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Applying the forward FFT gain up-front for fixed-point too

This makes us lose a bit of precision in the first steps, but our gain is more
precise because it's only rounded once. Overall, SNR is slightly improved.

--- a/celt/_kiss_fft_guts.h

+++ b/celt/_kiss_fft_guts.h

@@ -65,10 +65,6 @@

       do{ (m).r = ADD32(S_MUL((a).r,(b).r) , S_MUL((a).i,(b).i)); \

           (m).i = SUB32(S_MUL((a).i,(b).r) , S_MUL((a).r,(b).i)); }while(0)

-#   define C_MUL4(m,a,b) \

-      do{ (m).r = SHR32(SUB32(S_MUL((a).r,(b).r) , S_MUL((a).i,(b).i)),2); \

-          (m).i = SHR32(ADD32(S_MUL((a).r,(b).i) , S_MUL((a).i,(b).r)),2); }while(0)

-

 #   define C_MULBYSCALAR( c, s ) \

       do{ (c).r =  S_MUL( (c).r , s ) ;\

           (c).i =  S_MUL( (c).i , s ) ; }while(0)

--- a/celt/kiss_fft.c

+++ b/celt/kiss_fft.c

@@ -66,8 +66,6 @@

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

       {

          kiss_fft_cpx t;

-         Fout->r = SHR32(Fout->r, 1);Fout->i = SHR32(Fout->i, 1);

-         Fout2->r = SHR32(Fout2->r, 1);Fout2->i = SHR32(Fout2->i, 1);

          C_MUL (t,  *Fout2 , *tw1);

          tw1 += fstride;

          C_SUB( *Fout2 ,  *Fout , t );

@@ -131,12 +129,10 @@

       tw3 = tw2 = tw1 = st->twiddles;

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

       {

-         C_MUL4(scratch[0],Fout[m] , *tw1 );

-         C_MUL4(scratch[1],Fout[m2] , *tw2 );

-         C_MUL4(scratch[2],Fout[m3] , *tw3 );

+         C_MUL(scratch[0],Fout[m] , *tw1 );

+         C_MUL(scratch[1],Fout[m2] , *tw2 );

+         C_MUL(scratch[2],Fout[m3] , *tw3 );

-         Fout->r = PSHR32(Fout->r, 2);

-         Fout->i = PSHR32(Fout->i, 2);

          C_SUB( scratch[5] , *Fout, scratch[1] );

          C_ADDTO(*Fout, scratch[1]);

          C_ADD( scratch[3] , scratch[0] , scratch[2] );

@@ -227,7 +223,6 @@

       tw1=tw2=st->twiddles;

       k=m;

       do {

-         C_FIXDIV(*Fout,3); C_FIXDIV(Fout[m],3); C_FIXDIV(Fout[m2],3);

          C_MUL(scratch[1],Fout[m] , *tw1);

          C_MUL(scratch[2],Fout[m2] , *tw2);

@@ -336,7 +331,6 @@

       Fout4=Fout0+4*m;

       for ( u=0; u<m; ++u ) {

-         C_FIXDIV( *Fout0,5); C_FIXDIV( *Fout1,5); C_FIXDIV( *Fout2,5); C_FIXDIV( *Fout3,5); C_FIXDIV( *Fout4,5);

          scratch[0] = *Fout0;

          C_MUL(scratch[1] ,*Fout1, tw[u*fstride]);

@@ -614,6 +608,16 @@

     int fstride[MAXFACTORS];

     int i;

     int shift;

+#ifdef FIXED_POINT

+    /* FIXME: This should eventually just go in the state. */

+    opus_val16 scale;

+    int scale_shift;

+    scale_shift = celt_ilog2(st->nfft);

+    if (st->nfft == 1<<scale_shift)

+       scale = Q15ONE;

+    else

+       scale = (1073741824+st->nfft/2)/st->nfft>>(15-scale_shift);

+#endif

     /* st->shift can be -1 */

     shift = st->shift>0 ? st->shift : 0;

@@ -622,10 +626,13 @@

     /* Bit-reverse the input */

     for (i=0;i<st->nfft;i++)

     {

-       fout[st->bitrev[i]] = fin[i];

-#ifndef FIXED_POINT

-       fout[st->bitrev[i]].r *= st->scale;

-       fout[st->bitrev[i]].i *= st->scale;

+       kiss_fft_cpx x = fin[i];

+#ifdef FIXED_POINT

+       fout[st->bitrev[i]].r = SHR32(MULT16_32_Q15(scale, x.r), scale_shift);

+       fout[st->bitrev[i]].i = SHR32(MULT16_32_Q15(scale, x.i), scale_shift);

+#else

+       fout[st->bitrev[i]].r = st->scale*x.r;

+       fout[st->bitrev[i]].i = st->scale*x.i;

 #endif

     }