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Change CountDecimalDigit32() to simple implementation

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It is simple and pure C++. And it is found in performance test that it
is even faster than the original version, due to distribution of n. But
the performance gain is not obvious in RapidJSON.
This commit is contained in:
Milo Yip 2014-08-11 00:30:31 +08:00
parent 1900b7bace
commit c54915297b
1 changed files with 11 additions and 21 deletions
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32
include/rapidjson/internal/dtoa.h
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@ -218,27 +218,17 @@ inline void GrisuRound(char* buffer, int len, uint64_t delta, uint64_t rest, uin
}
inline unsigned CountDecimalDigit32(uint32_t n) {
static const uint32_t powers_of_10[] = {
0,
10,
100,
1000,
10000,
100000,
1000000,
10000000,
100000000,
1000000000
};
#ifdef _MSC_VER
unsigned long i = 0;
_BitScanReverse(&i, n | 1);
uint32_t t = (i + 1) * 1233 >> 12;
#elif __GNUC__
uint32_t t = (32 - __builtin_clz(n | 1)) * 1233 >> 12;
#endif
return t - (n < powers_of_10[t]) + 1;
// Simple pure C++ implementation was faster than __builtin_clz version in this situation.
if (n < 10) return 1;
if (n < 100) return 2;
if (n < 1000) return 3;
if (n < 10000) return 4;
if (n < 100000) return 5;
if (n < 1000000) return 6;
if (n < 10000000) return 7;
if (n < 100000000) return 8;
if (n < 1000000000) return 9;
return 10;
}
inline void DigitGen(const DiyFp& W, const DiyFp& Mp, uint64_t delta, char* buffer, int* len, int* K) {