiler.

[Bill Baxter]

Bill Baxter wrote:
> 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 ...

This exists in C++ with const reference parameters.  But I've not heard 
much gnashing of teeth over this.  Maybe people are getting worried 
about it more these days, but since I started learning C++ I've seen 
lots of code that uses const references all over the place.

But I can see that it is important to make the distinction in the header 
for those who are writing multithreaded code.

Which leaves us with two alternatives:
1) add some sort of const-reference construct to D
2) leave it as is.


I have to say that 2) is probably going to send me packing.  I've always 
assumed that D would get some sort of const reference someday but it 
sounds like you're saying you don't see a need.


But I think this raytracer example shows very clearly that using even 
moderately sized structs (i.e. 3-6 floats, not even that big) as 
parameters in critical loops can degrade performance significantly.

Switching to a class is not really an option.  That's going to be even 
slower given that intermediate vector calculations are needed all over 
the place.

So the other option is scope classes which someone suggested on the 
other thread.  Is this really an option?  I haven't played with it that 
much, so I don't really know what the issues are or what the syntax 
looks like in practice.  But I believe at least that it means every 
place you use LargeStruct you're going to have to say  'scope 
LargeStruct', right?  That's fairly ugly to have to say everywhere for 
something that's going to get heavy use.  Especially when in the end it 
really *is* a value type.  So it should be a struct.  Period.

--bb