Replace fp_sqrt function with one that only uses shift, or and sub.

Simply extends the current isqrt() to be able to do fractional bits
and improves the initial estimate using clz(). iqrt() itself is
no more and is equivalent to fp_sqrt(x, 0). The original also had
a small bug where the guess comparision should have been >=, not >.

Uses no large integer math or division and is very accurate
(simply returns a truncated fraction).

Change-Id: I2ae26e6505df1770dc01e56220f7385369f90ae9

1 files changed, 4 insertions, 4 deletions

diff --git a/apps/plugins/fft/fft.c b/apps/plugins/fft/fft.c
index 40c251de15..3e88722b23 100644
--- a/apps/plugins/fft/fft.c
+++ b/apps/plugins/fft/fft.c
@@ -624,16 +624,16 @@ static unsigned calc_magnitudes(enum fft_amp_scale scale)
                 }
                 else
                 {
-                    d = isqrt(d); /* linear scaling, nothing
-                                     bad should happen */
+                    d = fp_sqrt(d, 0); /* linear scaling, nothing
+                                          bad should happen */
                     d = fp16_log(d << 16); /* the log function
                                               expects s15.16 values */
                 }
             }
             else
             {
-                d = isqrt(d); /* linear scaling, nothing
-                                 bad should happen */
+                d = fp_sqrt(d, 0); /* linear scaling, nothing
+                                      bad should happen */
             }
         }