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Optimize COUNT_ZERO_MSBS macro

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Reorder the conditions according to the expected distribution of input
signal. This seems to make it almost as fast as the clz builtin using
the bsr instruction.

Signed-off-by: Erik de Castro Lopo <erikd@mega-nerd.com>
This commit is contained in:
Miroslav Lichvar
2012-05-07 13:00:12 +02:00
committed by Erik de Castro Lopo
parent d9fde55674
commit 5289b309cf
1 changed files with 5 additions and 3 deletions
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8
src/libFLAC/bitreader.c
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@@ -68,9 +68,11 @@ COUNT_ZERO_MSBS (uint32_t 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] ) \
(word) > 0xffffff ? byte_to_unary_table[(word) >> 24] : \
!(word) ? 32 : \
(word) > 0xffff ? byte_to_unary_table[(word) >> 16] + 8 : \
(word) > 0xff ? byte_to_unary_table[(word) >> 8] + 16 : \
byte_to_unary_table[(word)] + 24 \
)
#endif