ref: 7252c25849956e4ebec793cb5d3678049d0e7fc3
parent: ddea3a6fbfa5727e8d7c4d1a8943bb72862a90ee
author: Jean-Marc Valin <jmvalin@jmvalin.ca>
date: Sun Jun 19 10:44:55 EDT 2016
Fixes signed integer overflow in fixed-point Burg We just explicitly allow the overflow with silk_MLA_ovflw() since the result seems to be correct because the overflows cancel each other.
--- a/silk/fixed/burg_modified_FIX.c
+++ b/silk/fixed/burg_modified_FIX.c
@@ -150,8 +150,11 @@
C_first_row[ k ] = silk_MLA( C_first_row[ k ], x1, x_ptr[ n - k - 1 ] ); /* Q( -rshifts ) */
C_last_row[ k ] = silk_MLA( C_last_row[ k ], x2, x_ptr[ subfr_length - n + k ] ); /* Q( -rshifts ) */
Atmp1 = silk_RSHIFT_ROUND( Af_QA[ k ], QA - 17 ); /* Q17 */
- tmp1 = silk_MLA( tmp1, x_ptr[ n - k - 1 ], Atmp1 ); /* Q17 */
- tmp2 = silk_MLA( tmp2, x_ptr[ subfr_length - n + k ], Atmp1 ); /* Q17 */
+ /* We sometimes have get overflows in the multiplications (even beyond +/- 2^32),
+ but they cancel each other and the real result seems to always fit in a 32-bit
+ signed integer. This was determined experimentally, not theoretically (unfortunately). */
+ tmp1 = silk_MLA_ovflw( tmp1, x_ptr[ n - k - 1 ], Atmp1 ); /* Q17 */
+ tmp2 = silk_MLA_ovflw( tmp2, x_ptr[ subfr_length - n + k ], Atmp1 ); /* Q17 */
}
tmp1 = -tmp1; /* Q17 */
tmp2 = -tmp2; /* Q17 */