The LPC to LSF conversion scans a 1024-point grid for sign changes of two
cosine series and called libm cos for every series term, about 21,000 calls
per channel and frame (30-40 % of SILK and hybrid encode time). Compute
cos(m * w) for every grid point and multiple once, with the same expression
as before, so the values are bit-identical and the bitstream does not change.
Only the bisection steps still call cos.
A test compares the new root search against the previous one bit for bit on
random symmetric polynomials; a2nlsf rejects orders beyond the table.
Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
