Re: Will C++ compilers ever be better than C compilers?

>To summarize:
> C++ compilers will produce code with lower performance than
> C compilers on equivalent code, because of the extra features of
> C++. This will occur because:
> 1) less resources will be available for performance optimizations


> 2) more resources are needed to implement and test the same
> optimization in C++ compared to C.


> 3) much of the available optimization resources will be allocated to
> optimizations irrelevant to "C subset" code.


>Do you agree?


Not entirely. Especially with point 3.


>Can you provide concrete examples?


A few days back, while writing a blurb on the equivalence of
algorithms, I made the assumption that the VC++ 5.0 compiler wouldn't
be able to optimize the following code:


int multiply_the_slow_way(int a, int b)
{
        int c = 0;
        for(int i = 0; i < b; c += a, i++);
        return c;
}


int main(void)
{


        int a = 0;
        int b = 0;
        cin >> a;
        cin >> b;


        cout << multiply_the_slow_way(a,b) << end;


}


With optimizations on, the compiler generated the appropriate code --
it did an integer multiplication directly (I verified this by looking
at the generated assembler code). When I did this:


        cout << multiply_the_slow_way(10,2) << end;


it generated the constant 20 directly.


C++ compiler, mixed C/C++ code -- optimization recognized that my
function was multiplication and behaved accordingly.


Your mileage may vary.
--
Quinn Tyler Jackson


email: qjackson@wave.home.com
url: http://www.qtj.net/~quinn/
ftp: qtj.net
--

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