Re: Why is C# 450% slower than C++ on nested loops ??


"Stefan Simek" <simek.nospam@xxxxxxxxxxxxxxxxx> wrote in message
news:e2rqYVXDGHA.2912@xxxxxxxxxxxxxxxxxxxxxxx
> Peter Olcott wrote:
>> "Anders Borum" <anders@xxxxxxxxxxxxxx> wrote in message
>> news:edHCjaSDGHA.3528@xxxxxxxxxxxxxxxxxxxxxxx
>>
>>>Hello!
>>>
>>>
>>>>It must have been after the JIT, because C# beat C++ on some things, just
>>>>not the nested loops. The author of the link would know relatively obvious
>>>>things such as this.
>>>
>>>Unless somebody documents what methods have been used to measure the
>>>benchmarks, I am disposing the data altogether. Benchmarks should be
>>>verifiable ..
>>
>>
>> I searched high and low and finally found some stuff on unverified code. The
>> MSDN documentation mentions it yet does not discuss what it means. It could
>> be anything from security checks to tests for memory leaks. It doesn't say
>> what its purpose is.
>>
>>>--
>>>With regards
>>>Anders Borum / SphereWorks
>>>Microsoft Certified Professional (.NET MCP)
>>>
>>
>>
>>
> I guess you should google for 'define:verify'. Verifiable benchmarks have
> nothing to do with verifiable code. Anders just wanted the benchmarks to be
> verifiably *valid*, which they aren't by any means. A nested loop written the
> way it is in the benchmark is measuring nothing but a compiler's ability to
> optimize nested loops that do more or less nothing. Verifiable code means (in
> the .NET vocabulary) that the CLR can statically verify the code to ensure it
> will do nothing it is not expected to do. But enough of that, let's see what
> native assembly the

Yet I just found a Microsoft article that says that this sort of technology is
between 20 and 50 years away. Code is not expected to be incorrect. Proving that
code is not incorrect is technolgy that does not yet exist. I don't know what it
is verifying, but, correct code is not it. The posted benchmark was crucial to
my needs because my critical 100-line function is mostly loops.

> compilers generate for the loop and get over with it.
>
> <...>
>
> ha! I won't post the complete disassemblies (the C++ one is terribly cryptic
> so it would do no help), but I've found the following. The C++ compiler (8.0
> in my case, but I suspect 6.0 is doing the same) manages to pre-cache the
> additions in the outer loops, which the C# compiler doesn't. Thus, in the C#
> code all the additions that happen at
>
> x+=a+b+c+d+e+f;
>
> are evaluated for every single innermost loop, while c++ does something like
> the following:
>
> for (a = 0; a < n; a++)
> {
> for (b = 0, tmp_ab = a; b < n; b++, tmp_ab++)
> {
> for (c = 0, tmp_abc = tmp_ab; c < n; c++, tmp_abc++)
> {
> for (d = 0, tmp_abcd = tmp_abc; d < n; d++, tmp_abcd++)
> {
> for (e = 0, tmp_abcde = tmp_abcd; e < n; e++, tmp_abcde++)
> {
> for (f = 0, tmp = tmp_abcde; f < n; f++, tmp++)
> x += tmp;
> }
> }
> }
> }
> }
>
> If you compile this code in C#, the execution time is 6734 ms for .NET 2.0 and
> 8593 ms for .NET 1.1 versus 3656 ms for unmanaged C++ (8.0) on my machine. But
> note that the innermost loop in the C++ is only four instructions. This can
> hardly be matched to any real-life algorithm, and I can assure you that if
> there was anything more than 'nothing' in the inner loop, the performance
> difference would be much less than 80% (I expect it to drop far below 10% for
> most real-life algorithms).

Since this is not the same code that was posted on the benchmark, it is not a
valid test. If you are worried about arithmetic overflow then define X as a
double. I want to know why there was a 450% difference in the original
benchmark. When you change the code, this avoids rather than answers the
question. It is reported that 2005 does a much better job of optimization of
..NET code, yet, only with the C++, not the C# compiler.

>
> That said, I wouldn't expect C# (or .NET) to match pure native code
> performance for purely computational tasks like the one you describe is. C#
> can win easily in cases where a lot of dynamic allocation is involved, etc.,
> but it will probably never outperform optimized native C++ (or handwritten
> assembly) for computational tasks. If you need to squeeze every clock of your
> CPU, you will probably get best results using an optimizing compiler targeted
> for exactly the CPU the code will run on.

There is no reason (in theory) why .NET code would need to be any slower than
native code. The reason in practice seems to be that Microsoft just hasn't
gotten around to implementing every possible optimization in every language,
yet. The one tricky thing about optimization in .NET is that it is divided into
two different processes. Some of the optimizations at the compile step can screw
up some of the optimizations at the JIT step. The problem is that the compile
step does not know how many registers the target architecture has.

>
> I didn't want to write all this, but after reading through the long
> discussions on this matter without touching the importatnt points, I just
> decided to do so :)
>
> Just my 2c.
>
> Stefan


.

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