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libFLAC/fixed.c: Fix undefined behaviour

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Left shift if a negative integer such that the sign bit is affected is
(according to the C spec) undefined behaviour and the residual
calculations using the shift operator were hitting this.

Fortunately these same calculations using plain multiplication do not
invoke UB and according to benchmarking (on x86_64 linux) have the same
performance as the bit shift version.
This commit is contained in:
Erik de Castro Lopo
2015-08-29 05:21:43 +10:00
parent 25d387377a
commit 1b8af6bb45
1 changed files with 0 additions and 24 deletions
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24
src/libFLAC/fixed.c
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@@ -349,27 +349,15 @@ void FLAC__fixed_compute_residual(const FLAC__int32 data[], unsigned data_len, u
break;
case 2:
for(i = 0; i < idata_len; i++)
#if 1 /* OPT: may be faster with some compilers on some systems */
residual[i] = data[i] - (data[i-1] << 1) + data[i-2];
#else
residual[i] = data[i] - 2*data[i-1] + data[i-2];
#endif
break;
case 3:
for(i = 0; i < idata_len; i++)
#if 1 /* OPT: may be faster with some compilers on some systems */
residual[i] = data[i] - (((data[i-1]-data[i-2])<<1) + (data[i-1]-data[i-2])) - data[i-3];
#else
residual[i] = data[i] - 3*data[i-1] + 3*data[i-2] - data[i-3];
#endif
break;
case 4:
for(i = 0; i < idata_len; i++)
#if 1 /* OPT: may be faster with some compilers on some systems */
residual[i] = data[i] - ((data[i-1]+data[i-3])<<2) + ((data[i-2]<<2) + (data[i-2]<<1)) + data[i-4];
#else
residual[i] = data[i] - 4*data[i-1] + 6*data[i-2] - 4*data[i-3] + data[i-4];
#endif
break;
default:
FLAC__ASSERT(0);
@@ -391,27 +379,15 @@ void FLAC__fixed_restore_signal(const FLAC__int32 residual[], unsigned data_len,
break;
case 2:
for(i = 0; i < idata_len; i++)
#if 1 /* OPT: may be faster with some compilers on some systems */
data[i] = residual[i] + (data[i-1]<<1) - data[i-2];
#else
data[i] = residual[i] + 2*data[i-1] - data[i-2];
#endif
break;
case 3:
for(i = 0; i < idata_len; i++)
#if 1 /* OPT: may be faster with some compilers on some systems */
data[i] = residual[i] + (((data[i-1]-data[i-2])<<1) + (data[i-1]-data[i-2])) + data[i-3];
#else
data[i] = residual[i] + 3*data[i-1] - 3*data[i-2] + data[i-3];
#endif
break;
case 4:
for(i = 0; i < idata_len; i++)
#if 1 /* OPT: may be faster with some compilers on some systems */
data[i] = residual[i] + ((data[i-1]+data[i-3])<<2) - ((data[i-2]<<2) + (data[i-2]<<1)) - data[i-4];
#else
data[i] = residual[i] + 4*data[i-1] - 6*data[i-2] + 4*data[i-3] - data[i-4];
#endif
break;
default:
FLAC__ASSERT(0);