[Speex-dev] cheb_poly_eva using Clenshaw's recurrence formula
s9268716 at mail.inf.tu-dresden.de
Tue Nov 29 07:56:53 PST 2005
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) */
> /* Calculate the b_(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 */
> 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?).
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