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

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"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 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).



The snip you posted is meaningless without the value of n and the types of
the local variables, so please post the whole function.


Willy.



.

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