[music-dsp] C++ performance

[robert bristow-johnson]

On Oct 27, 2010, at 7:59 AM, Thomas Strathmann wrote:

> Some generic advice first:
>
> 1. Get to know your compiler and library and write "common sense" code
>   that the compiler knows how to deal with in an (near to) optimal  
> way.

this i agree with.

> 4. To reiterate: Don't think your cleverer than the people who wrote
>   the compiler and the runtime and don't try to port idioms from
>   other languages in the hope of making your code run faster.

i don't think i agree with this.

here is an example (a simple one-pole LPF with noise shaping):

//
//	transfer function:			H(z) = b0/(1 - (a1+1)*z^(-1))
//
//
void myProject::LPF(LPFBlock* this_filter, long* input)
     {
     register long* output_ptr	= &(this_filter->output[0]);

     register long   b0 = this_filter->b0;      // feedforward  
coefficient  Q8.24
     register long   a1 = this_filter->a1;      // feedback  
coefficient  Q8.24

     register long long  y1 = this_filter->y1; // previous output  
Q16.48, roundoff noise state in lower 24 bits

     register long   output_sample = (long)(y1>>24);	  // now is  
previous output sample, y[n-1]
	
     for (register int i=CHUNK_SIZE; i>0; i--)
         {
         y1 += (long long)b0 * (long long)(*input++);      // (y[n-1]  
+ b0*x[n]) * 2^24
         y1 += (long long)a1 * (long long)output_sample;   // (y[n-1]  
+ b0*x[n] + a1*y[n-1]) * 2^24
         output_sample = (long)(y1>>24);    // this truncation has  
simple roundoff noise shaping
         *output_ptr++ = output_sample;
         }

     this_filter->y1 = y1;                 // save state, including  
roundoff error state
     }

the use of register variables, the use of simple arithmetic lines with  
"+=" in the loop is there to make sure the compiler has no doubt what  
i want it to do.  the fact that i have "i" count down to zero rather  
than up to CHUNK_SIZE is to prevent the compiler from loading  
CHUNK_SIZE and subtracting, it only needs to compare to zero.

sometimes you *should* spell it out for the compiler.  i only wish the  
promotion to (long long) was unnecessary.  i wish that in C and C++,  
it was automatically understood that the type of the result of  
multiplying two N-bit numbers was a type with a 2N-bit number if a 2N- 
bit type is available.  that is one flaw of C or C++.

--

r b-j                  rbj at audioimagination.com

"Imagination is more important than knowledge."