[Ffmpeg-devel] [PATCH] flacenc - lpc and options

[Justin Ruggles]

Bobby Bingham wrote:
>>>+
>>>+/**
>>>+ * Levinson-Durbin recursion.
>>>+ * Produces LPC coefficients from autocorrelation data.
>>>+ */
>>>+static void compute_lpc_coefs(const double *autoc, int max_order,
>>>+                              double lpc[][MAX_LPC_ORDER], double *ref)
>>>+{
>>>+   int i, j, i2;
>>>+   double r, err, tmp;
>>>+   double lpc_tmp[MAX_LPC_ORDER];
>>>+
>>>+   for(i=0; i<max_order; i++) lpc_tmp[i] = 0;
>>>+   err = autoc[0];
>>>+
>>>+   for(i=0; i<max_order; i++) {
>>>+      r = -autoc[i+1];
>>>+      for(j=0; j<i; j++) {
>>>+          r -= lpc_tmp[j] * autoc[i-j];
>>>+      }
>>>+      r /= err;
>>>+      ref[i] = fabs(r);
>>>+
>>>+      err *= 1.0 - (r * r);
>>>+
>>>+      i2 = (i >> 1);
>>>+      lpc_tmp[i] = r;
>>>+      for(j=0; j<i2; j++) {
>>>+         tmp = lpc_tmp[j];
>>>+         lpc_tmp[j] += r * lpc_tmp[i-1-j];
>>>+         lpc_tmp[i-1-j] += r * tmp;
>>>+      }
>>>+      if(i % 2) {
>>
>>i&1 is faster then i%2
>>
> 
> 
> Though if i was unsigned, gcc (4.1.1 at least - I'd hope other
> versions too) is smart enough to generate the i&1 code in both cases.
> 

Somehow it doesn't feel right to rely on what certain compilers may or
may not do when I can just put the faster version right in the code.

-Justin