Limits how fast the CBR gain multiplier can increase

This fixes an overflow that was caused by having a huge gain

silk/float/encode_frame_FLP.c

@@ -94,7 +94,8 @@ opus_int silk_encode_frame_FLP(
     silk_float   res_pitch[ 2 * MAX_FRAME_LENGTH + LA_PITCH_MAX ];
     ec_enc       sRangeEnc_copy, sRangeEnc_copy2;
     silk_nsq_state sNSQ_copy, sNSQ_copy2;
-    opus_int32   seed_copy, nBits, nBits_lower, nBits_upper, gainMult_Q10, gainMult_lower, gainMult_upper;
+    opus_int32   seed_copy, nBits, nBits_lower, nBits_upper, gainMult_lower, gainMult_upper;
+    opus_int16   gainMult_Q8;
     opus_int16   ec_prevLagIndex_copy;
     opus_int     ec_prevSignalType_copy;
     opus_int8    LastGainIndex_copy2;
@@ -181,7 +182,7 @@ TOC(NSQ)
     } else {
         /* Loop over quantizer and entroy coding to control bitrate */
         maxIter = 5;
-        gainMult_Q10 = SILK_FIX_CONST( 1, 10 );
+        gainMult_Q8 = SILK_FIX_CONST( 1, 8 );
         found_lower = 0;
         found_upper = 0;
         for( iter = 0; ; iter++ ) {
@@ -234,7 +235,7 @@ TOC(ENCODE_PULSES)
             if( nBits > maxBits ) {
                 found_upper = 1;
                 nBits_upper = nBits;
-                gainMult_upper = gainMult_Q10;
+                gainMult_upper = gainMult_Q8;
                 if( found_lower == 0 && iter >= 3 ) {
                     /* Adjust the quantizer's rate/distortion tradeoff */
                     sEncCtrl.Lambda *= 1.5f;
@@ -242,7 +243,7 @@ TOC(ENCODE_PULSES)
             } else if( nBits < maxBits - 5 ) {
                 found_lower = 1;
                 nBits_lower = nBits;
-                gainMult_lower = gainMult_Q10;
+                gainMult_lower = gainMult_Q8;
                 /* Copy part of the output state */
                 silk_memcpy( &sRangeEnc_copy2, psRangeEnc, sizeof( ec_enc ) );
                 silk_memcpy( ec_buf_copy, psRangeEnc->buf, psRangeEnc->offs );
@@ -256,25 +257,26 @@ TOC(ENCODE_PULSES)
             if( ( found_lower & found_upper ) == 0 ) {
                 /* Adjust gain according to high-rate rate/distortion curve */
                 opus_int32 gain_factor_Q16;
-                gain_factor_Q16 = silk_log2lin( silk_LSHIFT( nBits - maxBits, 7 ) / psEnc->sCmn.frame_length + SILK_FIX_CONST( 16, 7 ) );
+                gain_factor_Q16 = silk_log2lin( silk_LSHIFT( nBits - maxBits, 7 ) / psEnc->sCmn.frame_length + SILK_FIX_CONST( 16, 7 ) );
+                gain_factor_Q16 = silk_min_32(gain_factor_Q16, SILK_FIX_CONST( 2, 16 ) );
                 if( nBits > maxBits ) {
                     gain_factor_Q16 = silk_max_32( gain_factor_Q16, SILK_FIX_CONST( 1.3, 16 ) );
                 }
-                gainMult_Q10 = silk_SMULWW( gainMult_Q10, gain_factor_Q16 );
+                gainMult_Q8 = silk_SMULWB( gain_factor_Q16, gainMult_Q8 );
             } else {
                 /* Adjust gain by interpolating */
-                gainMult_Q10 = gainMult_lower + ( ( gainMult_upper - gainMult_lower ) * ( maxBits - nBits_lower ) ) / ( nBits_upper - nBits_lower );
+                gainMult_Q8 = gainMult_lower + ( ( gainMult_upper - gainMult_lower ) * ( maxBits - nBits_lower ) ) / ( nBits_upper - nBits_lower );
                 /* New gain multplier must be between 25% and 75% of old range (note that gainMult_upper < gainMult_lower) */
-                if( gainMult_Q10 > gainMult_lower + silk_RSHIFT32( gainMult_upper - gainMult_lower, 2 ) ) {
-                    gainMult_Q10 = gainMult_lower + silk_RSHIFT32( gainMult_upper - gainMult_lower, 2 );
+                if( gainMult_Q8 > gainMult_lower + silk_RSHIFT32( gainMult_upper - gainMult_lower, 2 ) ) {
+                    gainMult_Q8 = gainMult_lower + silk_RSHIFT32( gainMult_upper - gainMult_lower, 2 );
                 } else
-                    if( gainMult_Q10 < gainMult_upper - silk_RSHIFT32( gainMult_upper - gainMult_lower, 2 ) ) {
-                        gainMult_Q10 = gainMult_upper - silk_RSHIFT32( gainMult_upper - gainMult_lower, 2 );
+                    if( gainMult_Q8 < gainMult_upper - silk_RSHIFT32( gainMult_upper - gainMult_lower, 2 ) ) {
+                        gainMult_Q8 = gainMult_upper - silk_RSHIFT32( gainMult_upper - gainMult_lower, 2 );
                     }
             }
 
             for( i = 0; i < psEnc->sCmn.nb_subfr; i++ ) {
-                pGains_Q16[ i ] = silk_LSHIFT_SAT32( silk_SMULWW( sEncCtrl.GainsUnq_Q16[ i ], gainMult_Q10 ), 6 );
+                pGains_Q16[ i ] = silk_LSHIFT_SAT32( silk_SMULWB( sEncCtrl.GainsUnq_Q16[ i ], gainMult_Q8 ), 8 );
             }
             psEnc->sShape.LastGainIndex = sEncCtrl.lastGainIndexPrev;