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Another aliasing hack in the SILK PLC with SMALL_FOOTPRINT

--- a/silk/PLC.c

+++ b/silk/PLC.c

@@ -181,7 +181,11 @@

     opus_int32 *sLPC_Q14_ptr;

     VARDECL( opus_int16, exc_buf );

     opus_int16 A_Q12[ MAX_LPC_ORDER ];

+#ifdef SMALL_FOOTPRINT

+    opus_int16 *sLTP;

+#else

     VARDECL( opus_int16, sLTP );

+#endif

     VARDECL( opus_int32, sLTP_Q14 );

     silk_PLC_struct *psPLC = &psDec->sPLC;

     opus_int32 prevGain_Q10[2];

@@ -188,8 +192,13 @@

     SAVE_STACK;

     ALLOC( exc_buf, 2*psPLC->subfr_length, opus_int16 );

-    ALLOC( sLTP, psDec->ltp_mem_length, opus_int16 );

     ALLOC( sLTP_Q14, psDec->ltp_mem_length + psDec->frame_length, opus_int32 );

+#ifdef SMALL_FOOTPRINT

+    /* Ugly hack that breaks aliasing rules to save stack: put sLTP at the very end of sLTP_Q14. */

+    sLTP = ((opus_int16*)&sLTP_Q14[psDec->ltp_mem_length + psDec->frame_length])-psDec->ltp_mem_length;

+#else

+    ALLOC( sLTP, psDec->ltp_mem_length, opus_int16 );

+#endif

     prevGain_Q10[0] = silk_RSHIFT( psPLC->prevGain_Q16[ 0 ], 6);

     prevGain_Q10[1] = silk_RSHIFT( psPLC->prevGain_Q16[ 1 ], 6);