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celt_lpc: avoid overflows when computing lpcs in fixed point

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The LPCs are computed in 32-bit, so increase the allowed range from +/-8
to +/-64 to avoid overflows caught during fuzzing. Before downshifting
back down to the +/-8 range in the final 16-bit output, perform bandwidth
extension to avoid any additional overflow issues.
This commit is contained in:
Felicia Lim 2021-02-22 22:29:14 -08:00
parent 16286a25fd
commit 7b05f44f4b
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GPG key ID: FDBB8534FAD78B08
5 changed files with 108 additions and 7 deletions
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54
celt/celt_lpc.c
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@ -57,10 +57,10 @@ int p
opus_val32 rr = 0;
for (j = 0; j < i; j++)
rr += MULT32_32_Q31(lpc[j],ac[i - j]);
rr += SHR32(ac[i + 1],3);
r = -frac_div32(SHL32(rr,3), error);
rr += SHR32(ac[i + 1],6);
r = -frac_div32(SHL32(rr,6), error);
/* Update LPC coefficients and total error */
lpc[i] = SHR32(r,3);
lpc[i] = SHR32(r,6);
for (j = 0; j < (i+1)>>1; j++)
{
opus_val32 tmp1, tmp2;
@ -82,8 +82,52 @@ int p
}
}
#ifdef FIXED_POINT
for (i=0;i<p;i++)
_lpc[i] = ROUND16(lpc[i],16);
{
/* Convert the int32 lpcs to int16 and ensure there are no wrap-arounds.
This reuses the logic in silk_LPC_fit() and silk_bwexpander_32(). Any bug
fixes should also be applied there. */
int iter, idx = 0;
opus_val32 maxabs, absval, chirp_Q16, chirp_minus_one_Q16;
for (iter = 0; iter < 10; iter++) {
maxabs = 0;
for (i = 0; i < p; i++) {
absval = ABS32(lpc[i]);
if (absval > maxabs) {
maxabs = absval;
idx = i;
}
}
maxabs = PSHR32(maxabs, 13); /* Q25->Q12 */
if (maxabs > 32767) {
maxabs = MIN32(maxabs, 163838);
chirp_Q16 = QCONST32(0.999, 16) - DIV32(SHL32(maxabs - 32767, 14),
SHR32(MULT32_32_32(maxabs, idx + 1), 2));
chirp_minus_one_Q16 = chirp_Q16 - 65536;
/* Apply bandwidth expansion. */
for (i = 0; i < p - 1; i++) {
lpc[i] = MULT32_32_Q16(chirp_Q16, lpc[i]);
chirp_Q16 += PSHR32(MULT32_32_32(chirp_Q16, chirp_minus_one_Q16), 16);
}
lpc[p - 1] = MULT32_32_Q16(chirp_Q16, lpc[p - 1]);
} else {
break;
}
}
if (iter == 10) {
/* If the coeffs still do not fit into the 16 bit range after 10 iterations,
fall back to the A(z)=1 filter. */
OPUS_CLEAR(lpc, p);
_lpc[0] = 4096; /* Q12 */
} else {
for (i = 0; i < p; i++) {
_lpc[i] = EXTRACT16(PSHR32(lpc[i], 13)); /* Q25->Q12 */
}
}
}
#endif
}