struct vs class benchmark (was Re: Give struct the status it deserves)

[Bruno Medeiros]

Derek Parnell wrote:
> 
> But I think my example does show that classes (allocated on the heap) 
> are slower than structs whether allocated on the heap or on the stack.
> 
> Classes are slower than structs. Prove otherwise. I'm happy to be shown 
> that I'm wrong.
> 
> --Derek Parnell
> Melbourne, Australia

Now this is odd. I didn't expect classes on the heap to be much slower 
than structs on the heap. After all, what is the extra overhead of classes?
It has the size overhead of having 2 more pointers (for ClassInfo and 
for the monitor) and the overhead of initializing those pointers when 
instantiating a class. Anything else? I thought not, however, I tried to 
run a test (see code below), where I would simulate a class creation 
with a struct, adding that overhead, yet the struct version is still 
twice as fast (with or without optimization)! Why is that?? The best 
that I could find was by looking at the assembly code, and that in the 
class version a _d_newclass function is called instead of _d_new for the 
struct version, yet does _d_newclass do more than _d_new other than just 
initialize the pointers?


// -------------- code starts -------------------
private import std.c.stdio;
private import std.c.time;
private import std.stdio;

const static uint n = 100000000;

struct PointStruct
{
	void * classinfo;
	void * monitor;

	void Construct(void *newclassinfo) {
	    this.classinfo = newclassinfo;
	    this.monitor = cast(void *) 0x98765;
	}
}

void doitStruct()
{
     PointStruct *p = new PointStruct;
     p.Construct(cast(void *) 0x12345678);
}


final class PointClass
{
}


void doitClass()
{
     PointClass p = new PointClass;
}

int main()
{
	writefln(PointClass.classinfo.init.length," : ", PointStruct.sizeof);

	clock_t time1 = clock();
	for (uint i = 0; i < n; ++i)
	{
		version(test_struct_heap) {		
			doitStruct();
		}
		version(test_class_heap) {		
			doitClass();
		}
	}

	double seconds = (clock() - time1);
	seconds /= CLOCKS_PER_SEC;
	writefln("Testing operation took %ss",  seconds);
	return 0;
}

// -------------- code ends -------------------

sh-2.04$ build main -version=test_struct_heap -Tbtsh -full
c:\devel\D.tools\dmd\bin\..\..\dm\bin\link.exe 
main,btsh.exe,,user32+kernel32,btsh.def/noi;

sh-2.04$ build main -version=test_class_heap -Tbtch -full
c:\devel\D.tools\dmd\bin\..\..\dm\bin\link.exe 
main,btch.exe,,user32+kernel32,btch.def/noi;

sh-2.04$ time ./btsh.exe
8 - 8
Testing operation took 18.204s

real    0m18.328s
user    0m0.015s
sys     0m0.015s

sh-2.04$ time ./btch.exe
8 - 8
Testing operation took 42.516s

real    0m42.625s
user    0m0.015s
sys     0m0.015s

-- 
Bruno Medeiros - CS/E student
http://www.prowiki.org/wiki4d/wiki.cgi?BrunoMedeiros#D