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ref: 8ab3e80ec46e8f811c5f880890488029261dea70
parent: 18335e8e4d6ddb289b31d8a4a362f5e15c24ff0a
author: Jean-Marc Valin <jmvalin@jmvalin.ca>
date: Sat Jul 30 18:40:48 EDT 2016 

Fixes an overflow in LPC_inverse_pred_gain_QA()

We now declare that anything that would overflow is not stable enough


--- a/silk/LPC_inv_pred_gain.c
+++ b/silk/LPC_inv_pred_gain.c
@@ -77,10 +77,21 @@
 
         /* Update AR coefficient */
         for( n = 0; n < (k + 1) >> 1; n++ ) {
+            opus_int64 tmp64;
             tmp1 = A_QA[ n ];
             tmp2 = A_QA[ k - n - 1 ];
-            A_QA[ n ] =            MUL32_FRAC_Q( tmp1 - MUL32_FRAC_Q( tmp2, rc_Q31, 31 ), rc_mult2, mult2Q );
-            A_QA[ k - n - 1 ] = MUL32_FRAC_Q( tmp2 - MUL32_FRAC_Q( tmp1, rc_Q31, 31 ), rc_mult2, mult2Q );
+            tmp64 = silk_RSHIFT_ROUND64( silk_SMULL( silk_SUB_SAT32(tmp1,
+                  MUL32_FRAC_Q( tmp2, rc_Q31, 31 ) ), rc_mult2 ), mult2Q);
+            if( tmp64 > silk_int32_MAX || tmp64 < silk_int32_MIN ) {
+               return 0;
+            }
+            A_QA[ n ] = ( opus_int32 )tmp64;
+            tmp64 = silk_RSHIFT_ROUND64( silk_SMULL( silk_SUB_SAT32(tmp2,
+                  MUL32_FRAC_Q( tmp1, rc_Q31, 31 ) ), rc_mult2), mult2Q);
+            if( tmp64 > silk_int32_MAX || tmp64 < silk_int32_MIN ) {
+               return 0;
+            }
+            A_QA[ k - n - 1 ] = ( opus_int32 )tmp64;
         }
     }