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Add GCC specific optimisation for log base 2 operations.

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Based on a patch from Cristian Rodríguez.
This commit is contained in:
Erik de Castro Lopo
2012-03-30 21:55:08 +11:00
parent 238b2676c2
commit 5c44cd7d0c
3 changed files with 45 additions and 5 deletions
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14
src/libFLAC/bitreader.c
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@@ -55,14 +55,24 @@ typedef FLAC__uint32 brword;
#else
#define SWAP_BE_WORD_TO_HOST(x) ENDSWAP_32(x)
#endif
#if defined(__GNUC__)
/* "int __builtin_clz (unsigned int x) If x is 0, the result is undefined" */
static inline uint32_t
COUNT_ZERO_MSBS (uint32_t word)
{
if (word == 0)
return 32;
return __builtin_clz (word);
}
#else
/* counts the # of zero MSBs in a word */
#define COUNT_ZERO_MSBS(word) ( \
(word) <= 0xffff ? \
( (word) <= 0xff? byte_to_unary_table[word] + 24 : byte_to_unary_table[(word) >> 8] + 16 ) : \
( (word) <= 0xffffff? byte_to_unary_table[word >> 16] + 8 : byte_to_unary_table[(word) >> 24] ) \
)
/* this alternate might be slightly faster on some systems/compilers: */
#define COUNT_ZERO_MSBS2(word) ( (word) <= 0xff ? byte_to_unary_table[word] + 24 : ((word) <= 0xffff ? byte_to_unary_table[(word) >> 8] + 16 : ((word) <= 0xffffff ? byte_to_unary_table[(word) >> 16] + 8 : byte_to_unary_table[(word) >> 24])) )
#endif
/*