libFLAC/fixed.c: Fix undefined behaviour
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.
diff --git a/src/libFLAC/fixed.c b/src/libFLAC/fixed.c
index 74b31b3..90c460c 100644
--- a/src/libFLAC/fixed.c
+++ b/src/libFLAC/fixed.c
@@ -349,27 +349,15 @@
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 @@
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);