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fixed-point: slight (but free) accuracy improvement in compute_band_energies()

Also moves the VSHR32() condition outside the loop just in case some compilers
don't optimize that properly.

--- a/celt/bands.c

+++ b/celt/bands.c

@@ -92,11 +92,11 @@

 #ifdef FIXED_POINT

 /* Compute the amplitude (sqrt energy) in each of the bands */

-void compute_band_energies(const CELTMode *m, const celt_sig *X, celt_ener *bandE, int end, int C, int M)

+void compute_band_energies(const CELTMode *m, const celt_sig *X, celt_ener *bandE, int end, int C, int LM)

 {

    int i, c, N;

    const opus_int16 *eBands = m->eBands;

-   N = M*m->shortMdctSize;

+   N = m->shortMdctSize<<LM;

    c=0; do {

       for (i=0;i<end;i++)

       {

@@ -104,18 +104,23 @@

          opus_val32 maxval=0;

          opus_val32 sum = 0;

-         j=M*eBands[i]; do {

-            maxval = MAX32(maxval, X[j+c*N]);

-            maxval = MAX32(maxval, -X[j+c*N]);

-         } while (++j<M*eBands[i+1]);

-

+         maxval = celt_maxabs32(&X[c*N+(eBands[i]<<LM)], (eBands[i+1]-eBands[i])<<LM);

          if (maxval > 0)

          {

-            int shift = celt_ilog2(maxval)-10;

-            j=M*eBands[i]; do {

-               sum = MAC16_16(sum, EXTRACT16(VSHR32(X[j+c*N],shift)),

-                                   EXTRACT16(VSHR32(X[j+c*N],shift)));

-            } while (++j<M*eBands[i+1]);

+            int shift = celt_ilog2(maxval) - 14 + (((m->logN[i]>>BITRES)+LM+1)>>1);

+            j=eBands[i]<<LM;

+            if (shift>0)

+            {

+               do {

+                  sum = MAC16_16(sum, EXTRACT16(SHR32(X[j+c*N],shift)),

+                        EXTRACT16(SHR32(X[j+c*N],shift)));

+               } while (++j<eBands[i+1]<<LM);

+            } else {

+               do {

+                  sum = MAC16_16(sum, EXTRACT16(SHL32(X[j+c*N],-shift)),

+                        EXTRACT16(SHL32(X[j+c*N],-shift)));

+               } while (++j<eBands[i+1]<<LM);

+            }

             /* We're adding one here to ensure the normalized band isn't larger than unity norm */

             bandE[i+c*m->nbEBands] = EPSILON+VSHR32(EXTEND32(celt_sqrt(sum)),-shift);

          } else {

@@ -150,16 +155,16 @@

 #else /* FIXED_POINT */

 /* Compute the amplitude (sqrt energy) in each of the bands */

-void compute_band_energies(const CELTMode *m, const celt_sig *X, celt_ener *bandE, int end, int C, int M)

+void compute_band_energies(const CELTMode *m, const celt_sig *X, celt_ener *bandE, int end, int C, int LM)

 {

    int i, c, N;

    const opus_int16 *eBands = m->eBands;

-   N = M*m->shortMdctSize;

+   N = m->shortMdctSize<<LM;

    c=0; do {

       for (i=0;i<end;i++)

       {

          opus_val32 sum;

-         sum = 1e-27f + celt_inner_prod(&X[c*N+M*eBands[i]], &X[c*N+M*eBands[i]], M*(eBands[i+1]-eBands[i]));

+         sum = 1e-27f + celt_inner_prod(&X[c*N+(eBands[i]<<LM)], &X[c*N+(eBands[i]<<LM)], (eBands[i+1]-eBands[i])<<LM);

          bandE[i+c*m->nbEBands] = celt_sqrt(sum);

          /*printf ("%f ", bandE[i+c*m->nbEBands]);*/

       }

--- a/celt/bands.h

+++ b/celt/bands.h

@@ -41,7 +41,7 @@

  * @param X Spectrum

  * @param bandE Square root of the energy for each band (returned)

  */

-void compute_band_energies(const CELTMode *m, const celt_sig *X, celt_ener *bandE, int end, int C, int M);

+void compute_band_energies(const CELTMode *m, const celt_sig *X, celt_ener *bandE, int end, int C, int LM);

 /*void compute_noise_energies(const CELTMode *m, const celt_sig *X, const opus_val16 *tonality, celt_ener *bandE);*/

--- a/celt/celt_encoder.c

+++ b/celt/celt_encoder.c

@@ -1519,7 +1519,7 @@

    if (secondMdct)

    {

       compute_mdcts(mode, 0, in, freq, C, CC, LM, st->upsample);

-      compute_band_energies(mode, freq, bandE, effEnd, C, M);

+      compute_band_energies(mode, freq, bandE, effEnd, C, LM);

       amp2Log2(mode, effEnd, end, bandE, bandLogE2, C);

       for (i=0;i<C*nbEBands;i++)

          bandLogE2[i] += HALF16(SHL16(LM, DB_SHIFT));

@@ -1528,7 +1528,7 @@

    compute_mdcts(mode, shortBlocks, in, freq, C, CC, LM, st->upsample);

    if (CC==2&&C==1)

       tf_chan = 0;

-   compute_band_energies(mode, freq, bandE, effEnd, C, M);

+   compute_band_energies(mode, freq, bandE, effEnd, C, LM);

    if (st->lfe)

    {

@@ -1651,7 +1651,7 @@

          isTransient = 1;

          shortBlocks = M;

          compute_mdcts(mode, shortBlocks, in, freq, C, CC, LM, st->upsample);

-         compute_band_energies(mode, freq, bandE, effEnd, C, M);

+         compute_band_energies(mode, freq, bandE, effEnd, C, LM);

          amp2Log2(mode, effEnd, end, bandE, bandLogE, C);

          /* Compensate for the scaling of short vs long mdcts */

          for (i=0;i<C*nbEBands;i++)