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Use more MAC16_16's and unroll a loop.

This splits out the non-arch-specific portions of a patch written
 by Aurélien Zanelli <[email protected]
 http://lists.xiph.org/pipermail/opus/2013-May/002088.html

I also added support for odd n, for custom modes.

0.25% speedup on 96 kbps stereo encode+decode on a Cortex A8.

--- a/celt/celt_lpc.c

+++ b/celt/celt_lpc.c

@@ -101,7 +101,7 @@

       opus_val32 sum = SHL32(EXTEND32(x[i]), SIG_SHIFT);

       for (j=0;j<ord;j++)

       {

-         sum += MULT16_16(num[j],mem[j]);

+         sum = MAC16_16(sum,num[j],mem[j]);

       }

       for (j=ord-1;j>=1;j--)

       {

@@ -161,11 +161,16 @@

    }

 #ifdef FIXED_POINT

    {

-      opus_val32 ac0=0;

+      opus_val32 ac0;

       int shift;

-      for(i=0;i<n;i++)

+      int n2;

+      ac0 = 1+n;

+      if (n&1) ac0 += SHR32(MULT16_16(xx[0],xx[0]),9);

+      for(i=(n&1);i<n;i+=2)

+      {

          ac0 += SHR32(MULT16_16(xx[i],xx[i]),9);

-      ac0 += 1+n;

+         ac0 += SHR32(MULT16_16(xx[i+1],xx[i+1]),9);

+      }

       shift = celt_ilog2(ac0)-30+10;

       shift = (shift+1)/2;

@@ -176,7 +181,7 @@

    while (lag>=0)

    {

       for (i = lag, d = 0; i < n; i++)

-         d += xx[i] * xx[i-lag];

+         d = MAC16_16(d, xx[i], xx[i-lag]);

       ac[lag] = d;

       /*printf ("%f ", ac[lag]);*/

       lag--;