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Converts denormalise_bands() to use 16-bit multiplications

--- a/celt/bands.c

+++ b/celt/bands.c

@@ -205,17 +205,30 @@

       for (i=start;i<end;i++)

       {

          int j, band_end;

-         celt_ener bandE;

-         opus_val32 g;

+         opus_val16 g;

          opus_val16 lg;

-         lg = ADD16(bandLogE[i+c*m->nbEBands], SHL16((opus_val16)eMeans[i],6));

-         bandE = PSHR32(celt_exp2(lg),4);

-         g = SHR32(bandE,1);

+#ifdef FIXED_POINT

+         int shift;

+#endif

          j=M*eBands[i];

          band_end = M*eBands[i+1];

+         lg = ADD16(bandLogE[i+c*m->nbEBands], SHL16((opus_val16)eMeans[i],6));

+#ifdef FIXED_POINT

+         /* Handle the integer part of the log energy */

+         shift = 16-(lg>>DB_SHIFT);

+         if (shift>31)

+         {

+            shift=0;

+            g=0;

+         } else {

+            /* Handle the fractional part. */

+            g = celt_exp2_frac(lg&((1<<DB_SHIFT)-1));

+         }

+#else

+         g = celt_exp2(lg);

+#endif

          do {

-            *f++ = SHL32(MULT16_32_Q15(*x, g),2);

-            x++;

+            *f++ = SHR32(MULT16_16(*x++, g), shift);

          } while (++j<band_end);

       }

       celt_assert(start <= end);

--- a/celt/mathops.h

+++ b/celt/mathops.h

@@ -190,6 +190,13 @@

 #define D1 22804

 #define D2 14819

 #define D3 10204

+

+static inline opus_val32 celt_exp2_frac(opus_val16 x)

+{

+   opus_val16 frac;

+   frac = SHL16(x, 4);

+   return ADD16(D0, MULT16_16_Q15(frac, ADD16(D1, MULT16_16_Q15(frac, ADD16(D2 , MULT16_16_Q15(D3,frac))))));

+}

 /** Base-2 exponential approximation (2^x). (Q10 input, Q16 output) */

 static inline opus_val32 celt_exp2(opus_val16 x)

 {

@@ -200,8 +207,7 @@

       return 0x7f000000;

    else if (integer < -15)

       return 0;

-   frac = SHL16(x-SHL16(integer,10),4);

-   frac = ADD16(D0, MULT16_16_Q15(frac, ADD16(D1, MULT16_16_Q15(frac, ADD16(D2 , MULT16_16_Q15(D3,frac))))));

+   frac = celt_exp2_frac(x-SHL16(integer,10));

    return VSHR32(EXTEND32(frac), -integer-2);

 }