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avcodec/ac3enc: fix undefined negative left shift

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This should fix the undefined behavior reported in:
https://trac.ffmpeg.org/ticket/4727.

I can reproduce this at runtime: simply stick in an abort call in
asym_quant to check if c < 0 and run FATE. I don't know ac3 so I can't
confirm if negative coefficients are intentional, but at the moment they
clearly are according to FATE.

This resolves the undefined behavior. Tested with FATE.

Signed-off-by: Ganesh Ajjanagadde <gajjanagadde@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
This commit is contained in:
Ganesh Ajjanagadde 2015-10-09 15:59:04 -04:00 committed by Michael Niedermayer
parent c4e23ca853
commit 979572365f
1 changed files with 1 additions and 1 deletions
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libavcodec/ac3enc.c
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@ -1183,7 +1183,7 @@ static inline int asym_quant(int c, int e, int qbits)
{
int m;
c = (((c << e) >> (24 - qbits)) + 1) >> 1;
c = (((c * (1<<e)) >> (24 - qbits)) + 1) >> 1;
m = (1 << (qbits-1));
if (c >= m)
c = m - 1;