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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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15
Python/pymath.c
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@ -79,18 +79,3 @@ round(double x)
return copysign(y, x);
}
#endif /* HAVE_ROUND */
static const unsigned int BitLengthTable[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
};
unsigned int _Py_bit_length(unsigned long d) {
unsigned int d_bits = 0;
while (d >= 32) {
d_bits += 6;
d >>= 6;
}
d_bits += BitLengthTable[d];
return d_bits;
}