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bpo-29782: Consolidate _Py_Bit_Length() (GH-20739)

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In GH-2866, _Py_Bit_Length() was added to pymath.h for lack of a better
location. GH-20518 added a more appropriate header file for bit utilities. It
also shows how to properly use intrinsics. This allows reconsidering bpo-29782.

* Move the function to the new header.
* Changed return type to match __builtin_clzl() and reviewed usage.
* Use intrinsics where available.
* Pick a fallback implementation suitable for inlining.
This commit is contained in:
Niklas Fiekas 2020-06-15 14:33:48 +02:00 • committed by GitHub
parent 25f38d7044
commit 794e7d1ab2
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6 changed files with 100 additions and 36 deletions
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47
Include/internal/pycore_bitutils.h
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@ -7,8 +7,8 @@
- _Py_bswap64(uint64_t)
*/
#ifndef Py_INTERNAL_BSWAP_H
#define Py_INTERNAL_BSWAP_H
#ifndef Py_INTERNAL_BITUTILS_H
#define Py_INTERNAL_BITUTILS_H
#ifdef __cplusplus
extern "C" {
#endif
@ -131,8 +131,47 @@ _Py_popcount32(uint32_t x)
}
// Return the index of the most significant 1 bit in 'x'. This is the smallest
// integer k such that x < 2**k. Equivalent to floor(log2(x)) + 1 for x != 0.
static inline int
_Py_bit_length(unsigned long x)
{
#if (defined(__clang__) || defined(__GNUC__))
if (x != 0) {
// __builtin_clzl() is available since GCC 3.4.
// Undefined behavior for x == 0.
return (int)sizeof(unsigned long) * 8 - __builtin_clzl(x);
}
else {
return 0;
}
#elif defined(_MSC_VER)
// _BitScanReverse() is documented to search 32 bits.
Py_BUILD_ASSERT(sizeof(unsigned long) <= 4);
unsigned long msb;
if (_BitScanReverse(&msb, x)) {
return (int)msb + 1;
}
else {
return 0;
}
#else
const int BIT_LENGTH_TABLE[32] = {
0, 1, 2, 2, 3, 3, 3, 3, 4, 4, 4, 4, 4, 4, 4, 4,
5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5
};
int msb = 0;
while (x >= 32) {
msb += 6;
x >>= 6;
}
msb += BIT_LENGTH_TABLE[x];
return msb;
#endif
}
#ifdef __cplusplus
}
#endif
#endif /* !Py_INTERNAL_BSWAP_H */
#endif /* !Py_INTERNAL_BITUTILS_H */