lavc/cook: get rid of wasteful pow in init_pow2table

The table is highly structured, so pow (or exp2 for that matter) can entirely
be avoided, yielding a ~ 40x speedup with no loss of accuracy.

sample benchmark (Haswell, GNU/Linux):
new:
4449 decicycles in init_pow2table(loop 1000),     254 runs,      2 skips
4411 decicycles in init_pow2table(loop 1000),     510 runs,      2 skips
4391 decicycles in init_pow2table(loop 1000),    1022 runs,      2 skips

old:
183673 decicycles in init_pow2table(loop 1000),     256 runs,      0 skips
182142 decicycles in init_pow2table(loop 1000),     512 runs,      0 skips
182104 decicycles in init_pow2table(loop 1000),    1024 runs,      0 skips

Reviewed-by: Clément Bœsch <u@pkh.me>
Signed-off-by: Ganesh Ajjanagadde <gajjanagadde@gmail.com>

libavcodec/cook.c

@@ -166,10 +166,17 @@ static float rootpow2tab[127];
 /* table generator */
 static av_cold void init_pow2table(void)
 {
+    /* fast way of computing 2^i and 2^(0.5*i) for -63 <= i < 64 */
     int i;
+    static const float exp2_tab[2] = {1, M_SQRT2};
+    float exp2_val = powf(2, -63);
+    float root_val = powf(2, -32);
     for (i = -63; i < 64; i++) {
-        pow2tab[63 + i] = pow(2, i);
-        rootpow2tab[63 + i] = sqrt(pow(2, i));
+        if (!(i & 1))
+            root_val *= 2;
+        pow2tab[63 + i] = exp2_val;
+        rootpow2tab[63 + i] = root_val * exp2_tab[i & 1];
+        exp2_val *= 2;
     }
 }