Using 64-bit acculumation for C0 in silk_burg_modified()

Signed-off-by: Jean-Marc Valin <jmvalin@jmvalin.ca>

silk/fixed/burg_modified_FIX.c

@@ -54,7 +54,7 @@ void silk_burg_modified(
     int                         arch                /* I    Run-time architecture                                       */
 )
 {
-    opus_int         k, n, s, lz, rshifts, rshifts_extra, reached_max_gain;
+    opus_int         k, n, s, lz, rshifts, reached_max_gain;
     opus_int32       C0, num, nrg, rc_Q31, invGain_Q30, Atmp_QA, Atmp1, tmp1, tmp2, x1, x2;
     const opus_int16 *x_ptr;
     opus_int32       C_first_row[ SILK_MAX_ORDER_LPC ];
@@ -63,27 +63,23 @@ void silk_burg_modified(
     opus_int32       CAf[ SILK_MAX_ORDER_LPC + 1 ];
     opus_int32       CAb[ SILK_MAX_ORDER_LPC + 1 ];
     opus_int32       xcorr[ SILK_MAX_ORDER_LPC ];
+    opus_int64       C0_64;
 
     silk_assert( subfr_length * nb_subfr <= MAX_FRAME_SIZE );
 
     /* Compute autocorrelations, added over subframes */
-    silk_sum_sqr_shift( &C0, &rshifts, x, nb_subfr * subfr_length );
-    if( rshifts > MAX_RSHIFTS ) {
-        C0 = silk_LSHIFT32( C0, rshifts - MAX_RSHIFTS );
-        silk_assert( C0 > 0 );
-        rshifts = MAX_RSHIFTS;
+    C0_64 = silk_inner_prod16_aligned_64( x, x, subfr_length*nb_subfr );
+    lz = silk_CLZ64(C0_64);
+    rshifts = 32 + 1 + N_BITS_HEAD_ROOM - lz;
+    if (rshifts > MAX_RSHIFTS) rshifts = MAX_RSHIFTS;
+    if (rshifts < MIN_RSHIFTS) rshifts = MIN_RSHIFTS;
+    
+    if (rshifts > 0) {
+        C0 = (opus_int32)silk_RSHIFT64(C0_64, rshifts );        
     } else {
-        lz = silk_CLZ32( C0 ) - 1;
-        rshifts_extra = N_BITS_HEAD_ROOM - lz;
-        if( rshifts_extra > 0 ) {
-            rshifts_extra = silk_min( rshifts_extra, MAX_RSHIFTS - rshifts );
-            C0 = silk_RSHIFT32( C0, rshifts_extra );
-        } else {
-            rshifts_extra = silk_max( rshifts_extra, MIN_RSHIFTS - rshifts );
-            C0 = silk_LSHIFT32( C0, -rshifts_extra );
-        }
-        rshifts += rshifts_extra;
+        C0 = silk_LSHIFT32((opus_int32)C0_64, -rshifts );
     }
+    
     CAb[ 0 ] = CAf[ 0 ] = C0 + silk_SMMUL( SILK_FIX_CONST( FIND_LPC_COND_FAC, 32 ), C0 ) + 1;                                /* Q(-rshifts) */
     silk_memset( C_first_row, 0, SILK_MAX_ORDER_LPC * sizeof( opus_int32 ) );
     if( rshifts > 0 ) {