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Cheap early detection of unstable filters

We check the DC gain before doing the full RC calculations

--- a/silk/LPC_inv_pred_gain.c

+++ b/silk/LPC_inv_pred_gain.c

@@ -117,14 +117,19 @@

     opus_int   k;

     opus_int32 Atmp_QA[ 2 ][ SILK_MAX_ORDER_LPC ];

     opus_int32 *Anew_QA;

+    opus_int32 DC_resp=0;

     Anew_QA = Atmp_QA[ order & 1 ];

     /* Increase Q domain of the AR coefficients */

     for( k = 0; k < order; k++ ) {

+        DC_resp += (opus_int32)A_Q12[ k ];

         Anew_QA[ k ] = silk_LSHIFT( (opus_int32)A_Q12[ k ], QA - 12 );

     }

-

+    /* If the DC is unstable, we don't even need to do the full calculations */

+    if ( DC_resp >= 4096 ) {

+       return 1;

+    }

     return LPC_inverse_pred_gain_QA( invGain_Q30, Atmp_QA, order );

 }