About structs and performant handling

[Ali Çehreli]

On 05/08/2013 05:57 AM, Namespace wrote:

 > On Saturday, 9 March 2013 at 23:07:50 UTC, Ali Çehreli wrote:
 >> On 03/09/2013 12:19 PM, Namespace wrote:
 >>
 >> > But structs with a bigger size aren't that performant if you
 >> pass them
 >> > as copy or move them (See also my small benchmark:
 >>
 >> I have started working on the DConf presentation about copy and move
 >> semantics in D. I have done exactly the same type of tests and was
 >> surprised how faster pass-by-reference can be.
 >
 > Hey, could you please publish your slides of your DConf presentation? :)

Yes, they are here:

   http://acehreli.org/AliCehreli_copy_move_D.pdf

But I see that you have been after some test results between by-copy vs. 
by-ref parameters. I have decided not to include them in the 
presentation for various reasons: not enough presentation time, not 
trusting my synthetic tests enough to be sure that I was covering all 
aspects of the behaviors of modern cpu architectures, etc. I can say 
that by-value was almost always slower in my little test.

However, as others have been saying, by-ref can be slower if a lot of 
the members of the struct are accessed through that reference, 
effectively defeating the CPU caches. For me to see that by-ref was 
slower, I had to define huge structs, put them in huge arrays, and touch 
all of the members of random elements of that array:

// s1 and s2 are random elements of huge arrays
void foo(ref const(S) s1, ref const(S) s2)
{
     // use all members of s1 and s2
}

Only then by-ref was slower than by-value. Let me build more trust in my 
test before opening it to discussion on the forums.

Ali