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MINOR: tools: add generic pointer hashing functions

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There are a few places where it's convenient to hash a pointer to compute
a statistics bucket. Here we're basically reusing the hash that was used
by memory profiling with a minor update that the multiplier was corrected
to be prime and stand by its promise to have equal numbers of 1 and 0,
and that 32-bit platforms won't lose range anymore.

A two-pointer variant was also added.
This commit is contained in:
Willy Tarreau 2022-09-07 10:56:03 +02:00
parent 6a28a30efa
commit 4a3907617f
1 changed files with 41 additions and 0 deletions
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41
include/haproxy/tools.h
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@ -1070,6 +1070,47 @@ static inline uint statistical_prng_range(uint range)
return mul32hi(statistical_prng(), range ? range - 1 : 0);
}
/* returns a hash on <bits> bits of pointer <p> that is suitable for being used
* to compute statistic buckets, in that it's fast and reasonably distributed
* thanks to mixing the bits via a multiplication by a prime number and using
* the middle bits on 64-bit platforms or remixing the topmost with lowest ones
* on 32-bit. It provides ~2588 unique values (~1510 non-colliding) at 100%
* fill ratio for 12 bits, ~1296 (~756 non-colliding) at 100% fill ratio for 11
* bits, ~648 (~378 non-colliding) at 100% fill ratio for 10 bits, ~163 (95 non
* colliding) at 100% fill ratio for 8 bits, hence 1-1/e and 1/e respectively.
* It must be inlined so that <bits> is always a compile-time constant.
*/
static forceinline uint ptr_hash(const void *p, const int bits)
{
unsigned long long x = (unsigned long)p;
x *= 0xc1da9653U;
if (sizeof(long) == 4)
x ^= x >> 32;
else
x >>= 33 - bits / 2;
return x & (~0U >> (-bits & 31));
}
/* Same as above but works on two pointers. It will return the same values
* if the second pointer is NULL.
*/
static forceinline uint ptr2_hash(const void *p1, const void *p2, const int bits)
{
unsigned long long x = (unsigned long)p1;
unsigned long long y = (unsigned long)p2;
x *= 0xc1da9653U;
y *= 0x96531cadU;
x ^= y;
if (sizeof(long) == 4)
x ^= x >> 32;
else
x >>= 33 - bits / 2;
return x & (~0U >> (-bits & 31));
}
/* Update array <fp> with the character transition <prev> to <curr>. If <prev>
* is zero, it's assumed that <curr> is the first character. If <curr> is zero
* its assumed to mark the end. Both may be zero. <fp> is a 1024-entries array