[Speex-dev] cheb_poly_eva using Clenshaw's recurrence formula

[Björn Thalheim]

Hello,

Matthieu Poullet schrieb:
> static float cheb_poly_eva(spx_word32_t *coef, float x, int m, char *stack)
> {
>  int k;
>  float b0, b1, tmp;
>  int m2=m>>1;
> 
>  /* Initial conditions */
>  b0=0; /* b_(m+1) */
>  b1=0; /* b_(m+2) */
> 
>  x*=2;
> 
>  /* Calculate the b_(k) */
>  for(k=m2;k>0;k--)
>  {
>     tmp=b0;                           /* tmp holds the previous value of b0 */
>     b0=x*b0-b1+coef[m2-k];    /* b0 holds its new value based on b0 and b1 */
>     b1=tmp;                           /* b1 holds the previous value of b0 */
>  }
> 
> return(-b1+x/2*b0+coef[m2]);
> }
> 
> I don't really know if it's a little improvement or not, so I'd like
> to have feedback from the Speex experts here.

I'm far from beeing an expert in anything, but I've recently had a
look at the function as it is used in speex-1.1.10.
Both implementations do some assignments and then go m/2 times
through a loop.
The current implementation does in this loop two multiplications and
two additions. The function described above only uses two simple
assignments and 1 multiplication and one addition.
Seems like less operations to me. As I'm sure the return values are
the same (which should be tested before any refactoring), I suppose
the suggestion is more effective.
Maybe a new implementation should use the algorithm above plus the
explicit ALLOC which is used all over speex (why actually?).

Ciao,

Björn


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